5 . . . " TOF | . . :: ORNL P : : C 2 : . liga · . L ' . . * _ 1366 et. op ... . : , 145 SO 6 ? IU . MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF SEANDARDS -1963 ... - . - - ---- ----- - - ... - - - . . . - - - - LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representa- tion, expressed or implied, with respect to the accuracy, completeness, or usefulness! of the information contained in this report, or that the use of any information, appa- ratus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, inethod, or process disclosed in this report. 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The report we prepared u La Actor , ar prostda neceu to, may taformation purnust to emplogo.cot or contract sployee or contractor of the Companion, or employee of such contractor preparos, or contractor of the Coca.lacion, or vaplore of much contractor, to the meat that Ao med bathe above, "per co xting oo baball of the Commisaloa" lacludes way uw of my wormats appuntas, tothod, or procon disclosed to this report of may lalarualon, appunto, method, or procos. daclou B. Asmuss may habuitor will respect to the wo of, or lor damage resulting from ebo of the faformation contaipad lo this report, or that the we rasty or reproseatation, utpreaned or impiad, with roject to be accu. down tor the Cosimlasloo, sor Lay period actin og badell of the Commonsi la this report my oot latrice or Govenosat sponsored work. Nollber Abe United - LEGAL NOTICE * INTRAXED SPECTROSCOPIC STUDY OF CONDENSED PHAJE TRANSITIONS IN CF Ralph G. Steinhardt, Jr. and Walter Ne il sen, Hollins College, Virginia, 0. S. A. and Henry W. Morgan and Percy Staats, Oak Ridge National Laboratory Oak Ridge, Tennessee, 0. S. A. Abstract · Significant features of the infrared spectrum of CF4 have beon studied from 112°K in the liquid state to approximately 20°K in the solid state. The fundamental absorption V, and the combina- tion bands (vy ) and free pi show strong discontinuities in band-width at the second-order transition aear 76°k, but display no observable change in band-width at the melting point. Furthermore, the band ( + ) displays apparent rotational fine structure in the liqrid state as well as in the solid state above the transition tempe rature, but does not show fine structure at tempe ratures below the transition tempo ra ture. These observations appear to confirm directly the notion that the second-order transition involves the onset of restricted or hindo red molecular rotation, and that the .melting process involves primarily the destruction of the long- range order characteristic of the solid stat HE PUBLIC IS APPROVEO. PROCEDURES PATENT CLEARANCE OBTAINED. RELEASE OF AREION FILE IN THE RECOVNS SECTION, *Research Jointly sponsored by the U. S. Atomic Energy Commission tador contract with Union Carbide Corporation and by Hollins College. Sommaire Les caractéristiques essentielles du spectre infrarouge de CF, sont ó tudiées depuis l'état liquide à une température de 112°K jusqu'à l'état solide à une température de 20°x. L'absorption fondamentale aussi bien que les bandes de combinaison (v + et lat, + Ved permettent d'observer des discontinuités marquantes dans les largeurs respectives des bandes au point de trisition autour de 76°x; cependant, elles.ne présentent aucun changement notoire de largour au point de fusion. En outre, la bande (ty + ) prósente uno structure qui parait rotative, à l'état liquide de même qu'à l'état solide au dessus de la température de transitiun, mais ne préseate pas une telle structure à des températures iurérieures à la température de transition. Ces observations semblent offrir une confirmation directe de l'hypothèse que os transitions de cet ord re sont le résultat d'un début de rotat. l.ori moléculaire limitée et que la fusion est principalement le résultat du bouleversement de l'ordre étendu caractéristique de l'état solide. Introduction carbon tet ide I An infrared investigation of the liquid and solid states of carbon tetrafluoride was carried out as part of a general program or the study of simple liquids. Previous inf rared studies of the molecule BF, in the pure liquid state, as a solution in liquid xenon, and as a solid, have been reported (1, 2). BF, may be represented as piane symmetrical molecule (D3h) in which the spa bonding permits inter-molecular interaction to occur through the vacant p-orbitals perpendicular to the molecular plane.. However, CF, with spy bonding and with a tetrahedral structure, is a simple molecule which can be treated to good approximation as a spherical assembly. Dispersion forces are the primary intermolecular forces operative in an assembly of molecules of this type. The tetrafluoride was chosen for primary study from among the other members of the halide series because this compound shows no 180 tope effects and has been previously studied through both calorimetric 13) and NMR (4) techniques. Furthermore, the melting and boiling points as well as the other transition tempe ra. tures are in a convenient range ros experimental work. Experimental . . .. ..... CF, was observed as liquid