NOTES
Sept., 1963
1919
NEW HIGH PRESSURE FORM OF CALCIUM
PARTIAL MOLAL VOLUMES OF SULFUR HEXAFLUORIDE
DISILICIDE BYM. S. SILVERMAN AND J. R. SOULEN
BY H. HIRAOKA AND J. H. HILDEBRAND Department of Chemistry, University of California, Berlceleg, (lalifornia Received March 16, 1965
Research and Deuelopment Laboratories, Pennsalt Chemicals Corporation, Wyndmoor, Pennsylvania Received Awril 86. 196s
Several of our recent investigations have served to emphasize the fact that in regular solutions the partial molal entropy of solution at constant pressure includes, in addition to the entropy of dilution, a significant increment of entropy of expansion. This was brought out strikingly in the cases of solutions of iodine1 and stannic iodide,2 and of gases in various solvents.a It was evident, therefore, that measurements of the partial molal volume of SFBin several solvents were necessary in order to interpret its unusual solubility relations as reported by Archer and H i l d e b r a ~ ~ d . ~ The dilatometer was of the type devised by Horiuti5 and used by Jolley and Hildebrand3 with the improvement in technique adopted to main constant pressure described by Walkley and Hildebrand.6 From 4 to 6 additions of gas mere made to each solvent and the total volume increment was plotted against the mole6 of gas added. The points fell on straight lines with very small scatter. Different runs for the same system agreed closely. We estimate the probable error as 0.8%, except in the case of CS2, where it is a little larger because of low solubility. The mole fraction of dissolved gas was so small in each case that the partial molal volumes given in Table I are virtually the limiting values at infinite dilution. TABLE I SFe A S D E N T R O P Y O F SO&UTION SAMEMOLEFRACTION, AT 25"
P A R T I A L M O L A L FrOLUMES O F FROM
1 ATMOSPHERE TO Solvent
CJ%iCzFs CClzF'CClFz i-C*H,,
n-CJLs cc1, C6H6 CS,
-
-
61
6.1 73 6 9 74 8 6 9 2 10 0
v2,
00.
94.0 101.4 104.2 105.5 104.0 105.5 I26
S2(10-4) &(l atm.)
... 1.0 1 7 2.0 2 5 3.8 4.1
Included in Table I are figures for the entropy of transferring SF6 from gas a t 1 atm. to solutions at the same mole fraction, calculated as described by Jolley and Hildebrand.3 The strong increases in both V2 and As, in descending the table contrast with the reverse trend in both quantities in solutions of argon, indicating, as does the excess over unity of the slope of the SFa line in Fig. 2 of the paper by Archer and Hildebrand, departures of SF6 from the geometric mean relation in mixtures with nonfluoride solvents. Theoretical treatment of these results in terms of intermolecular forces will be undertaken in a later paper. Acknowledgments.-This research was supported by the Atomic Energy Commission. We are indebted to the Minnesota Mining and Manufacturing Co. for the supply of C ~ F I I C ~ H S . (1) J. H. Hildehrand, J . P h y s . Chem., 64, 370 (1960). (2) E. B. Smith and J. Walkley, Trans. Faraday Soc., 66, 1276 (1960). (3) J. E. Jolley and J. H. Hildebrand, J . Am. Chem. Soo., 80, 1050 (1958). See also J. H. Hildebrand and R. L. Scott, "Regular Solutions," PrenticeHall, Inc., New York, N. Y., 1962, PP. 34 ff,, 44, 45, Chapter 8, p. 153. (4) G. Archer and J. H. Hildebrand, J. Phys. Chem., 67, 1830 (1963). (6) J. Horiuti, Sei. P a p e r s l n s t . Phys. Chem.. Res. (Tokyo), 17, 126 (1931). (6) 5 . Walkley and J. HaHildebrand, J . Am. Chem. Soc., 81,4439 (1859).
Calcium disilicide is known to have a hexagonal layer type structu:re,l as do graphite and boron nitride. At very high pressures and temperatures the latter compounds form the more dense cubic polymorphs, diamond2 and borazon.3 CaSiz has now been transformed at high pressures to a new, more dense modification which appears to be tetragonal. Commercial grade CaSi2 obtained from K and K Laboratories was used as one starting material. This contained several per cent iron as an impurity. Mixtures of semiconductor grade silicon, spectroscopic grade graphite, and CaO prepared by heating 99.9% CaC03 were also used. The apparatus was of the tetrahedral anvil type developed at the National Bureau of standard^.^ The sample holder, heating assembly, and calibration techniques were similar to those described previ~usly,~ except that a boron nitride insulating capsule was used between the graphite heater and the sample. I n carrying out the runs, CaSi2 or a CaO Si C mixture was first compressed to the desired pressure between 8 and 87 kilobars; temperature then was raised and held at the desired level, which ranged up to 1800". After 2 to 10 min. the electrical power was shut off and the sample was; decompressed after the product had reached ambient temperature. With CaSiz as th.e starting material, striking changes were noted in the solid resulting from a number of runs. Under microscopic examination it appeared that essentially 100% conversion to a reflective, coral material had occurred from the original gray form. In concentrated HC1 the latter reacted with vigorous effervescence whereas the new form, even when finely ground, underwent only slight reaction. Density of the hexagonal starting maLerial was 2.47 f 0.02 g . / ~ m . ~in, good agreement with 2.46 g./cm. reported previously.1 Measured densities of the high pressure form ranged from 2.60 to 2.76 :k 0.02 g . / ~ m . ~ The . highest value was obtained from a product formed under the most severe combination of condit'ions, 87 kilobars and 1500*, and thus must be closest to correct for the new form. XRay diffraction films of many of the samples gave a n identical powder paittern which is characteristic of the new form and is completely different from that of the original material. The strongest lines are listed in Table I. These can be indexed on the basis of a tetragonal structure with a = 6.23 A. and c = 4.52 8. Density calculated from these constants, assuming 3 CaSi2 per unit cell, is 2.73 g./cm.a, compared with 2.76 for the highest density product. Taking the iron impurity into account, produlct analysis showed about 2.6% Fe and a compositioii approximately Ca~,~~Feo.o&3i2.00.
+ +
(1) J. Bohm and 0. Hassel, 2. anoro. allgem. Chem., 160, 152 (1927). (2) F. P. Bundy, H. T. Hall, H. M. Strong, and R. H. Wentorf, Jr., n'ature, 176, 51 (1955). (3) R. H. Wentorf, Jr., J Chem. Phys., 26, 956 (1957). (4) E. C. Lloyd, U.0. Hutton, and D. P. Johnson, J . Res. Nut?. B u r . Std., 6aC, 59 (1858). (6) J, R d Soulen and M. $3, Silverman, J . Polymer ,%..,. Ly 823 (1963).
