■ h ^ 4- «fe? -: •• * v^V V-^V V*- T V r ^°- : - •:> ^ ^ o V jy c o » o „ *<$> o V "O, A** • <5n & ♦ ^W/V ^ J> »V "^ 'o, "'^.T*" A o O ii> J I 1C 9069 Bureau of Mines Information Circular/1986 Aluminum Fluxing Salts: A Critical Review of the Chemistry and Structures of Alkali Aluminum Halides By Charles A. Sorrell, John G. Groetsch, Jr., and D. M. Soboroff UNITED STATES DEPARTMENT OF THE INTERIOR f^t^ J &ti £u/ , faiAjxjxuc « Library of Congress Cataloging in Publication Data: Sorrel!, Charles A • Aluminum fluxing salts. (Bureau of Mines information circular ; 9069) Bibliography: p. 34-37. Supt. of Docs, no.: I 28.27 : 9069. 1. Aluminum— Metallurgy. 2. Alkali aluminum ha lides. 3. Flux (Metallurgy). I. Groetsch, J. G. (John G. ), II. Soboro ff, D. M. (David M.). III. Title. IV. Series: In formation circular (United States. Bu- reau of Mines) ; 9069. TN295.U4 [TN775] 622s [669'. 7221 85-600303 CONTENTS Page Abstract , , Introduction , Crystal chemistry of the alkali aluminum halides , The alkali halides , Aluminum chloride , Aluminum fluoride , , Cryolite , Na 3 A1F 6 Potassium cryolite , K 3 A1F 6 Elpasolite , K 2 NaAlF 6 Chiolite, Na 5 Al 3 F li+ KAIF^ and NaAlF^ NaAlCl^ and KALCl^ Summary of crystal chemistry Phase equilibria Melting points and transition temperatures Alkali halides Aluminum halides Alkali hexaf luoroaluminates Chiolite Alkali tetraf luoroaluminates Alkali tetrachloroaluminates Alkali halide systems The alkali haloaluminates Sodium halide-cryolite systems The alkali chloroaluminates Mixed alkali halides Equilibria in the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 Summary of phase equilibria data Phase relations in the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 Experimental procedures Solid solubility in the elpasolite phase Subsolidus equilibria Powder diffraction data for KAlF tt and K 3 A1F 6 Conclusions References ILLUSTRATIONS 1. The crystal structure of A1C1 3 , projected on the (010) plane 2. One layer of A1C1 3 , projected on the (001) plane 3. The structure of hexagonal A1F 3 , projected on the (0001) plane 4. The structure of hexagonal A1F 3 , projected on the (1120) plane 5. The structure of cryolite, Na 3 AlF 6 , projected on the (001) plane 6. The structure of K-cryolite, K 3 A1F 6 , projected on the (001) plane 7. An alternative structure for K-cryolite, assuming space group P4/mnc 8. The structure of elpasolite, K 2 NaAlF 6 , assuming space group Pa3 9. The structure of one layer of chiolite, Na 5 Al 3 F 1 i + , projected on the (001) plane 10. The structure of chiolite, projected on the (100) plane 11. The structures of NaAlF 4 and KAIF4 12. The structure of NaAlCl 4 , projected on the (001) plane 1 2 3 3 5 6 7 9 10 12 15 16 16 17 17 17 18 18 18 18 19 19 20 20 21 21 23 27 27 28 28 29 30 32 34 5 6 7 8 9 11 11 13 14 15 16 17 ii ILLUSTRATIONS — Continued Page 13. The system NaF-KF and the system NaCl-KCl 19 14. The systems NaF-NaCl and NaF-KCl 20 15. The system NaF-AlF 3 and the system NaCl-AlCl 3 20 16. The system KF-A1F 3 and the system KCI-AICI3 21 17. The system NaF-Na 3 AlF 6 and the system NaCl-Na 3 AlF 6 21 18. The system NaCl-KCl-AlCl 3 22 19. The reciprocal system NaCT-KCl-NaF-KF , redrawn from the data of reference 47 22 20. The reciprocal system NaCl-KCl-NaF-KF , redrawn from the data of reference 25 22 21. The system NaCl-KCl-Na 3 AlFg-K 3 AlFg , redrawn from the data of reference 8.. 23 22. Liquidus surfaces of the faces of the compositional prism for the system NaCl-KCl-NaF-KF-Na 3 AlF 6 -K 3 AlF 6 , redrawn from the data of reference 36.... 24 23. The NaF-KCl-K 2 NaAlF 6 section through the compositional prism 25 24. The NaF-NaCl-K^NaAlFg section through the compositional prism 25 25. The KF-NaCl-K 2 NaAlF 6 section through the compositional prism 25 26. The KF-KCl-K 2 NaAlF 6 section through the compositional prism 25 27 . The Na 3 A1F 6 -K 3 A1F 6 binary diagram 26 28. The system NaF-KF-AlF 3 26 29. The system NaCl-NaF-ALF 3 27 30. The system NaCl-KCl-NaF 27 31. X-ray lattice measurements for samples in the system Na 3 AlFg-K 3 AlF 6 28 32. X-ray intensity ratio measurements for samples in the system Na 3 AlF 6 - K 3 A1F 6 29 33. Surfaces of the compositional prism for the system NaCl-KCl-AlCl 3 -NaF-KF- A1F 3 , showing subsolidus compatibility relationships 29 34. Subsolidus compatibility tetrahedra in the system NaCl-KCl-AlCl 3 -NaF-KF- A1F 6 31 35. Subsolidus compatibility in the portion of the system NaCT-KCl-AlCl 3 -NaF- KF-A1F 3 corresponding to those in reference 36 31 36. Subsolidus compatibility tetrahedra in the volume bounded by NaCl-KCl- AlF 3 -Na 3 AlF 6 -K 3 AlF 6 31 37. Subsolidus compatibility tetrahedra in the volume bounded by NaCl-KCl- A1C1 3 -A1F 3 31 38. Section through the compositional prism of the system NaCl-KCl-AlCl 3 -NaF- KF-A1F 3 at the chloride-fluoride molar ratio of 1:1 32 TABLES 1. Crystallographic data and sources of powder diffraction data for crystal- line phases in the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 4 2. Measured and calculated interionic distances in the alkali halides 5 3. Interionic distances in the cryolite structure 8 4. Interionic distances in K 3 A1F 6 10 5. Interionic distances in elpasolite 12 6. Interionic distances in chiolite 15 7. Interionic distances in KA1F 4 16 8. Interionic distances in NaAlF 4 16 9. Interionic distances in NaAlCl 4 17 10. Powder diffraction data for KALF4 33 ALUMINUM FLUXING SALTS: A CRITICAL REVIEW OF THE CHEMISTRY AND STRUCTURES OF ALKALI ALUMINUM HALIDES By Charles A. Sorrell, 1 John G. Groetsch, Jr., 2 and D. M. Soboroff 3 ABSTRACT This Bureau of Mines publication reviews the structural characteris- tics of crystalline phases and phase equilibria data for the system NaCl-KCl-AlCl3-NaF-KF-AlF3 , which encompasses a large number of molten salt fluxes currently used in aluminum recycling. Its purpose is to provide guidelines for research into the relationships between molten salt compositions and their physical properties, notably vapor pres- sures, densities, surface tensions, and viscosities, knowledge of which is essential to maximizing fluxing efficiencies and metal recovery and minimizing hazardous emissions and disposal problems. In addition, this report describes experimental determinations of sub- solidus compatibility relationships in the system, which is of the qua- ternary reciprocal type, containing 12 different stable 4-phase assem- blages in the solid state. The compatibility diagram serves to define the important compositional planes across which important changes in properties are likely to occur. ^Geologist. ^Chemical engineer. ^Research supervisor. Avondale Research Center, Bureau of Mines, Avondale, MD . INTRODUCTION This publication combines a review of technical information from the literature with experimental data associated with remelting of scrap aluminum and recycling of dross. Bureau of Mines research in this area is not new; in 1916, Gillett ( 16)4 published a lengthy summary of the problem and reviewed the technology available at that time. The problems en- countered in melting aluminum are conse- quences of its extreme reactivity, ex- ceeded only by those of the alkali and alkaline earth metals. Were it not for the fact that a thin, transparent alumi- num oxide film forms on the surface of the metal immediately on exposure to the atmosphere and serves as an effective barrier to further reaction, aluminum would find few practical applications. During melting of scrap aluminum, for- mation of an oxide layer on the surface of the molten metal is unavoidable unless melting is accomplished in a high vacuum or completely inert atmosphere, neither of which is a feasible method. Reactions with the furnace atmosphere form not only aluminum oxide, AI2O3 , but also aluminum nitride, A1N, aluminum carbide, Al^C^, and a range of oxide and oxynitride spi- nels. Fortunately, even though the den- sities of these phases are greater than that of molten aluminum, some remain on the surface of the melt and can be skimmed prior to removal of the melt from the furnace. This skim, or dross, can contain as much as 85 pet metallic alumi- num trapped as droplets; these droplets are coated with a thin but remarkably strong layer of oxides which prevents the droplets from coalescing and combining with the metal in the bath. Larger alu- minum producers sell the skim or dross to smaller, independent operators who recy- cle it to recover a portion of the metal content. There are other problems with melting aluminum scrap. Because the material, which may include beverage cans, borings and turnings, foil, etc., has a high Underlined numbers in parentheses re- fer to items in the list of references at the end of this report. specific surface area, it oxidizes read- ily, and the oxide coating prevents the metal from coalescing into