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 \0| UNITEDSTATESATOMICENERGYCOMMISSION 
 
 THE STRUCTURE OF URANIUM HYDRIDE AND DEUTERIDE 
 
 by 
 R. E. Rundle 
 
 Iowa State College 
 
 This document consists of 8 pages. 
 Date Declassified: February 10, 1947 
 
 This document is for official use. 
 
 Its issuance does not constitute authority 
 
 for declassification of classified copies 
 
 of the same or similar content and title 
 
 and by the same author. 
 
 Technical Information Division, Oak Ridge Directed Operations 
 Oak Ridge, Tennessee 
 
THE STRUCTURE OF URANIUM HYDRIDE AND DEUTERIDE 
 
 By.R. E. Rundle 
 
 Uranium metal reacts with hydrogen to form only one hydride.* This hydride, established as a 
 compound by X-ray diffraction,' has been found chemically to have the composition UHj.^ The hydride 
 of uranium, if existing data on other metallic hydrides is reliable, is unique in that it does not fall in 
 the class of volatile hydrides, salt-like hydrides, or interstitial solution hydrides. It is a metal-like 
 hydride of perfectly definite composition and with a structure completely unrelated to any of the three 
 forms of uranium metal. It has a structure in which the bonding between uranium and hydrogen must 
 play a predominant role, since metal- metal bonds of any strength are almost completely lacking. 
 
 UNIT CELL AND X-RAY DENSITY 
 
 Debye-Scherrer powder diagrams of UH, have been made using CuKa radiation, a camera of 
 5-cm radius, and samples sealed in thin-walled glass capillaries. These diagrams may be inter- 
 preted in terms of a primitive cubic lattice, a = 6.63 A. The density of the hydride has been meas- 
 ured by helium displacement and found to be 10.95 g/cc.^ There are then eight uranium atoms in 
 the cubic unit. 
 
 A precise determination of the unit cell has been made using a symmetrical, self-focusing 
 powder camera of 5-cm radius and unfiltered CuKa radiation. Samples were prepared from purest 
 Ames uranium and carefully purified hydrogen (purified by decomposition of UHj.t The samples of 
 Table 1 were prepared at about 1 atm hydrogen pressure and at relatively low (200 to SOOt) tem- 
 peratures. Due to small particle size these samples produced satisfactory but not particularly 
 sharp reflections in the back reflection region. 
 
 Other samples, prepared by W. Tucker and P. Figard, were made at pressures up to 1800 psi and 
 temperatures up to 500 to 600°C. These samples showed considerable growth in particle size and 
 produced very sharp maxima in the back reflection region. The lattice spacing, as determined from 
 these samples (Table 2), is more precise but within the limit of experimental error of the spacing of 
 the low pressure hydride. 
 
 Uranium metal has been heated with UH3, and two phases are maintained. Changes in spacing 
 of neither uranium metal nor of hydride were great enough to be detected. For example, a sample 
 50% U metal and 50% UH3 gave a hydride spacing of 6.630+0.002 A. Neither hydrides prepared at 
 high hydrogen pressures nor hydrides prepared with excesses of metal showed any alteration in the 
 UH3 spacing. It must be concluded that the composition of UHj is perfectly definite, that there is no 
 appreciable solubility of hydrogen or uranium in the hydride at ordinary temperatures and pressures. 
 
 We consider the best value of the hydride spacing to be 6.6310 i 0.008 A. The X-ray density, 
 calculated in accordance with the recommendations of Jette and Foote,* is 10.92 g/cc, in excellent 
 agreement with the experimental value obtained by helium displacement.' 
 
 ♦Other physical and chemical reports on the hydride will be made in J. Am. Chem. Soc. by 
 F. Spedding, A. Newton, J. Warf et al. 
 
 ■f This method of purification, first described by A. Newton, has been shown to be very effective.' 
 
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 Table 1. Lattice constant of UH prepared at low pressure. 
 