and solid in a low-tempe ra ture infrared liquid cell which has been reported previously (5). This . cell (Fig. 1) is of a conventional Dewar design, with two access tubes to allow filling of the liquid compartment. The cell windows for this experiment were CsBr (permitting observations to frequencies .... . as low as 250 cm**) with a thermocouple embedded in one of the windows for temperature measurement. The cell was cooled by liquid CH4 (110°x), 02 (90°K), N, 177°K), Ax (86°), or H2 (20°K). Tem- peratures between 60% and 76°% were obtained by controlled evacua. tion of the liquid nitrogen in the coolant compartment. Tempe ra. tore control within + 1°8 could be ma inta ined in this manner for periods of at least twenty minutes. The sample was obtained from a cylinder or commercial-grade carbon totrafluoride (DuPont Freon 14), and was twice distilled at low pressures before being condensed into the infrared cell. The complete spectra in the gas, liquid, and solid phases showed no traces of any impurity bands. Infrared spectra were studied on a Perkin-Elmer Model 21 spectrophotomo ter using sodium chloride and cesium bromide prisms, and at higher resolution on a Perkin-Elmer Model 521 grating spectrophotometer. The precise slit functions were not measured, though it is known that the grating studies were conducted at a resolution of one wave number or better. Results and Discussion Carbon te i rafluoride has ta symme try and thus possesses o ine characteristic vibrational modes, several of which are degenerate as ind icated in Table I. V, and V, which are totally symmetrical, are inf rared-ioactive and were not obsered in any of the recorded 8péctra. Vy is the asymmetric C-F stretching motion and possesses such an intense absorption that it could pot be studied. A 10 micron sample thickness, the smallest thickness which could be used in the liquid cell, gave complete absorption over a 100 cm-t region. The high vapor pressure of both liquid and s011d CF4 in the neighbosm hood of the melting and transition tempe ra tures caused thin deposited films to evaporate so rapidly as to preclude infrared study. For this band, a prohistirely thin sample would be necessary to provide resolution of the fundamental and the seve ral combination and over- tone bands which appear within the 100 cm-1 absorption region. The fundamental was observed with a peak absorption of about 80% with a 10 micron sample thickness. This thickness was used for all of the stud ies on CF, and also provided sufficient absorption for significant measurements to be made on the combination bånds fty +74) and (). Using CH4, 02, and Ar as coolants, CF4 was observed in the i iquid state. Using liquid N2; the sample was studied as a solid, and the conversion from liquid to solid could be observed visibly in the cell. The solid above the second-order transition tempera. ture was highly transparent in the visible region. By pumping on the liquid nitrogen coolant, the temperature was lowe red and the second-order phase transition was noted visually as a change from a transparent to a highly scattering solid. This was due to the volume change which occurs at the second-order transition point as has been reported for CHA (6). In the infra red spectrum this change produces both scattering and a marked Christiansen filter effect. Upon warming, the sample returned to full transparency, and this cycle could be repeated many times, At no temperature was there any indication of asymmetry or structure in the band except that both scattering and the Christiansen filter effect can be noted in the 70°K spectrum. A smooth decrease in band width was measured down to 76°k, at which tompe rature a marked dis- continuity occurred. This is illustrated in the graph shown in Fig. 3. Although not indicated in the graph, there was no further The combination band (V, + VA) is shown in Fig. 4 (ordinates displaced). There is evidence of asymmetry in the band contour at 111°K which is not observed in the liquid phase at 90°K or in the solid phase at boºk. In the solid at 70°K below the second-order transition) there is again a definite structure to the band. This structure is shown in greater detail in Fig. 5 which is the band contour at 20°k. It is not believed that the asymmetry at 111°K in the liquid phase is related to the low-temperature band structure. The plot of half-widtL* vs. tempe rature for this bend shown in Fig. 3 and again indicates a smooth decrease at tempe ra tures down to 76°K, at which an increase occurs. This increase is apparently due to the structure which appears in the band below this temperature but which cannot be resolved under the instrumental resolution presently available. (Vg + ) is shown in Fig. 6 (ordinates displaced). Under high resolution both the liquid phase and the solid phase above the second-order transition show pronounced structure on both sides of *By "half-width" is meant "width at aalr-intensity." structure in a pure liquid or a puro solid. As the sample 18 coolod through the transition tompe rature this structure disappears, leaving a band comparable in width to