a pool. In practice, therefore, the scrap is added to a molten metal heel in a charging well and is pushed beneath the surface as rap- idly as possible. It has become standard practice to use a cover of molten salt on the surface of the liquid metal in the charging well to serve the multiple pur- poses of protecting the aluminum from further oxidation, stripping the oxide film from the molten metal so the drop- lets can coalesce, and holding the solid particles in suspension so that a clean metal can be recovered. The mechanisms by which the salt "flux" strips the oxide from the metal and holds the solids in suspension are not well un- derstood. Sully (60) provided a reason- able explanation of the mechanism for suspension of solids based on the obser- vation that 90 wt pet NaCl-10 wt pet CaF 2 molten salts loaded with 10 to 15 wt pet alumina of various types behaved as thix- otrophic fluids, through which the set- tling velocities of solids were nearly zero. It was thought for many years that the fluxing salt dissolved the aluminum oxide from the surface of the metal, but it now appears that this is not a major factor. Phillips showed that, though the solubility of aluminum oxide in molten cryolite, Na 3 AlFg , is appreciable (45), addition of NaCl to the melt decreases the solubility to very low levels (46). It has since been suggested (65-66) that the low interfacial tension between the salt and molten aluminum is the primary cause of the stripping. Because the salt wets the metal and the oxide particles and, in turn, the metal does not wet the oxide, the stripping action is very ef- fective. It is also believed that the presence of fluorides in the salt is ben- eficial in lowering the interfacial ten- sion, enhancing the stripping action. For this reason, fluxing salts commonly are made up of chlorides with a small percentage of fluoride, normally cryo- lite, Na3AlFg , or fluorspar, CaF2» In the most recent review of the chem- istry and properties of salt fluxes used in secondary aluminum production pro- cesses, Rao (49) listed 122 flux composi- tions from the literature and 36 patented compositions , nearly all of which are mixtures of halides. The most commonly used fluxes are mixtures of NaCl and KC1 in approximately equal amounts with 3 to 5 wt pet cryolite added. The chloride mixture has a melting point near the min- imum liquidus temperature of ~645° C, just below the melting point of aluminum. This fact is important in the recycling of drosses in rotary kilns because, dur- ing heating of the charge, the salt melts and coats the aluminum before the metal melts and thereby protects the metal from further oxidation. The fluoride is added to enhance the fluxing action, though some operators obtain acceptable results without the fluoride. Of the 158 compo- sitions listed by Rao, 49 are formulated from various mixtures of NaCl, KC1, AICI3 , NaF, KF, and A1F 3 or from mixtures of compounds of these halides; the major- ity of the other compositions listed con- tain one or more of those six halides with small percentages of other salts. Study of the literature indicates that no truly thorough investigation of any fluxing salt system has been done. In the case of the alkali aluminum halides , the system itself has not even been de- fined. Changes made in fluxing salts since 1916 have used trial and error as bases , and all the salt properties and relevant reactions have not been deter- mined. There are abundant data on liqui- dus temperatures, vapor pressures, densi- ties, surface tensions, viscosities, and reactions with ambient atmospheres for small compositional ranges within many systems, but there is no complete charac- terization of an entire system. In view of the fact that incongruent vaporiza- tion, reactions with the metal and the suspended solids, and hydrolysis reac- tions with combustion products all are likely to occur, it is reasonable to ex- pect that molten salt compositions can vary rapidly with time over broad ranges , so it is essential that the whole chemi- cal system be defined and characterized. This information is also necessary to facilitate development of processes for recycling or safe disposal of spent salt slags (34) . Because of the wide use of compositions within the system NaCl-KCl- AlCl 3 -NaF-KF-AlF3 in present-day aluminum recycling, it has been selected as the first to be studied. This report is restricted to a discus- sion of structures and melting character- istics, obtained from the literature, and determination of compatibility relation- ships based on Bureau of Mines experimen- tal research. CRYSTAL CHEMISTRY OF THE ALKALI ALUMINUM HALIDES Because several researchers have shown relationships between melting and vapori- zation characteristics and structure of the crystalline solids, and because some of the structural characteristics of the solids have been observed in the molten state, data on the crystalline phases re- ported to be stable in the system NaCl- KCl-AlCl 3 -NaF-KF-AlF 3 are assembled here. Crystallographic data for the 14 phases in the system with references for the structural descriptions are listed in table 1. THE ALKALI HALIDES The alkali halides — NaCl (halite, rock salt), KC1 (sylvite), NaF (villiaumite) , and KF — are isostructural, crystallizing in the familiar rock salt structure, space group Fm3m (No. 225), with ions lo- cated in the following positions: 11. Alkali : (4a) 000; -^0 22 V-; 2 2' 22 Halide 111 (4b) - ^-; ^00; 0^0; 00^ 222 2 2 2 The alkali ions are coordinated with six halide ions on the corners of regular oc- tahedra; conversely, the halide ions are coordinated with six alkali ions (6:6 coordination) so that each octahedron shares all eight faces with adjacent oc- tahedra. Interionic distances, calcu- lated from the lattice parameters in ta- ble 1, are listed in table 2. Comparison TABLE 1. - Crystallographic data and sources of powder diffraction data for crystalline phases in the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 Crystalline phase System and space group Cell parameters 1 Calculated density, g/cm 3 References PDF card 2 NaCl Cubic, Fm3m a = a = a = a = a = b = c = 3 = 5.6402 6.2431 4.6342 5.347 5.92, 10.22, 6.16, 108°. 2.163 1.987 2.802 2.524 2.498 3, 3, 48 48 28, 61 63 , 63 , 63 62 5-628 KC1 4-587 NaF KF 4-793 4-726 AICI3 » 22-10 AIF3 a = a = 5.039, 58.5°. 3.197 27, 54, 58 9-138 a = c = 4.927, 12.445. 3.197 27, 54, 58 NAp Na 3 AlF 6 a = b = c = 3 = 5.46, 5.61, 7.80, 90.18°. 2.918 35, 43, 59 25-772 K 3 A1F 6 Tetragonal, 14/mmm or a = c = 5.944, 8.468. 2.867 6, 20, 22, 59 3-615 K 2 NaAlF 6 Cubic, Pa3 or Fm3m. . . . a = 8.1120 3.013 21, 38, 59, 62 22-1235 Na 5 Al 3 F 11+ Tetragonal, P4/mnc. . . . a = c = 7.0142, 10.400. 2.997 4, 42 30-1144 KAIF4 Tetragonal, P4/mmm. . . . a = c = 3.550, 6.139. 3.049 5, 44 2-595 NaAlF^ a = c = 3.48, 6.29. 2.746 15, 17, 24, 37 19-1243 NaAlCl,, Orthorhombic, P2 1 2 1 2 1 . a = b = c = 10.36, 9.92, 6.21. 1.996 1, 53 23-649 KAICI^ a = b = c = 3 = 7.23, 10.48, 9.25, 93.3°. 1.973 53 23-468 Nap Not applicable. ! Unit cell parameters reported in angstroms at room temperature (20°-26° C). 2 Powder Diffraction Files, compiled by the Joint Committee on Powder Diffraction Standards, International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, PA 19081. FIGURE 1. - The crystal structure of AIC 1 3/ projected on the (010) plane. Small atoms are Al; large atoms are CI. TABLE 2. - Measured and calculated interionic distances in the alkali halides, MX Crystal Measured Calculated 1 Crystal Measured Calculated 1 M-X X-X M-X X-X M-X X-X M-X X-X NaCl 2.802 3.147 3.988 4.450 2.76 3.24 3.62 3.62 NaF 2.367 2.674 3.277 3.781 2.31 2.69 2.72 2.72 Calculations were made using the following radii: 1.81 A; F", 1.36 of measured and calculated interionic distances shows a rather stable configu- ration, with the alkali-halide values near the potential minimum and little anion-anion repulsion. The alkali ha- lides are, therefore, very stable phases, as will be shown later. Na + , 0.95 A; K + , 1.33 A; CI", ALUMINIUM CHLORIDE The layer structure of aluminum chlo- ride, AICI3, is shown in figures 1 and 2. The structure is monoclinic, space group C2/m (No. 12), with atoms in the follow- ing positions: Al : (4g) ± (oyO; y'y + y,0^) ; y=0.167 Cl(l) : (4i) ± xOz; y+x.p x=0.226; z=0.219 Cl(2) : (8j) xyz; xyz; y+x,y+y,z; 1 1 Y + x,j-y,z x=0.250; y=0.175; z=0.781 The structure consists of a distorted close-packed arrangement of chloride ions in which all the octahedral sites are empty in one layer and two-thirds of the octahedral sites are occupied by alumi- num ions in adjacent layers. The layers are held together by weak van der Waals bonds. Within the bonded layers, the oc- tahedra share all six corners with adja- cent octahedra. The AlClg octahedra are all identical, slightly distorted, with