 
 No. of lines 
 
 Spacing 
 (A) 
 
 Estimated error 
 (one film) 
 
 Sample I 
 Sample I* 
 
 8 
 
 5 
 
 6.6323 
 6.6324 
 
 ±0.0010 
 .0009 
 
 ♦Measured by another observer. 
 
 Table 2. Lattice constant of UH prepared at 1800 psi H 
 
 Sample 
 
 No. lines 
 
 Spacing 
 
 Estimated error 
 
 
 
 (A) 
 
 (one film) 
 
 A 
 
 12 
 
 6.6308 
 
 ± 0.0008 
 
 B 
 
 12 
 
 6.6317 
 
 .0008 
 
 C 
 
 12 
 
 6.6312 
 
 .0005 
 
 D 
 
 12 
 
 6.6319 
 
 .0008 
 
 E 
 
 19 
 
 6.6306 
 
 .0008 
 
 At 
 
 12 
 
 6.6302 
 
 .0014 
 
 Bt 
 
 12 
 
 6.6304 
 
 .0009 
 
 ct 
 
 12 
 
 6.6301 
 
 .0017 
 
 Dt 
 
 12 
 
 6.6317 
 
 .0011 
 
 I Measured by another observer. 
 
 LATTICE CONSTANT OF THE DEUTERIDE 
 
 The deuteride, UE^, has been prepared at low pressures. Precision diagrams, taken as before 
 with CuKa radiation, lead to a lattice constant, a = 6.620 ± .002 A, more than 0.01 A smaller than the 
 hydride spacing. This change in lattice constant is far greater than the experimental error. The 
 X-ray density of the deuteride is 11.11 g/cc. 
 
 SPACE GROUP AND STRUCTURE.' 
 
 Powder diagrams of UH3 show many absences. These absences, which do not appear even on 
 the most intense powder diagrams, seem to be systematic and most extraordinary. They include 
 reflections with the following forms of Miller indices: 
 
 (4n, 4n' + 2, 4n" + 2); (4n, 4n', 4n" + 2); (4n + 2, 4n' + 2, 2n" + 1); 
 
 (2n + 1, 2n' + 1, 2n" + 1); (4n, 2n' + 1, 2n" + 1) 
 
 All other possible reflections have been observed with good intensity on powder diagrams made 
 with CuKa radiation. 
 
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 These absences correspond to no special set of equivalent positions in the cubic system, and it 
 can be shown that no set of eight equivalent uranium atoms with any parameters in any space group 
 will lead to these absences. The same can be sho'Afn for any two sets of four equivalent positions 
 for the uranium atoms. 
 
 The space groups Ojl,, O^ and Tj provide two equivalent positions plux six equivalent positions 
 which combined lead to a structure that requires the absences noted and no others. These posi- 
 tions are: two atoms in ^^, ^t 000, \ij, and six atoms in (c) at^Oi, jiO, Oji;, sOi, Oil, HO, (or the 
 six equivalent positions may be taken via ii'cm the positions here given). 
 
 The parameterless structure provides good agreement between observed and calculated inten- 
 sities (Table 3). The observed intensities were visually estimated from powder diagrams. Calcu- 
 lated intensities were corrected for Lorentz, polarization and multiplicity factors. No absorption 
 correction nor temperature correction was made, so that the ratio of calculated and observed in- 
 tensities changes with angle, but in a regular fashion. The absences noted demand the paramster- 
 less structure within very narrow limits, so it seems unnecessary to carry the intensity calcu- 
 lations further. 
 
 DISCUSSION OF THE STRUCTURE 
 
 The metal positions of UH3 are shown in Figure 1. The atoms in the six equivalent positions, 
 (c), form three perpendicular but nonintersecting linear arrays of atoms running parallel to the 
 three cubic axes. The atoms are spaced at half the cube edge, or at 3.316 A; each (c) atom pos- 
 sesses two other (c) atoms as nearest neighbors at this distance. This is by far the shortest metal- 
 metal bond -in the compound, and apparently is the only metal- metal bond of any strength in the 
 structure. 
 