the fundamontal ti this həll-width is maintained at temperatures down to 20°x. The fine structure of this band appears as the sample temperature 18 raised by 1888 than 0.5°K through the second-order transition point. Careful measurements of band half-widths as functions or tempe ra ture show no discontinuities at the melting point; this is consistent with the commonly-held view that the solid-liquid transi. tion destroys long-range order only (7,8). There are definite changes at 76°K, the second-order transition temperature reported by Aston, et al (4), but no pronounced spectral change was observed at 55°x, at which these authors reported a strong discontinuity in the NMR line width. However, no careful study was made of the spectrum at 55°K because of difficulties in temperature control in this region. The structure of solid carbon tetrafluoride, unfortunately, 18.not known. However, the structure of the (V2 + qa) band may tentatively be assumed to be due to site-splitting in the phase below the transition in which it is assumed that rotational notion • destroys the symmetry of the crystalline site. The structure observed in the liquid and solid phases in the 19, +) band indicates rotational motion in both the liquid and in the solid phase above the second-order transition point. It will be noted that the structure of (ty + y) is somewhat more pronounced in the liquid than in the solid state. The spacings and intensities • ...- .- .-. ..... -.... . . .. .. . . . . . . .. . .... . ... in no way suggest free rotation but rather indicate a potential . barrier (i.e., binde red or restricted rotation), somewhat higher in the solid than in the liquid. The spacing between the fine structure peaks is approximately 3 cm-1, which is not inconsistent with such motion. This structure appears at a sharply defined tempe rature, and can be associated with the large decrease in NMK bandwidth found by Aston, et al at 70°K, at which they postulate that rotational motion occurs about all axes and under these con- ditions is "practically" free. The structure in ( + wy) proves that either binde red or restricted rotational motion is occurring in the liquid and in the . . .. . ..... previously cited studies of CF, and other similar molecules. It is not understood why such structure does not appear in the funda. . mental me but only in the combination band (ot, + Vol. However, the suggestion of structure in ivo + ) in the liquid state implies the possibility that rotational motion in condensed phases of CF4 is somehow to be associated only with syrometrical vibrational modes. Acknowledgements The authors are grateful for the support of the Petroleum Research Fund of the American Chemical Society, the Hollins College Faculty Travel and Research Fund, the Virginia Academy of Science, and the Oak Ridge Institute of Nuclear Studies. One of us, W. N., 18 grateful for the award of an Undergraduate Research Participant. ship from the National Science Foundation through a grant to Hollins college. a v. .-.. (1) R. G. Steinhardt, Jr., M. Jordan, G. E. S. Fetsch, J. Chem. Phys. (in press), (2) Ibid., Conforence on the Liquid State, The Physical Society of London, September, 1963. (3.) A. Euckes and E. Schroder, 2. physik. Chem., B. 41, 307 (1958). (4) J. G. Aston, Q. R. Stottlemeyer, and G. R. Murray, J. Ara. Chem. Soc., 82, 1281 (1960). (5) H. W. Morgan, P. A. Staats and R. G. Steinhardt, Proceedings of the Society for Applied Spectroscopy, 5th Annual Meeting, Cleveland, Ohio, September 1964. To appear in the Review of Scientific Instruments. (0) A. Schallanach, Proc. Roy. Soc. (London) A171, 569 (1939). (7) M. F. Crawford, A. 2. Welch, and J. H. Harrold, Canadian J. Phys., 30, 82 (1952). (8) G. E. Ewing, J. Chem. Phys., 40, 179 (1964). . . . . . . ..... no.-.... . .... -.-- Fundamental 630 126,5 437 904 Frequency (cm!) TABLE I asymmetric bend a symmetric stretch symmetric bend symmetric stretch Motion Triple Triple Double None Degeneracy .. - ...::::..........-..-.com voc-su...-som- Figure 1. Low Temperature Infrared Cell for Liquids and Solids. - - - - - - . .. ORNL-OWG 63-4381R3 NAS USA - RADIATION SHIELD - OUTER WALL FILLING TUBES COOLANT WINDOWS - FLANGE - GOLD SPACER - BOLTS (8 ON PERIMETER) INDIUM GASKET Figure 2. Absorption Band or the Fundamertal, ve ............ Liquid, 110°K ---- Solia, 80°K Solid, 70°K ORNL-DWG 65-4724 600 cm 660 cm.' 630 cm BAND-CARBON TETRAFLUORIDE Figure 3. Graph, cmd Half-width vs Temperature. ORNL-DWG 65-5366 - . + TEMPERATURE - OK 0 ? 2 cm+ HALF-WIDTH 4 6 8 10 cm' HALF-WIDTH Figure 4. Absorption Band of the combination, (vx + y). ............ Liquid, 110°C Liquià, 90°K Solid, 80°K Solla, 70°x 1080cm 1070 cm ORNL-DWG 65-4722 1060 cm 1050cm . . . (v.tw) BAND-CARBON TETRAFLUORIDE Figure 5. Combination rand, (vg + .), at 20°K. - - | ORNC-DWG 65-5365 1060 cm | | 그 ​1070 cm | | (vytv) BAND-CFL 20K Figure 6. Absorption Band of the Comnination, (va * V). ........ 110°K ----... 8°K 70°: 1570 cm 1560 cm 1550cm 1540 cm' ORNL-DWG 65-5367 4530 cm 4520 cm PUMPED N2 METHANE (0,+) BAND - CARBON TETRAFLUORIDE END . 1 DATE FILMED 9/3/165 * i-...2 h CA