two Cl~ ions at 2.29 A, two at 2.32 A, and two at 2.33 A from the Al 3+ ion. The octahedral edges are much shorter than the sum of the anionic radii, 3.62 A, with two Cl-Cl distances of 3.10 A, two of 3.28 A, and eight of 3.33 A. This is because the radius ratio, 0.28, is too small for a stable octahedral coor- dination. A structure with tetrahedral coordination is not possible, however, because of the requirements of the elec- trostatic valence rule. The structure is, therefore, a compromise between con- flicting physical requirements; the high vapor pressures and low melting point, as discussed later, are a consequence of the unstable structure. Aluminum chloride is essentially isostructural with gibbsite, A1(0H) 3 . ALUMINUM FLUORIDE Unlike the layer structure of A1C1 3 , the structure of aluminum fluoride, AIF3 , is a continuous, three-dimensional FIGURE 2. - One layer of AICI3, projected on the (001) plane, showing pseudohexagonal symmetry. framework of AlFg octahedra, with shared corners, and is much more stable. It is rhombohedral, space group R32 (No. 155) , with atoms in the following positions: Al : (2c) xxx; xxx; x=0.237 F(l) : (3d) Oxx; xOx; xxO; x=0.430 F(2) : (3e) ^xx; x|x; xx^; x=0.070 The corresponding hexagonal coordinates are — Al : (6c) OOz; OOz; Rh; z=0.237 F(l) : (9d) xOO; 0x0; xxO; Rh; x=0.430 F(2) : (9e) xo|; 0x-|; xx|; Rh; x=0.570 The structure, as shown in figures 3 and 4, is a deformed version of the simple cubic Re0 3 structure, with a slight expansion along one [111] axis. It is a defect cubic close-packed arrangement of F~ ions, with one-fourth of the sites va- cant. All the available octahedral sites are occupied by Al 3+ ions. The octahedra share all six corners with adjacent octa- hedra, forming a three-dimensional frame- work structure. The octahedra are dis- torted, with three F" ions at 1.707 A and three at 1.889 A from the central Al 3+ ion. The 12 edges of the octahedra are formed by F~ ions 2.537 A apart. The F-F distances are somewhat less than the sum of the radii, 2.72 A, and are most likely an indication of considerable polari- zation of the F~ ion. The stability of A1F 3 , as compared with AICI3, is ob- viously a consequence of the stable octahedral coordination and the three- dimensional linkage. CRYOLITE, Na 3 ALF 6 Cryolite, Na 3 AlF 6 , is monoclinic, space group P2/n (No. 14); this places the unit FIGURE 3. - The structure of hexagonal AIF 3 , projected on the (0001) plane. Small atoms are Al, large atoms are F. FIGURE 4. - The structure of hexagonal AIF 3 , pro- jected on the (1120) plane. cell in an alternate setting of space group P2 1 /c. The atoms are located at — Al : (2a) 000; 111 222 Na(l) : (2b) Oo|; —0 Na(2) : (4c) ± xyz; -+ x ,--y,-+z x=0.50; y=0.945; z=0.24 F(l) : (4c) x=0.065; y=0.06; z=0.22 F(2) : (4c) x=0.71; y=0.16; z=0.03 F(3) : (4c) x-0.15; y=0.28; z=0.94 The structure, shown in figure 5, is a pseudocubic close-packed arrangement of F" ions with one-fourth of the sites va- cant. The octahedral sites are occupied by Al 3+ and one-third of the Na + ions in an ordered, alternating arrangement, forming a framework structure with all octahedra sharing corners with adjacent octahedra. The octahedral framework of the cryolite structure is a distortion of the AIF3 structure, with half the sites occupied by Na + ions, rather than Al 3+ ions. The remaining Na + are in inter- stitial sites with a highly distorted octahedral coordination. Interionic dis- tances in the cryolite structure are listed in table 3. The Al-F and Na-F distances are comparable to those in AIF3 and NaF , respectively, and the F-F dis- tances in the AlFg octahedron are slight- ly less than the sum of the F~ radii, in- dicating a strongly bonded structure. As is indicated later, the high-temperature stability of cryolite is comparable with those of NaF and A1F 3 . TABLE 3. - Interionic distances in the cryolite structure Ions Number of ions Interionic distance, A 2 4 1.783 1.834 2 4 2.332 2.237 Na-F , interstitial. . . 1 2 2 1 2.211 2.338 2.466 2.778 F-F, A1F 6 octahedron. 2 2 2 4 2 2.500 2.552 2.562 2.594 2.617 F-F, NaF octahedron.. 2 2 2 2 2 2 2.985 2.988 3.206 3.242 3.333 3.449 FIGURE 5. - The structure of cryolite, Na3AlF6, projected on the (001) plane. Small atoms are Al, intermedi- ate-sized atoms are Na, large atoms are F. POTASSIUM CRYOLITE, K 3 A1F 6 The exact structure of I^AlFg , commonly referred to as K-cryolite, has not been determined. It was originally described as cubic (6) and is still indexed as such in the incomplete data in the Powder Diffraction Files. (See table 1.) Later work showed, however, that it is tetra- gonal, with parameters near those listed in table 1. If it is assumed that the structure is a tetragonal distortion of the structure originally assigned to it, the correct space group is 14/mmm (No. 139) , with atoms located at — 10 Al (2a) 000; 111 222 K(l) : (2d) 00^; || 1«3 K(2) : (4d) Oil; |oi; ^ 13_ 24 F(l) : (4e) ± (oOz; 1,I,I +Z ) ; z=0. 20 F(2) : (8h) ±r xxO; xxO; y+x.j 1 hx 'T ; i i X, X,— *2 *2 x-0.20 The structure, based on these data, is shown in figure 6. Though the lattice parameters and symmetry are different from those of cryolite, Na3AlF 6 , the structure of Na 3 AlF 6 is quite similar. It consists of a framework of alternating AlFg and KF 6 octahedra, with the remain- ing K + ions in interstitial sites. Un- like the Na + ion, which occupies the highly distorted interstitial octahedral site, the larger K + props the structure open and occupies a distorted cubo- octahedral position. The interionic dis- tances are listed in table 4. The Al-F distances are significantly less than in cryolite (table 3), the K-F distances are less than in KF (table 2), and the F-F distances in the AlFg octahedra are greater than in cryolite (table 3). This indicates the structural data are not very accurate. The correct space group is probably P4/mnc (No. 128), and the structure is a slight distortion of that shown in figure 6. This alternative structure is shown in figure 7. The dif- ferences between the structures are very slight and not significant in terms of the stability of the phase; it is to be expected that high-temperature properties of l^AlFg will be comparable to those of Na 3 AlF 6 , as proves to be the case. TABLE 4. - Interionic distances in K 3 A1F 6 Number Interionic Ions of distance, ions A 4 1.681 2 1.694 4 2.540 2 2.522 K-F , interstitial. . . . 4 3.002 8 3.012 F-F, A1F 6 octahedron. 4 2.378 8 2.386 F-F, KF 6 octahedron.. 4 3.566 8 3.580 ELPASOLITE, K 2 NaAlF 6 The structure of elpasolite, as orig- inally described (38), was placed in space group Pa3 (No. 205), with atoms at — Al: (4a) 000; ±0^s ±|o ; o|L 11 Na: (4b) ill; 0^0; 0o|; ^00 K: (8c) ± fxxx, -+x,-i-x,x; x,-j+x,-|-x; I- x ,x,-UxJ ; x=0.25 F: (24d) ± f xyz; zxy; yzx; ^-+x,--y,z; -+z,--x,y; 11 _ _ 1 1 _ 1 1 -^ + y,-j-z,x; x,-+y,--z; z,-+x,--y; _ 1 1 1 _ 1 1 _1 1 _ 1 \ y, "2 'T x; "T x »y»7 +Z ' ^- z » x »7 + y; Y y ' z >~2 ) ; x=0.22; y=0.03; z=0.01 FIGURE 6. - The structure of K-cryolite, K 3 AIF 6 , projected on the (001) plane. Space group is assumed tobe 14/mmm. Small atoms are Al; intermediate-sized atoms are K; large atoms are F. '80 ><*^ so w FIGURE 7. - An alternative structure for K-cryolite, assuming space group P4 mnc. 12 The structure thus defined is shown in figure 8. As is the case with cryolite, the structure consists of a framework of alternating A1F 5 and NaF 6 octahedra linked by shared corners in all three dimensions. The K + ions are all located in interstitial cubo-octahedral sites, as is the case with K^AlFg. The higher sym- metry is a consequence of the ordering of Na + and K + ions in octahedral and cubo-octahedral sites, respectively. The similarity between the elpasolite struc- ture and the structures of Na3AlFg and K 3 A1F 6 (figs. 5-7) may be seen in the tetragonal subcell indicated by dot-dash lines in figure 8. The interionic dis- tances are listed in table 5. The metal- fluoride distances are comparable to those in cryolite and those in A1F 3 , NaF, and KF , so it is reasonable to expect the high-temperature properties to be compar- able with those phases, as proves to be the case. There is some question in the litera- ture about the details of the structure of elpasolite. Though Menzer (38) placed the structure in space group Pa3, several other authors (21 , 59 , 62 ) contend that X-ray and paramagnetic resonance data show the correct space group to be Fm3m. The atomic coordinates listed above show that the differences between the two Al (1): (2a) 000; 111 222 Al (2): Na ( 1 ) : Na (2): F (1): F (2): (4c) 0^0; ^00; 0^-; i-O^ 2 ' 2 ' 22' 2 2 (2b) 00J; ||0 TABLE 5. - Interionic distances in elpasolite Number Interionic Ions of distance, ions A 6 1.805 6 2.289 K-F, interstitial.... 3 2.656 3 2.777 3 3.005 3 3.113 F-F, AlFg octahedron. 