 In addition, each (c) atom has four nearest (a) atoms as neighbors at 3.707 A. These neighbors 
 form a tetrahedron flattened out along one two-fold axis. Each (a) atom has twelve nearest neigh- 
 bors, (c), at 3.707 A. These twelve neighbors have the arrangement of a deformed icosahedron. 
 
 The metal- metal distances are known in the a and V (high temperature) forms of uranium. In 
 the former,' each metal atom has two neighbors at 2.76 A, two at 2.85 A, four at 3.27 A, and four at 
 3.36 A. In the body- centered, r form of the metal, each metal atom has eight nearest neighbors at 
 2.97 A.' Accordingly, the 3.316 A spaci:^ in the hydride represents metal- metal bonds of a 
 strength corresponding to the weaker bonds in the a -metal structure. Since there are but two such 
 bonds per (c) atom and none for the (a) atoms, the metal- metal bonding in uranium hydride is rel- 
 atively unimportant. This might also t3 concluded from the density which decreases from 19 to 
 about 11 on formation of the hydride. 
 
 In no sense of the word can uranium hydride be thought of as an interstitial solution. Its com- 
 position is definite, the metal arrangement unique and unrelated to that of any metal structure, and 
 metal-metal bonds are practically nonexistent in this hydride. 
 
 In the absence of structural data, however, uranium hydride would doubtless be listed as an 
 interstitial solution, since in appearance and conductivity it still resembles a metal. 
 
 A PROPOSAL CONCERNING THE NATURE OF THE HYDRIDE 
 
 As we have seen, the metallic properties of uranium hydride are quite inconsistent with an 
 ionic structure, and the chemical ana physical properties and the structure of the hydride do not 
 correspond to an interstitial solution. The lack of important metal- metal bonds suggests that the 
 
 * Apparently gamma-uranium was obtained by chance earlier.' 
 
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 Figure 1. Uranium positions in the hydride. 
 
 Table 3. Intensities of powder reflections from uranium hydride. 
 
 Indices 
 
 Intensity calculated 
 
 Intensity observed* 
 
 (100) 
 
 
 
 
 (110) 
 
 
 
 
 (111) 
 
 
 
 
 (200) 
 
 
 1427 
 
 (210) 
 
 
 4147 
 
 (211) 
 
 
 3274 
 
 (220) 
 
 
 
 
 (300) 
 
 (221) 
 
 
 
 (310) 
 
 
 
 
 (311) 
 
 
 
 
 (222) 
 
 
 416 
 
 (320) 
 
 
 1068 
 
 (321) 
 
 
 1889 
 
 (400) 
 
 
 772 
 
 (410) 
 
 (322) 
 
 
 
 (411) 
 
 (330) 
 
 
 
 (331) 
 
 
 
 
 (420) 
 
 
 524 
 
 (421) 
 
 
 964 
 
 (332) 
 
 
 451 
 
 (422) 
 
 
 
 
 (500) 
 
 (430) 
 
 
 
 (510) 
 
 (431) 
 
 
 
 
 
 
 
 w 
 s 
 
 MS 
 
 
 
 
 
 
 
 
 
 w 
 
 M 
 
 VS 
 
 M 
 
 
 
 
 
 
 
 MW 
 
 MS 
 
 MW 
 
 
 
 
 
 
 
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 Table 3. (Continued). >• 
 
 
 
 S 
 
 MS 
 
 MS 
 
 
 
 
 
 
 
 MW 
 
 W 
 
 MS 
 
 
 
 
 
 
 
 C 
 
 W 
 
 MS 
 
 M 
 
 MW 
 
 
 
 
 
 
 
 W 
 
 MS 
 
 S 
 
 
 
 
 
 
 
 
 
 s 
 vs 
 
 w 
 • 
 
 
 
 * S = strong; M = medium; W = weak; V = very 
 
 important bonds in the structure are metal- hydrc^en bonds, and these bonds must be such as to 
 leave the structure with metallic properties. 
 