6 2.306 6 2.778 F-F, NaFg octahedron. 6 3.127 6 3.343 choices are very slight. If the x, y, and z coordinates of the fluoride ion were shifted from 0.22, 0.03, and 0.01 to 0.25, 0.00, and 0.00, respectively, the symmetry would be changed from Pa3 to Fm3m, so the differences are insignificant. CHIOLITE, Na 5 Al 3 F 11+ Chiolite is tetragonal, space group P4/mnc (No. 128), with atoms located at — 1 . 11 1 1 (8g) ± I x,^+x,-^; ^+x,x,^; ^- x ,x,f-; x,^+x,f ); x=0.275 1 . "'2 'h (4e) ± (oOz; 1,1,1+z^ ; z =0.185 (8h) ± (xyO; -U x i-x,j; yxO ; I + y,i +x I); x=0.07; y=0.25 13 xyz; xyz; I +x ,i-y,I + z; ~x,I + y i + z; yxz; yxz; -|+y,-|+x; -Uz ; I-y,I- x ,i+z x=0.21; y=0.535; z=0.12 a. FIGURE 8. - The structure of elpasolite, K 2 NaAIF6, assuming space group Pa3. In order of increasing size, the atoms are Al, Na, F, and K. 14 One of the complex layers making up the structure is shown in figure 9. The layer is composed of two kinds of A1F 6 octahedra; half the octahedra share four corners, and the other half share two corners with adjacent octahedra. One- third of the Na + ions are located within the layer, surrounded by eight equidis- tant F~ ions. The remaining Na + ions are located between the layers, as shown in figure 10, surrounded by F~ ions in a highly distorted octahedral arrangement. There is no octahedral linkage between layers; the Na-F bonds serve to hold the FIGURE 9. - The structure of one layer of chiolite, Na 5 AI 3 F 14 , projected on the (001) plant 15 0|0-0 (g£) -$© — 75 (© 25 93 07 75 25 FIGURE 10. - The structure of chiolite, projected on the (100) plane. layers together. The interionic dis- tances, listed in table 6, are comparable to those in A1F 3 , cryolite, and NaF, in- dicating stabilities comparable with those phases. KA^ AND NaAlF^ The potassium and sodium tetrafluoro- aluminates are isostructural, space group P4/mmm (No. 123), with atoms located at — Na or K: (la) 000 111 Al: (Id) 222 F(l): (2e) 0?±; jo| F(2): (2h) 11 11_ n „. -— z; — -z; z=0.21 22 * 22 ' TABLE 6. - Interionic distances in chiolite Ions Number of Interionic distance, ions A Al(l)-F, octahedron.. 6 1.821 Al(2)-F, octahedron.. 2 4 1.821 1.946 Na(l)-F, intralayer. . 8 2.399 Na(2)-F, interlayer. . F-F, Al(l)-F 2 2 1 1 12 2 4 4 2 2.207 2.273 2.582 2.611 2.575 F-F, Al(2)-F 2.496 2.553 2.773 2.987 The structure, shown in figure 11, con- sists of layers of AlFg octahedra, each sharing four corners with adjacent octa- hedra. All the Na + or K + ions are lo- cated between the layers, surrounded by eight F~ ions on the corners of a tetra- gonal prism. The interionic distances are listed in tables 7 and 8. The Al-F and F-F distances are comparable with those in A1F 3 and cryolite, but the Na-F and K-F distances are significantly greater than in the corresponding alkali fluorides and hexaf luorides (tables 2-5). The K-F distance in KAlF^ is 5.5 pet greater than in KF , leading to weak bond- ing between the layers, accounting for the low melting point (5, 44). The Na-F distance in NaAlF^ is 18 pet greater than in NaF; this is in accord with the obser- vation that NaAlF^ is unstable with re- spect to chiolite, Na 5 Al 3 F 14 , and has probably been formed only as a metastable phase via vapor deposition (5, 24, 37), although it has been reported as a stable phase (17). 16 TABLE 7. - Interionic distances in KAIF^ FIGURE 11. - The structures of NaAIF 4 and KAIF 4 . Alkali ions are shown as intermediate-sized circles; small atoms are Al; large atoms are F. NaAlCl^ AND KA1C1 4 Ther structure of NaAlCl^ has been as- signed to the orthorhombic space group P2i2i2i (No. 18), with atoms located in the general positions — (4a): xyz; --x,y,z; -+x,--y,2; x,-+y,--z with the following coordinates: Atom X y '* X Na 0.128 0.207 0.677 Al .039 .485 .204 Cl(l) .031 .490 .552 Cl(2) .148 .316 .105 Cl(3) .348 .024 .923 CK4) .379 .336 .577 The structure, shown in figure 12, is made up of independent AlCl^ tetrahedra held together by Na + ions in the inter- stitial positions. The interionic dis- tances are listed in table 9. The Al-Cl distances are somewhat less than the Ions Number of ions Interionic distance, A Al-F 4 2 8 8 4 1 1.775 K-F 1.783 2.822 2.510 2.514 2.578 TABLE 8. - Interionic distances in NaAlF^ Ions Number of ions Interionic distance, A Al-F. 4 2 8 8 4 1 1.740 Na-F, 1.824 2.793 F-F, F-F, 2.461 2.521 2.642 octahedral distances in AICI3, indicating a more stable coordination. Only two of the Na-Cl distances, however, are near the value of 2.802 A in the NaCl struc- ture (table 2), explaining the low melt- ing point for the phase. X-ray diffraction analysis of the structure of KAICI^ has not been done. The unit cell is monoclinic, rather than orthorhombic, but Raman spectroscopy (53) indicates an independent tetrahedral structure, probably a distortion of the NaAlCl^ structure. SUMMARY OF CRYSTAL CHEMISTRY The crystal structures of phases in the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 are es- sentially of two groups: (1) those in which all the metal-halide distances are such that the interionic potential energy is near the minimum and the linkage of 17 ®^e . 0... 0^0 J£ 0^0 TABLE 9. - Interionic distances in NaAlCl, Ions 1 Interionic distance, A T03 © ©T03 © ©T© Al-Cl, tetrahedron. FIGURE 12. - The structure of NaAICI 4 , projected on the (001) plane. Small atoms are Al; intermediate- sized atoms are Na; large atoms are F. AIF4 tetra- hedra are shown in the lower unit cells. Na-Cl, interstitial. Cl-Cl, tetrahedral edges, 2.113 2.121 2.132 2.163 2.793 2.877 2.964 3.054 3.081 3.290 3.388 3.468 3.486 3.494 3.509 3.543 *1 ion in all instances. octahedral units, through shared faces or corners, produces a three-dimensional network, and (2) those in which geomet- ric and electrostatic conditions are such that neither of those two conditions can be met, so that some ions are in sites too large or the development of a three-dimensional network is not pos- sible. The former group, consisting of the alkali halides, A1F 3 , cryolite, K-cryolite, elpasolite, and chiolite, have relatively high melting points and low vapor pressures. The latter group, consisting of AICI3 and the alkali tetraf luoroaluminates and tetrachloro- aluminates , have relatively low melting points and high vapor pressures. The re- lationship between crystalline structure and the structure and properties of the liquid phase is not clear, but it is rea- sonable to expect a dependence on the ra- tios of the different ions, particularly as those ratios will affect the distribu- tion of aluminum halide octahedral and tetrahedral species in the raelt. PHASE EQUILIBRIA A large body of literature describing the melting characteristics of composi- tions within the system NaCl-KCl-AlCl 3 - NaF-KF-AlF 3 exists, but it is by no means complete, nor is it entirely reliable. There are numerous disagreements as to melting temperatures and many contradic- tions as to the compositions of the crys- talline phases. Following is a summary of the most important information now available. MELTING POINTS AND TRANSITION TEMPERATURES Alkali Halides The chlorides and fluorides of sodium and potassium are remarkably similar, not only in structure, but also in terras of thermal properties. The melting points reported are 800° C (30) to 805° C (_U) for NaCl; 722° C (47) to 774° C (11) for 18 KC1; 990° C (47) to 994.5° C (23) for NaF; and 850° C (47) to 858.4° C (23) for KF. Equilibrium vapor pressures for these phases are relatively low, and they show little tendency to react with atmos- pheric moisture to hydrolyze to the oxide and the acid. The high melting points and low vapor pressures are consequences of the intrinsic stability of the rock salt structure; the ions are in stable coordination and the polyhedral sharing results in a strongly bonded, isotropic structure. Aluminum Halides The aluminum halides are strikingly different in structure and properties. The unbonded layer structure (figs. 1-2) and the unstable octahedral coordination of AICI3 result in a very unstable phase. Melting points of 190.2° C (26) and 193.3° C (30^) have been reported; these values were obtained from visual observa- tion of crystallization in sealed glass tubes at pressures greater than 760 torr. At atmospheric pressure, AICI3 sublimes at 180.2° C, and the triple point for co- existence of solid, liquid, and vapor has been placed at 192.6° C and 1,715 torr (_56 ) . In the presence of atmospheric moisture, AICI3 hydrolyzes readily to form A1(0H) 3 and HC1 , so it is necessary to conduct experimental work in a com- pletely dry environment. The three-dimensional octahedral frame- work and stable coordination of AIF3 result in a much more stable phase. The melting point has been reported as 990° C in a sealed container ( 44 ) , but this value cannot be correct because the phase sublimes at 1,265° C at 760 torr ( 50 , 64) ; the triple point has not yet been determined. AIF3 also hydrolyzes read- ily in the presence of atmospheric moi- ture to form A1(0H) 3 or A1 2 3 , depending on temperature, and HF. Alkali Hexaf luoroaluminates Cryolite, Na 3 AlF 6 , undergoes a reversi- ble transition from monoclinic to cubic symmetry at 561° C (35); melting tempera- tures between 1,000° C (36) and 1,009° C ( 10) have been reported. The relatively high melting point and low vapor pres- sures are consequences of the three- dimensional octahedral network formed by alternating AlFg and NaFg octahedra. K-cryolite, K 3 A1F 6 , which is structurally very similar to cryolite, also has a rel- atively high melting point, 985° C (44) to 986° C (36). Steward ( 59 ) reported K 3 A1F 6 to be tetragonal, pseudocubic at 25° C, with the c/a ratio changing gradu- ally to 300° C, at which temperature it is strictly cubic. Elpasolite, which has been given the formula K 2 NaAlF 6 , has not been well studied. Only one value of the melting point, 932° C, has been reported (36), and no property measurements have been made. As indicated later in this report, the phase has a wide range of composi- tion and is strictly cubic. It is to be expected, because of the chemical and structural similarity between elpasolite, cryolite, and K-cryolite, that the prop- erties will be similar across the whole system Na 3 AlF 6 -K 3 AlF 6 . Chiolite Chiolite, Na 5 Al3F 1 i + , melts incongruent- ly at 741° C to cryolite and liquid con- taining 30 wt pet AIF3 (13). The rela- tively high melting point is a result of the stable coordination of the Na + ion between the octahedral layers, but the structure is considerably less stable than that of cryolite. Alkali Tetrafluoroaluminates The phase KAlF^ has definitely been es- tablished as a stable phase in the sys- tem KF-AIF3. It melts incongruently at 574° C to form AIF3 and a liquid only slightly richer in KF (44). The symmetry KA1F, as a function of temperature of is the subject of some disagreement. Grjotheim (17) reported the phase to be tetragonal at 25° C, with a transition to cubic symmetry at 327° C; Phillips (44) later reported it to be cubic at 25° C, with a transition to orthorhombic symme- try at approximately -15° C. The sodium analog, NaAlF^, which is isostructural with KAIF4, is much less stable, and its existence as a stable 19 phase at atmospheric pressure has been questioned. Grjotheim (17) , for example, determined a melting point of 775° C, using thermal analyses. Foster (13), however, did a detailed study which indi- cated it did not form as a stable phase. Other work (1_0, 15, 24, 37, _39) indicates that it forms only as a metastable phase, most readily by vapor deposition, and is easily dissociated into Na 5 Al 3 F 1 i + and A1F 3 . The layer structure of KA1F 4 and NaAlFt^ is such that, though the K + ion is large enough to maintain a stable struc- ture, the Na + ion is not able to achieve a stable Na-F bond distance. Alkali Tetrachloroaluminates As indicated by the structural analy- sis (1_) , the Na-Cl bond distances are considerably greater than the sum of the ionic radii; the resultant instability of the phase is shown by the low melt- ing point and high vapor pressures. The phase melts incongruently at 152° C (26) or 153° C (30) to NaCl and a liquid only slightly richer in A1C1 3 . The KAICI^ phase has a higher melting point, 250° C ( 26 , 55) , also incongruent, form- ing KC1 and a liquid only slightly richer in AICI3 . The higher melting point of KAICI^ is a result of the larger radius of the K + ion, which is more stably coor- dinated with the CI" ions. ALKALI HALIDE SYSTEMS 5 The systems NaCl-KCl ( 11 , 52 ) and NaF- KF (22), shown in figure 13, illustrate clearly the chemical differences between fluoride and chloride systems. NaCl and KC1 form a complete solution series with a minimum liquidus temperature of 645° C at ~50 mol pet KCl; the solid solutions dissociate spinodally at lower tempera- tures, however. The consolute point is at ~500° C and ~35 mol pet KCl. Dissoci- ation of the solid solutions proceeds at a rapid rate, with metastable equilibrium attained within an hour even at 25° C. 5 These and other diagrams in this re- port have been drawn from data of the original authors or redrawn from those in "Phase Diagrams for Ceramists" (31-33). 1,000 1 r - _NaF 1 1 i 1 r 1 1 900 NaF and liquid KF 800 NaCl ^\72i;^^^^ KF and liquid^^-^^ 700 ^^^-^-^ 1 -^-^\~64£__^- : ^^^-- \ - 600 (Na.K)CI solid solutions 500 - 400 \. 300 2 solid solutions N. - 200 100 1 1 1 1 1 1 1 1 1 NaF NaCl 40 50 6C HALIDE. mol pet KF KCl FIGURE 13. - The system NaF-KF, drawn from the data of reference 23, and the system NaCl-KCl, drawn from the data of references 11 and 52. NaF and KF exhibit only slight solid so- lution on the KF-rich side, 5 mol pet NaF in KF; a eutectic is located at 721° C, 60 mol pet KF. The differences may be attributed to the fact that the F~ ion has a strong polarizing effect on the ca- tions, making the structure much less adaptable to substitutional solid solu- tion. The liquidus minimum, 645° C, in the system NaCl-KCl is just below the melting point of pure aluminum, 660° C, so a 50:50 mix of the two salts makes an ideal base composition for fluxing salts. Interactions between NaF and the alkali chlorides, NaCl (18) and KCl (47), are very similar, as shown in figure 14. Both are simple eutectic systems, with no solid solubility; the eutectics and liquidus curves are quite close. The ab- sence of substitutional solid solution between CI" and F~ is another indication of their intrinsic differences and the extent to which the F~ ion determines the properties of molten salts. The remaining combination of mixed salts, NaCl-KF , is an unstable system; the phases react completely and irrevers- ibly to form NaF and KCl (47). 20 1,100 I T 1 — i 1 r 1 i 1 yxx - 900 - 800 NaF liquid NaCI 700 ; ^ 68 S>6^^' - ~\64B>^ 73% 600 500 " 400 2 solids - 300 ■ 200 - - 100 1 1 1 1 1 1 1 - 30 40 50 60 CHLORIDE, mol pel 100 NaCI NaF KCI NaCI FIGURE 14. - The systems NaF-NaCI and NaF-KCI, drawn from the data of references 18 and 47. THE ALKALI HALO ALUMI NATES The systems NaF-AlF 3 (_10, L3) and NaCl- A1C1 3 (30) , shown in figure 15, strik- ingly illustrate the fundamental differ- ence between the effects of AIF3 and ALCI3 on the properties of salt systems. The structural stability of cryolite, Na 3 AlF 6 , with a melting point higher than NaF, is the dominant factor in the system NaF-AlF 3 ; the eutectic is at 888° C, -13 mol pet AIF3. Chiolite, Na 5 Al 3 F lt+ , melts incongruently to the more stable cryolite and a liquid with ~41 mol pet A1F 3 ; the eutectic between chiolite and A1F 3 is at 694° C, -46 mol pet A1F 3 . The liquidus curve for A1F 3 has not been determined; the high vapor pressures over AlF 3 -rich liquids are such that sublimation of solid AIF3 in the assemblage probably oc- curs at a composition near the eutectic at atmospheric pressure. The unstable coordination of the Al 3+ ion with CI" in A1C1 3 and the unstable coordination of the Na + ion with CI" in the NaAlCl 4 structure result in a very unstable system in NaCl-AlCl 3 . The 40 50 HALIDE, mol pel FIGURE 15. - The system NaF-AIF3, drawn from the data of references lOand 13, and the system NaCI- AICI3, drawn from the data of reference 30. incongruent melting point of NaAlCl^ is 152°-153° C, and the eutectic between NaAlCl^ and A1C1 3 is at 107.2° C, 61.4 mol pet AICI3. The liquid miscibility gap, which has not been defined, extends from 191.3° C upward. The systems KF-AIF3 (44) and KC1-A1C1 3 ( 26 , 40 , 55) , shown in figure 16, are quite similar to the sodium analogs. There is no potassium analog of chiolite, however, and KAIF^ is a stable solid be- low 574° C. The greater stability of the K + ion in the structure of KAlCl^ results in a melting point of 250° C, as compared with 152°-153° C for NaAlCl^. SODIUM HALIDE-CRYOLITE SYSTEMS The NaF-rich portion (Na 3 AlF 6 ) of the system NaF-AlF 3 , shown previously in fig- ure 15, as reported by Fuseya (14) , and the system NaCl-Na 3 AlF 6 (29) are shown for comparison in figure 17. Addition of NaCI to cryolite has a greater effect on liquidus temperatures than does addition of NaF. This is possibly a result of the competition between the Al 3+ ion and the 21 ,000 I I I I I I 1 1 1 900 K,AIF S \ S^ 800 / AlF . and liquid 700 - 600 - x H _ KAIF, \ 500 — \ - 400 KCI and liquid \ - 300 \ / \ / 2 liquids ' ?no KAICI. 1 1 J \^^^"~"aICI, liquid 100 I I I 1 1 1 1 KF KCI 40 50 60 HALIDE, mol pet 100 AlF, AICI, FIGURE 16.