 Moreover, though the melting point of the hydride is unknown it is certainly fairly high for a 
 compound with such weak metal- metal bonds. At high hydrogen pressures the hydride has been 
 taken above 600°C, and it is certain that the melting point is much higher. In addition, since the 
 metal breaks up into a powder on formation of the hydride, it must be that the hydride is brittle. 
 The particles of the hydride produced at high temperatures were large enough to confirm this 
 property. 
 
 The properties cited certainly must mean that covalent bonding of hydrogen to individual 
 urani'..-n atoms, leading to a molecular crystal, is out of the question. It is also inconsistent with 
 the two very different types of uranium atoms in the structure. 
 
 (511) 
 
 (333) 
 
 X 
 
 
 
 (520) 
 
 (432) 
 
 
 855 
 
 (521) 
 
 
 
 538 
 
 (440) 
 
 
 
 506 
 
 (522) 
 
 (441) 
 
 
 
 
 (530) 
 
 (-ias, 
 
 
 
 
 (531 
 
 
 
 
 
 (600) 
 
 (442) 
 
 
 270 
 
 (610) 
 
 
 
 208 
 
 (611) 
 
 (532) 
 
 
 605 
 
 (620) 
 
 
 
 
 
 (621) 
 
 (540) 
 
 (443) 
 
 
 
 (541) 
 
 
 
 
 
 (533) 
 
 
 
 
 
 (622) 
 
 
 
 175 
 
 (630) 
 
 (542) 
 
 
 544 
 
 (631) 
 
 
 
 349 
 
 (444) 
 
 
 
 233 
 
 (700) 
 
 (632) 
 
 
 
 
 (710) 
 
 (550) 
 
 (543) 
 
 
 
 (711) 
 
 (551) 
 
 
 
 
 (640) 
 
 
 
 180 
 
 (720) 
 
 (641) 
 
 
 558 
 
 (721) 
 
 (633) 
 
 (552) 
 
 714 
 
 (642) 
 
 
 
 
 
 (722) 
 
 (544) 
 
 
 
 
 (730) 
 
 
 
 
 
 (731) 
 
 (553) 
 
 
 
 
 (650) 
 
 (643) 
 
 
 646 
 
 (732) 
 
 (651) 
 
 
 905 
 
 (800) 
 
 
 
 237 
 
 (810) 
 
 (740) 
 
 (652) 
 
 
 
 (811) 
 
 (741) 
 
 (554) 
 
 
 
 (733) 
 
 
 
 
 
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 The physical properties of the hydride, such as melting point and brittleness, are in keeping 
 with a valence-type compound, i.e., a continuous structure held together by covalent bonds.* Since 
 metal- metal bonds are absent or weak, the continuous structure can be providea only by metal- 
 hydrogen bonds, and by these only if hydrogen bridges metal atom to metal atom. 
 
 To provide electrical conductivity it is also necessary to have a continuous structure. Without 
 good metal-metal bonds this, too, can only be provided by a metal -hydrogen— metal bridge struc- 
 ture. 
 
 Finally, the nearest metal- metal distances in the hydride are, sxcept for the 3.316 A spacing, 
 too long to be metal- metal bonds and too short to provide space for hydrogen between metal atoms 
 except in a bridge-type structure. Indeed, if one tries to find sensible places for hydrogen without 
 the use of a bridge structure, the uranium hydride structure appears Incomprehensible. 
 
 The structure can be understood in terms of electron deficient "half bonds" of the type recently 
 proposed to explain the bonding in the boron hydrides, aluminum alkyl dimers,' and certain inter- 
 stitial carbides, nitrides, and metallic oxides.'" In accordance with this proposal, hydrogen may use 
 its Is-orbital for the formation of two bonds (but with one electron pair for the two bonds). Sir.ue 
 s-orbitals lack directional properties, we shoiild expect the U-H-U bond angle 1^ to be 180° because 
 of the small size of hydrogen. Circumstances imder which electron deficient "half bonds" are to be 
 expected are described elsewhere.', lo In this case we should expect them if uranium has fewer 
 valence electrons than stable bond-orbitals. That this condition is reasonable for uranium hydride 
 we shall attempt to demonstrate. 
 