- The system KF-AIF3, drawn from the data of references 26 and 44, and the system KCI-AICI3, drawn from the data of references 26, 40, and 55. FIGURE 17. - The system NaF-Na 3 AIF 6 , drawn from the data of reference 14, and the system NaCI-Na 3 AlF 6 / drawn from the data of reference 29. Na + for the F~ ions to form more stable octahedral units than can be formed with the Cl~ ions; in any case, the observa- tion can be made that, in general, addi- tion of chlorides to fluorides causes greater decreases in liquidus tempera- tures than does addition of one fluoride to another fluoride. THE ALKALI CHL0R0ALUMINATES The ternary system NaCl-KCl-AlCl 3 has not been well studied because of the ex- perimental difficulties of working with very volatile, easily hydrolyzed samples. Barton (2_) presented some preliminary data, but the high-AlCl 3 portion of the system was not covered and the completed study was apparently never published. Midorikawa (40-41) determined the peri- tectics and eutectics in the systems NaCl-AlCl 3 and KCI-AICI3 and liquidus isotherms in the low-melting portion of the ternary system. Figure 18 (top) shows a hypothetical interpretation of the system, based on the ternary data of Midorikawa (41); the bottom presents the complete system, based on binary data shown in figures 13, 15, and 16 and on the data of Barton (2). It must be em- phasized that the diagram is hypothetical and that the equilibria in the high-AlCl 3 portion of the system would be valid only at pressures >760 torr. The assumption has been made that the immiscible liq- uids observed in the binary systems also exist as ternary liquids above 193° C and that NaAlCl^ and KAICI^ form a complete solution series, at least just below sol- idus temperatures. This latter assump- tion is not in agreement with the work of Chikanov (9), who shows NaALCl I+ -KAlCl 4 as a simple binary system with a eutectic at 125° C, -32 mol pet KA1C1 4 . MIXED ALKALI HALIDES Compositions containing all four alkali halides — NaCl , KCI, NaF , and KF — do not melt in accordance with the character- istics of a quaternary system. This is because NaCl and KF are unstable in com- bination; they react completely and 22 AICI, NaAICI KAICI. FIGURE 18. - The system NaCI-KCI-AICI 3 . Upper diagram redrawn from the data of reference 41. Lower diagram represents the system as inferred from the data of references 2, 19, and 41. Irreversibly at all temperatures to form NaF and KC1 , the combination of small cations and anions and of large cations and anions lowering the lattice energies because of the more favorable radius ra- tios. Phase equilibria can be shown, therefore, as two ternary diagrams with only one common binary, in this case NaF- KC1. Liquidus temperatures for the ter- naries were determined by Polyakov (47 ) , as shown in figure 19, and by Ishaque (25) , as shown in figure 20. Both au- thors indicated no solid solubility ex- cept for NaCl-KCl. Both diagrams show simple ternary eutectic relationships; the binary liquidus temperatures and eu- tectics are in reasonably good agreement with other authors' data. FIGURE 19. - The reciprocal system NaCI-KCI- NaF-KF, redrawn from the data of reference 47. NaF 986° S80° NaCI 801° X x N \ XI v \ \ \ X ' v \ \ x' \ \ \ A. \ \ \ 900- \ \ \ \ \ I \ / \ \ \ \ \ \ / \ \ / \ X / \ / ~*^^ "*' >. ) ~--800---" \ / / 6124- / / / / y/ \\ ~~~ 700 -" \ x. ^«60° \ X. \ X. ^ X*. \ — > \ .. / N \ \ KF 860° KCI 772° FIGURE 20. - The reciprocal system NaCl-KCl- NaF-KF, redrawn from the data of reference 25. 23 EQUILIBRIA IN THE SYSTEM NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 In addition to gaps in the information available about the system, there is con- siderable uncertainty about the composi- tions of the phases and their crystal- lization behavior. This is most obvious in the case of the binary relationships between Na 3 AlF 6 and K 3 A1F 6 . In 1962 Buk- halova (_7) , in the first detailed study within the system, reported that a com- plete solid solution exists between Na 3 AlF 6 and K 3 A1F 6 and that phases in that binary exist in stable equilibria with NaCl-KCl solid solutions. Bukha- lova's liquidus isotherms are shown in figure 21. No indication was given of the compositions of coexisting solid so- lutions, of the existence or nonexistence of a subsolidus mlscibility gap, or of the crystallographic parameters of the solid solution. The experimental method was "visual polythermal, " which consists of heating samples to a constant temper- ature and making a visual observation to determine if complete melting has FIGURE 21. - The system NaCI-KCI-Na 3 AIF 6 - K3AIF6, redrawn from the data of reference 8. occurred; if it has not, the temperature is increased and another observation is made, and this is continued until the sample appears to be completely liquid and the liquidus temperature is placed between the temperatures of the two final observations. Three years later, Bukhalova (7) and Mal^tsev ( 36 ) presented crystallization data for Na 3 AlFg and K 3 A1F 6 in equilib- rium with NaF, KF , NaCl , and KC1 , indi- cating that there is no solid solution between Na 3 AlF 6 and K 3 A1F 6 but that el- pasolite, K 2 NaAlF 6 , is present as a stoi- chiometric compound with a melting point of 932° C. To complicate the picture further, Edoyan (12) , in the same year, reported six stoichiometric compounds between Na 3 AlF 6 and K 3 AlF 6 :2Na 3 AlF 6 * K 3 A1F 6 ; 5Na 3 AlF 6 -3K 3 AlF 6 ; Na 3 AlF 6 »K 3 AlF 6 ; 3Na 3 ALF 6 «5K 3 AlF 6 ; Na 3 AlF 6 «2K 3 AlF 6 ; and 2Na 3 AlF 6 «5K 3 AlF 6 . Edoyan based the phase identification on the melting dia- gram and provided no characterization of the compounds. Edoyan also reported that addition of 14 to 15 wt pet K 3 A1F 6 low- ers the melting point of Na 3 AlFg from 1,000° C to 832° C, almost 100° C below the lowest liquidus temperatures indi- cated by Mal'tsev (36). The data of Mal'tsev are shown in fig- ures 22-26. The crystallization volumes are described in terms of a compositional prism, the faces of which are shown by the three squares and two triangles of figure 22. Within the prism, they reported four stable crystallization tetrahedra: K 2 NaAlF 6 -Na 3 AlF 6 -Na 3 F 3 -Na 3 Cl 3 K 2 NaAlF 6 -Na 3 Cl 3 -Na 3 F 3 -K 3 Cl 3 K 2 NaAlF 6 -Na 3 F 3 -K 3 Cl 3 -K 3 F 3 K 2 NaAlF 6 -K 3 AlF 6 -K 3 Cl 3 -K 3 F 3 They also provided liquidus diagrams for four ternary sections: K 2 NaAlF 6 - Na 3 F 3 -K 3 Cl 3 (fig. 23); K 2 NaAlF 6 -Na 3 F 3 - Na 3 Cl 3 (fig. 24); K 2 NaAlF 6 -K 3 F 3 -Na 3 Cl 3 24 Na 3 AIF 1.000' 913° K 2 NaAIF, 932° 926° Na 3 AIF K 3 AIF, 986° K 2 NaAIF 6 926° K 3 AIF S FIGURE 22. - Liquidus surfaces of the faces of the compositional prism for the system NaCI-KCI-NaF-KF- t^^AIF^-r^AIF^, redrawn from the data of reference 36. (fig. 25); and K 2 NaAlF 6 -K3F3-K 3 Cl3 (fig. 26) . None of the diagrams shows ternary equilibria, however. Figures 23, 24, and 26 show primary crystallization fields for K 3 A1F 6 or Na 3 AlF 6 even though they were described as "tetrahedrating sec- tions." Figure 25 shows primary crystal- lization fields for both Na 3 AlF 6 and K 3 A1F 6 as well as for NaF. This indi- cates that the binary system KF-NaCl is not a true binary, and the existence of liquidus curves for NaCl and KF directly contradicts the data for that binary as shown in figure 22. It appears likely, therefore, that the primary crystalliza- tion fields are not well established and that the crystallizing phases were, in many cases, identified incorrectly. The "diagonal reciprocal" relationships shown in figure 22 appear to be question- able; the Alkemade lines between K^aAlFg and the alkali halides cross phase bound- aries in three of the four cases, with KC1, NaCl, and KF. This requires that elpasolite, K 2 NaAlF 6 , be a congruently melting phase in some sample compositions and an incongruently melting phase in others; this is possible only if elpaso- lite has a range of compositions. In view of the gently sloping liquidus sur- faces, shallow phase boundary troughs, and inadequate characterization of the 25 FIGURE 23. - The NaF-KCI-K 2 NaAIF 6 section through the compositional prism shown in figure 22. FIGURE 24. • The NaF-NaCI-K 2 NaAlF 6 section through the compositional prism shown in figure 22. FIGURE 25. - The KF-NaCI-K 2 NaAIF 6 section through the compositional prism shown in figure 22. FIGURE 26. - The KF-KCI-K 2 NaAlF 6 section through the compositional prism shown in figure 22. 26 crystalline phases, it is necessary to regard the data only as a general guide to aid in determination of temperatures of complete melting in the system. Figure 27 shows the binary system Na 3 AlF 6 -K3AlF 6 , constructed from liquidus temperatures taken from figure 22 and solid solution information presented later in this report. The upper portion depicts an interpretation of the data, plotted on the same scale as binary dia- grams shown earlier, to emphasize the flatness of the liquidus. The lower por- tion shows the same interpretation, ex- panded on the temperature axis to show the eutectics more clearly. The inset shows an alternative interpretation, with a peritectoid for the equilibrium among the limiting elpasolite solid solution, the limiting l^AlFg solid solution, and liquid. This interpretation could ac- count for incongruent melting of elpaso- lite in some cases. Some very meticulous experimental work would be necessary to determine relationships in this system. The system studied by Mal'tsev (36^) is not the complete system involved in for- mulation of aluminum fluxing salts be- cause Na3AlF 6 and K3AIF6 are not true components, being binary compounds of NaF yoo N..AIF 40 so ao K.AIF.. mo) pel and AIF3 and of KF and A1F 6 , respective- ly. It provides no information about melting temperatures in cases where AIF3, AICI3, or other materials with Al-F ra- tios of <1:6 are added to the flux, for example. To provide a complete picture, the system must be defined as NaCl-KCl- AlCl 3 -NaF-KF-AlF 3 . Unfortunately, little information is available about this sys- tem other than that just described, which constitutes only a part of it. As a first effort to provide such in- formation, figure 28 shows a hypothetical ternary diagram of the system NaF-KF- ALF3 , constructed from the data of Buk- halova (36), which have been redrawn as a triaxial plot at the bottom, and from the data shown in figures 13, 15, and 16. To simplify the system and forego the uncer- tainty regarding the melting behavior of K 2 NaAlF 6 or solid solutions thereof, the 750° C peritectic was changed to a eutec- tic in the subsystem K 2 NaAlF 6 -K 3 AlF 6 -KF; AIF, KAIF, 574° Na,AI,F Na,AIF 1,009' K,AIF, 985° FIGURE 27. - The Na 3 AlF 6 -K 3 AlF 6 binary diagram, drawn from ternary diagram of reference 36. FIGURE 28. - The system NaF-KF-AIF 3 . Lower dia- gram represents data of reference 36, redrawn as a tri- axial graph. Upper diagram is hypothetical, based on binary equilibria data. 27 the phase boundaries within the subsystem K 2 NaAlFg-Na 3 AlFg-NaF were also changed to a more realistic, though hypothetical, configuration. Enough data are available to show that the Na 3 AlF 6 -K 3 AlF 6 binary represents an important dividing line and that addition of A1F 3 to compositions along that line causes a very sharp decrease in liquidus temperatures and ternary eutectics below the 560° C and 694° C eutectics on the binaries. Two other ternary diagrams within the system are available. The system NaCl- NaF-AlF 3 (29), shown in figure 29, indi- cates that NaCl-Na 3 AlFg is a true binary and that addition of A1F 3 to mixtures of NaCl and Na 3 ALFg can lower the liquidus temperature to a 626° C eutectic. Figure 30 shows that NaCl-KCl-NaF is a true ternary (51) . SUMMARY OF PHASE EQUILIBRIA DATA The melting behavior of compositions in salts containing NaCl, KC1 , A1C1 3 , NaF , KF, and A1F 3 has not been thor- oughly described in the literature; some generalizations are possible, however. PHASE RELATIONS IN THE SYSTEM Experimental research (5 7) has been carried out to address (1) the defini- tion of the correct system in terms of It is noted, for example, that crystal- line compounds with AlFg octahedral or alkali halide octahedral linkages are relatively stable, with high melting points and low vapor pressures, so that salt compositions within the volume bounded by NaCl, KC1 , NaF, Na 3 AlF 6 , and K 3 A1F 6 form reasonably stable melts. These compositions all have F-Al ratios >6. Addition of A1C1 3 or other phases with a F-Al ratio <6, however, lowers liquidus temperatures, drastically in- creases vapor pressures, and promotes hy- drolysis. Relative stabilities of the crystalline compounds in the system indi- cate that the Al 3+ ion is stable only in the AlFg octahedral configuration; in salts with F-Al ratios >6, the excess F + ions are bound strongly to Na + and K + ions, but in salts with F-Al ratios <6, the excess Al 3+ ions form highly volatile aluminum chloride species , predominantly the neutral dimer Al 2 Clg. In the system NaCl-KCl-ALCl 3 -NaF-KF-AlF 3 , therefore, the boundary between stable and unstable salt mixtures is the plane NaCl-KCl- Na 3 AlF 6 -AlFg. NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 subsolidus compatibility relationships and (2) the characterization of elpaso- lite in terms of its compositional range. FIGURE 29. - The system NaCI-NaF-AlF 3 , redrawn from the data of reference 29. FIGURE 30. - The system NaCl-KCl-NaF, redrawn from the data of reference 51. 28 EXPERIMENTAL PROCEDURES Reagent-grade NaCl , KC1 , NaF , KF, and A1F 3 were dried in air at 350° C for at least 24 h prior to weighing and mixing. Anhydrous AICI3 was used as received. All chemicals were obtained from MCB Chemicals, Cincinnati, OH. 6 All composi- tions were prepared in 50-g batches, with each component weighed to the nearest 0.01 g, and were stored in airtight bot- tles. Only AICI3 and KF presented han- dling problems because of hydration in air. Special care was taken to weigh these components rapidly and to seal the samples immediately. For determination of subsolidus compatibility relation- ships, ~2-g samples of the compositions were sealed in evacuated borosilicate tubes and heated at 550°±10° C for 8 to 24 h. After heating, the tubes were cooled and broken, and the reacted sam- ples were immediately pressed into pel- lets, using starch as a binder, and covered with plastic wrap to prevent ab- sorption of moisture. During the study, it was found that compositions with no free AICI3 or KF in the equilibrium assemblages could be melted in air without excessive vaporiza- tion losses and could be handled in air without moisture absorption, so several samples with the composition KAIF^ and 16 samples on the Na 3 AlF 6 -K3AlF 6 join were prepared by melting in air, in porcelain crucibles, and quenched by pouring the liquids into a stainless steel beaker im- mersed in water. The quenched samples were ground with mortar and pestle and annealed in air at 800° C. Phases in the covered samples were identified by X-ray dif f ractometry , using CuKa radiation at a scanning rate of 1° 20 per minute. Detailed X-ray data for KAIFlj and the quenched and annealed sam- ples on the Na 3 AlF 6 -K3AlF 6 join were ob- tained by scanning at 0.25° 20 per min- ute, using a quartz internal standard for precision measurement of diffraction angles. Intensities of the diffraction 6 Reference to specific products does not imply endorsement by the Bureau of Mines . lines were measured by counting squares under the recorded diffraction peaks. SOLID SOLUBILITY IN THE ELPASOLITE PHASE The essentially identical structures of cryolite, elpasolite, and K-cryolite seem to be amenable to accommodation of vari- able amounts of cations of different sizes through slight distortions of the anion network and symmetry changes. This speculation, coupled with the importance of the cryolite series in crystallized fluxing salts, prompted a study of the solid solubility limits in the system Na 3 AlF 6 -K 3 AlF 6 . Detailed lattice and in- tensity measurements of the 16 compo- sitions quenched from the melts and an- nealed at 800° C are shown in figures 31 and 32. The limited number of samples and the scatter in the lattice measure- ments do not permit accurate determina- tion of solubility limits, but the data show rather sharp changes in the parameters of cryolite and K-cryolite, 8.440 t 1 r 1 r ] 8.430 - °< {2* 8.106 z LU S UJ DC w 8.100 < UJ KEY /\ Quenched O Annealed K-cryolite "a" ' UJ o 8.094 Elpasolite a A y 2.756 - T^fi^yT Cryolite d(211) 2.746 1 1 J L 50 K3AIF., mol pet 100 FIGURE 31. - X-ray lattice measurements for sam- ples in the system Na 3 AlF 6 -r<3AlF 6 . 