 If we now consider which bonds in the structure must be the bridge bonds, U-H-U, the result 
 is quite simple. The 3.316 A distance, as we have seen, corresponds to a metal-metal bond. The 
 3.707 A spacing is quite satisfactory for the bridge bonds, and all other distances are too large for 
 such bonds. There are 12 such bridge bonds to each uranium atom of the set (a), or 24 per unit 
 cell. There are then 24 hydrogens per unit cell, making the overall formula UH,. 
 
 In the structure proposed here each atom of type (a) is bonded to twelve atoms of type (c) by 
 hydrogen bridges. Each atom of type (c) is bonded directly to two other type (c) atoms and by hy- 
 drogen bridges to four atoms of type (a). 
 
 This structure is consistent witii the use of six orbitals of uranium for bond formation for 
 both types of uranium atoms in the structure. The configuration about (c) uranium atoms is that 
 of a tetrahedron flattened along one two-fold axis and with two extra bonds directed along the 
 shortened two-fold axis. There are twelve bonds from type (a) atoms. These bonds are directed 
 toward the corners of a nearly regular icosahedron (see Figure 2). If (a) type atoms use but six 
 bond orbitals then they must use orbitals which are directed in two directions and must use each 
 orbital to form two "half bonds." If this is th3 case it is understandable that the six orbitals 
 chosen for bond formation by the two types of atoms are unlike. If uranium can furnish at least 
 six stable bond-orbitals and will furnish at most four valence electrons in hydride formation, which 
 seems reasonable, then co^ditions for "half bonds" are fulfilled. g,, 
 
 Of course, the structure is not entirely clarified by the proposal made here. The types of 
 orbitals used for bond formation by uranium are not clear in case of either (a) or (c) type atoms. If, 
 as Pauling has suggested," when uranium furnishes four valence electrons, the 7s-orbital contains 
 an electron pair, th2n the orbitals available for bond formatior are the five 6d-orbitals, the three 
 p-orbitals and quite possibly the Sf-orbitals.J To obtainbetter bonding it would doubtlessbe possible 
 
 ♦ This terminology follows that of F. Seitz.* 
 
 tThe bond is drawn directly through hydrogen to indicate one electron pair, since U-H-U 
 would normally imply two electron-pair bonds. 
 
 J The elements in this part of periodic table have rare-earth-like properties, suggesting the 
 stabilitv of the 5f-orbitals." 
 
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 [7 
 
 tO shift the electrons from the 7s-orbital to other almost equally stable levels. The best six hybrid 
 bond-orbitals available to uranium are difficult to obtain from the wealth of atomic orbitals avail- 
 able. This is a problem beyond the scope of the proposal made here, and that it is difficult to solve 
 is not a detraction from the present proposal. 
 
 The proposed structure provides uranium hydride with a continuous, valence-type bonding 
 (Figure 2) which will accounc for high melting point and brittleness, and with a resonating system 
 of "half bonds" which o/iuld lead to high polar izability, metallic luster, and high electrical con- 
 ductivity, much as these properties ai' '?iand in graphite. Moreover, it provides a satisfactory 
 explanation for the uranium- uranium distances in the hydride and suggests satisfactory positions 
 for precisely the correct number of hydrogen atoms. 
 
 The author is quite aware that it is impossible to defend the proposed structure except insofar 
 as the proposal provides an explanation of the unusual properties, composition and metal arrange- 
 ment of the hydride. This proposal is offered here because it seems unlikely that an M-H-M 
 bridge is unique to uranium hydride. Many other metallic hydrides have rather similar properties,, 
 and though structural information is not yet complete enough to cite another good example of this 
 type, it seems to the author that most "interstitial solution" compounds need further study. iJearly 
 all such compounds, including hydrides, are brittle, hard, and high melting, properties quite incon- 
 sistent with the weakening of metal- metal bonds unless replaced with better bonds. A number of 
 metallic hydrides fulfill the conditions for "half bonds" as described before.'° 
 
 Twelve nearest neighbors (c) about uranium 
 atom (a). All twelve distances are equal (3.707A) 
 and the (c) atoms lie at the corners of a nearly 
 regular icosahedron. In accordance with the 
 proposal presented in the paper, each of twelve 
 bonds is an U-H-U bridge. 
 
 Two nearest (c) atoms (3.316A) and four near- 
 est (a) atoms (3.707A) about any (c) atom. The 
 shorter bond is an ordinary U-U bond and the 
 longer bond is a U-H-U bridge, according to 
 the proposal of this paper. The hydrogen atom 
 is shown in the longer bond in this case. 
 
 Fig. 2. Coordination about uranium atoms in the hydride. 
 
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 SUMMARY 
 
 Uraiium forms a metallic hydride, UI^, a compound of definite composition, unique metal ar- 
 rangement unrelated to that of the metal itself, and almost lacking in metal- metal bonds. 
 
 The hydride is cubic, a = 6.631 A, with eight uranium atoms per unit cell at positions (a) 000, 
 HI, and (c)r 5O, Oil, |Oi,l|0, Orl, |0| of the space group O^, 0% or Tj. The X-ray density is 
 10.92 g/cc. The deuteride spacing: is 6.620 A, definitely smaller than for the hydride. 
 
 It is proposed that the hydrogens form U-H-U bridges between metal atoms of type (a) and 
 (c) in the structure, where the bridge contains one electron pair for the two bonds. This structure 
 accounts for the physical properties, unique metal arrangement, and formula of the hydride, and is 
 consistent with a recently proposed theory of electron deficient structures.''*" 
 
 ACKNOW LEDGMENTS 
 
 The author wishes to express his appreciation to the members of the Ames section of the 
 Metallurgical Project for both samples and analytical data. Especial thanks are due A. S. Wilson 
 for aid in the precise determination of hydride and deuteride spacings, to R. Nottorf, Dr. A. Newton, 
 W. Tucker, and F. Figard for special sample preparations and to Dr. 1. B. Johns and Dr. F. H. 
 Spedding. 
 
 REFERENCES 
 
 1. Rundle, R., Metallurgical Project Report, CT-609, p 30, April 1943. 
 
 2. Battelle Memorial Institute, Metallurgical Project Report, CT-818, July 1943. 
 
 3. Newton, A., J. Warf , O. Johnson, and R. Nottorf, Metallurgical Project Report, CC-1201, 
 
 January 1944. 
 
 4. Jette, E. and F. Foote, J. Chem. Phys. 3:605 (1935). 
 
 5. Jacobs, C. and B. Warren, J. Am. Chem. Soc. 59:2588 (1937). 
 
 6. Wilson, A. and R. Rundle, Metallurgical Project Report, CT-1775, May 1944. 
 
 7. McLennan, J. and R. McKay, Trans. Roy. Soc. Canada, m, 24:1 (1930). 
 
 8. Seitz, F., Modern Theory of Solids, Chapter I, McGraw-Hill Bo<* Co.,New York, 1940. 
 
 9. Rundle, R., Electron deficient compounds, J. Am. Chem. Soc: 69:1327-1331 (1947). 
 
 10. Rundle, R., A new interpretation of interstitial compounds, submitted for publication in J. Am. 
 
 Chem. Soc. 
 
 11. Pauling, L., The Nature of the Chemical Bond, Second Edition, p 413, Cornell University Press, 
 
 Ithaca, N. Y., 1945. 
 
 12. Chem. and Eng. News 24:161 (1946). 
 
LORIDA 
 
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