29 50 K,AIF e , mol pet 100 FIGURE 32. - X-ray intensity ratio measurements for samples in the system t^^AIF^-t^AIF^. AlClj AICI, AIF, AIF, AIF, FIGURE 33. - Surfaces of the compositional prism for thesystemNaCI-KCI-AlCI 3 -NaF-KF-AIF 3/ show- ing subsolidus compatibility relationships. indicating mutual solubility of <5 mol pet in the end members. The intensity ratios provide a more reliable esti- mate of the solubility limits in elpaso- lite, whose compositional range is placed between 40±5 mol pet and 75±5 mol pet K^AlFg. The ratios for the annealed sam- ples are not significantly different from those of the quenched samples, so it is concluded that no appreciable unmixing occurs at 800° C or at room temperature. It might be expected that limited solu- bility occurs in the NaAlCl^-KAlCl^ se- ries and that there is limited solubility in KAIF^ and NasA^Fm, but these solu- bilities have not been determined. There is also a possibility of chloride-fluo- ride substitution, but this has not been studied. SUBSOLIDUS EQUILIBRIA To determine the order of the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 , it was nec- essary to identify the irreversible ex- change reactions between mixed halides. X-ray analyses of reaction products formed in sealed evacuated tubes showed the following binary reactions: NaCl + KF + NaF + KC1 (1) 3 NaF + AICI3 ->• 3 NaCl + A1F 3 (2) 3 KF + AICI3 ■»• 3 KC1 + AIF3 (3) Reaction 1 verifies previous work (25, 36, 47). Reactions 2 and 3 indicate re- ciprocal systems which, coupled with the chloride and fluoride ternaries, show that all phases and crystallization vol- umes can be represented graphically with- in a trigonal compositional prism whose faces are shown in figure 33. Once the reciprocal diagonals were established, the other tie lines were fixed by geomet- ric necesssity, as shown. The following binary reactions were also observed to occur in three component mixtures: KC1 + NaCl + AICI3 ->- NaCl + KAICI4 (4) NaF + 2KF + 2A1F 3 ->• A1F 3 + K 2 NaAlF 6 (5) Reaction 4 fixes the binary joins in the chloride ternary as NaAlCl^-KAlCl^ and NaCl-KAlCl^ , and reaction 5, coupled with previous work (13), fixes the binary 30 joins in the fluoride ternary as shown in figure 3. These were verified by subse- quent experimental determination of the resulting quaternary assemblages. To establish the binary joins within the compositional prism, the follow- ing ternary reactions were verified experimentally: 2NaF + 3KF + A1F 3 ■+ NaF (6) + KF + K 2 NaAlF 6 5NaCl + 5KC1 + NaF + 2KF + A1F 3 (7) ->• 4NaCl + 5KC1 + K 2 NaAlF 6 KC1 + NaF + 4KF + A1F 3 -► KC1 (8) + 2KF + K 2 NaAlF 6 In addition, six compositions with dif- ferent percentages of NaCl , KC1 , and A1F 3 were heated in sealed tubes and were found to undergo no net reaction. Reac- tions 6, 7, and 8 establish the exist- ence of stable joins between elpasolite, K 2 NaAlF 6 , and the component alkali ha- lides. This, coupled with the proven ex- istence of NaCl-KCl-AlF 3 as a stable ter- nary, requires that the compositional prism be divided into 12 compatibility tetrahedra, as shown in figure 34. To verify those conclusions, 12 composi- tions, one each within the 12 tetrahedra, were prepared and reacted in sealed evac- uated tubes. For ease of visualization, the prism has been divided into three volumes, as illustrated in figures 35-37, and- indicated below. The reactions ob- served follow: 1. NaCl + 7KF + 2A1F 3 -»- KC1 + KF + K 3 A1F 6 + K 2 NaAlF 6 (9) 2. 2NaCl + 5KF + A1F 3 ■»■ 2KC1 + NaF + KF + K 2 NaAlF 6 (10) 3. 3KC1 + 4NaF + A1F 3 ->- 2NaCl + NaF + KC1 + K 2 NaAlF 6 (11) 4. 2KC1 + 7NaF + 2A1F 3 ->- 2NaCl + NaF + Na 3 AlF 6 + K 2 NaAlF 5 (12) 5. NaCl + 7KF + 3A1F 3 + KC1 + KA1F 4 + K 3 A1F 6 + K 2 NaAlF 6 (13) 6. 2KC1 + UNaF + 5A1F 3 ■+ 2NaCl + Na 3 AlF 6 + Na 5 Al 3 F 11+ + K 2 NaAlF 6 (14) 7. NaCl + 4NaF + 3A1F 3 ■*■ KC1 + A1F 3 + KAIF^ + K 2 NaAlF 6 (15) 8. 2KC1 + 8NaF + 5A1F 3 -»• 2NaCl + A1F 3 + Na 5 Al 3 F 11+ + K 2 NaAlF 6 (16) 9. NaCl + NaF + 2KF + 2A1F 3 + NaCl + KC1 + A1F 3 + K 2 NaAlF 6 (17) 10. NaCl + 2KC1 + A1C1 3 + A1F 3 ->- NaCl + KC1 + A1F 3 + KA1C1 4 (18) 11. 2NaCl + KC1 + 2A1C1 3 + A1F 3 ■*■ A1F 3 + NaCl + NaAlCl 4 + KAICI4 (19) 12. NaCl + KC1 + 3A1C1 3 + A1F 3 + A1C1 3 + A1F 3 + NaAlCl,, + KA1C1 4 (20) As a further aid to visualization, fig- ure 38 shows the 12 tetrahedra in a sec- tion through the center of the prism; in this representation, points represent bi- the first time (57). nary assemblages, lines represent ternary assemblages, and triangles or parallelo- grams represent quaternary assemblages. The tetrahedra are numbered in accordance with the numbers of the reactions listed above. Tetrahedra 1-4 confirm the subsolidus compatibility reported by Mal'tsev (36) . Tetrahedra 5-12 have been determined for POWDER DIFFRACTION DATA FOR KAIF^ AND K 3 A1F 6 There is some diagreement regarding the symmetry of KAIF^. Brosset (5_) 31 Na 3 CI 3 Na 3 F 3 K,CI, Na 3 CI K 3 CI 3 Na 3 AIF, K,AIF R KAIF. AIF, FIGURE 34. - Subsolidus compatibility tetrahedra in the system NaCI-KCI-AICI 3 -NaF-KF-AIF 6 . Na,F. K,AIF t FIGURE 35. - Subsolidus compatibility in the por- tion of the system NaC I -KCI-AIC 1 3 -NaF-KF-A I F 3 coj^, responding to those in reference 36. Na 3 CI K 3 CI 3 Na 3 AIF Na 5 AI 3 F,; K 3 AIF 6 AIF, FIGURE 36. - Subsolidus compatibility tetrahedra in the volume bounded by NaCI-KCI-AIF 3 -Na 3 AIF6- K 3 AIF 6 . Na 3 CI 3 K,CI, AIF 3 FIGURE 37. - Subsolidus compatibility tetrahedra in the volume bounded by NaCI-KCI-AICI 3 -AIF 3 . 32 AICI 3 - AIF 3 NaAICI 4 - AIF KAICI, - AIF3 Na.CL - AIF 3W13 *-»ii 3 3CI3 - AIF3 KAIF 4 - K3CI, Na 5 AI 3 F 14 - Na 3 CI Na 3 AIF 6 - Na 3 CI 3 K 3 AIF 6 - K 3 CI 3 Na,CI, - Na 3 F 3" 3 K,CI, - Na,F, K3CI3 - K3F3 FIGURE 38. - Section through the compositional prism of the system NaCI-KCI-AlCl3-NaF-KF-AIF3 at the chloride-fluoride molar ratio of 1:1. originally assigned it to the tetragonal system and determined the structure on that basis. Phillips (44) , however, re- ported the symmetry as orthorhombic at low temperatures, with a reversible transition to cubic at about -15° C. Grjotheim (17) reported it to be tetra- gonal, with a transition to cubic at 327° C. The powder data obtained in this study indicate tetragonal symmetry; be- cause the data in the Powder Diffraction file are incomplete, detailed data are listed in table 10, along with calculated intensities based on Brosset's structure. The calculated and observed intensities are in sufficiently good agreement to indicate that the reported structure is essentially correct. The tetragonal symmetry of ^AlFg at 25° C as reported by Steward (59) , with unit cell parameters as in table 1, was confirmed by this study. CONCLUSIONS Available data on crystal structures, powder diffraction data, and phase equilibria data for the entire system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 have been as- sembled into one publication. Bureau of Mines determination of the subsolidus 33 TABLE 10. - Powder diffraction data for KAIF^ hkl Calculated 1 PDF card 2-595 Observed d, A I d, A I d, A I 001 6.155 8.1 NR NR 6.146 9.8 100 3.570 12.3 3.57 20 3.570 6.6 101 002 3.088 3.077 71.1 28.9 | 3.08 100 3.084 100.0 110 2.525 28.4 2.52 50 2.525 27.4 111 102 2.336 2.331 40.3 23.4 \ 2.32 70+ 2.333 53.8 003 2.051 4.6 NR NR 2.051 5.5 112 1.952 .1 NR NR ND ND 200 103 1.785 1.779 41.0 34.8 1.779 100 1.782 82.3 201 1.714 1.0 NR NR ND ND 210 1.597 1.7 NR NR NM 3.9 113 1.592 .9 NR NR ND ND 211 202 1.546 1.544 16.9 12.6 | 1.540 70 I 1.542 57.1 004 1.539 22.0 1.538 50 J 212 1.417 5.5 NR NR NM 3.2 104 1.413 .1 NR NR ND ND 203 1.347 3.1 NR NR NM 2.4 114 1.314 3.7 1.313 20 NM 4.3 220 213 1.262 1.260 10.1 18.6 > 1.258 70 1.260 25.2 221 1.236 .2 NR NR ND ND 005 1.231 <.l NR NR ND ND 300 1.190 1.3 NR NR ND ND 301 222 1.168 1.168 3.1 .2 > 1.165 50 ND ND 204 105 1.165 1.164 11.8 1.3 | 1.163 50+ 1.166 15.7 310 1.129 3.8 NR NR NM 5.6 d = interplanar spacing; 1 not measurable with precision; ^pacings calculated for a calculated for z = 0.21, with = X-ray diffraction intensity; NR = not reported. = 3.570 A; c = 6.154 A at 22 c no temperature correction. ND = not detected; NM = C. Relative intensities compatibility relationships provides quidelines for systematic research into the properties of molten salts in the system. Based on information in the literature, the compatibility relationships described in this work, and observations made dur- ing the research, the system can be di- vided into three distinct crystallization volumes. Compositions within each of the volumes will melt to form molten salts with properties within ranges quite dif- ferent from those in the other volumes. Molten salts within the volume bounded by NaCl, KC1, NaF, KF , Na 3 AlF 6 , and K 3 A1F 6 (fig. 35) are relatively stable, with high melting points and low vapor pres- sures; these salts have a F-Al ratio >6 so that all the Al 3+ ions are in stable A1F 6 octahedra. Detailed property deter- minations are needed for a thorough un- derstanding of the relationships among salt composition, molten salt structures, properties, and fluxing efficiencies. Molten salts within the volume bounded by NaCl, KC1 , A1F 3 , Na 3 AlF 6 , and K 3 A1F 6 , shown in figure 36, are considerably less stable, with lower melting points and higher vapor pressures. The stable crys- talline phases within the volume are 34 NaCl, KC1, and all the fluorides of alu- minum and the alkalies; the F-Al ratio is <6 and, though the Al 3+ ion can be in octahedral coordination, the melting tem- peratures and other properties depend on the interactions of the alkali ions and the A1F 6 octahedra. Molten salts within the volume bounded by NaCl, KC1 , A1C1 3 , and A1F 3 , shown in figure 37, crystallize to form only one stable fluoride, A1F 3 , which is a vola- tile phase, along with the even more vol- atile phases A1C1 3 , NaAlCl^ , and KAICI^. The F-Al ratio is <3, and the Al 3+ ions necessarily are forced into unstable co- ordination with the halide ions. 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