COMMUNICATIOXS TO THE EDITOR
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U
2 1.6 : 1.4 ';j 1.2 -
E 1.0 +
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w
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I
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25
i
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36 40 45 50 55 60 Time after sample introduction, min.
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Fig. 1.-Typical synchronized katharometer and pulse-integrator recorder traces for Hz-HT-Tz sample (3.1 std. ml., N ~ = T 6.85 X 10-13, AET = 7.5 X 1 O - I o mc. per std. ml., N T ~= 8.44 X AT^ = 9.2 X lo-" mc./std. ml.) with helium carrier at a flow rate of 90 ml./min. through two 8-ft. ferric oxide on alumina columna in series a t 77°K.
described previously but were not used for direct hydrogen-tritium gas mixture counting in conjunction with prior chromatographic resolution of the isotopic components. No shielding other than from photo effects was employed and a residual background count rate of 150200 c.p.m. was observed. The scaler-pulse integratorrecorder system mas sufficiently sensitive to count adequately, amplify, integrate, and record total changes in the background of 30 c.p.m. over a several minute interval; hence no shielding was deemed necessary for the tritium detection. This system for detecting low-level tritium activity combined with the gas chromatographic method of concentration of the radioactive gas should make possible the determination of the tritium content of natural water with a minimum of electrolytic concentration and a relatively lorn cost for detection equipment. This application is under investigation. I n addition, deuterium content can be determined simultaneously. 6 Acknowledgment.-The authors are grateful to the United States Atomic Energy Commission for support of this research. (6) H. A. Smith and P. P. Hunt, J . P h y 8 . Chem., 64, 383 (1960); P. P. H u n t and H. A. Smith, ibid.. 66, 87 (1961); E. H. Carter, Jr., and H. A. Smith, in preparation.
E. H. CARTER,JR. CHEMISTRY DEPARTMENT HILTONA. SMITH THEUNIVERSITY OF TENXESSEE TENNESSEE KNOXVILLE, RECEIVED DECEMBER 17, 1962
A SECOND CRYSTALLINE PHASE OF XEXON TETRAFLUORIDE Sir: From an X-ray diffraction study of the new compound xenon tetrafluoridell a second monoclinic phase has been
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found in addition to that described by Siegel and Gebert.2 The modification of XeF4 described here is apparently the less stable form a t room temperature since it is produced somewhat less readily, although both condense from the vapor near room temperature. The greater density of this second phase probably indicates that it is a lower-temperature polymorph. Crystals of this form of XeF4 are visually distinguishable, by their pyramidal habit, from the platelets of the low-density form. The XeF4 was prepared by reaction of the elements,' identified, and its purity verified by infrared analysis in the apparatus described by D. F. Smith3; then it was distilled into evacuated thin-walled quartz tubing, where crystals of both phases were grown simultaneously and studied with a microscope and by X-ray diffraction. Single-crystal precession photographs, taken with hfoK X-rays, provided the data for determination of the unit cell and space group. The unitcell dimensions are: a = 6.64 + 0.01, b = 7.33 & 0.01, c = 6.40 0.01, and p = 92'40' i 5'; the probable space group is P21/c. Four XeF4 molecules per unit cell give a calculated density of 4.42 g./cm.a. This value is remarkably higher than the value of 4.07 for the other phase. The general hlcl reflections are strong only when h,lc,l are all odd or all even, implying a face-centered arrangement of the heavy atoms. Accordingly, the Xe atoms are placed in the following special positions of P2dC
and the fluorines must occupy the general positions
with four crystallographic kinds of fluorine atoms. The location of Xe atoms at centers of symmetry4 requires the XeF4 molecules to be centrosymmetric and planar, as was previously shown for the other phase (2). Further work on the structure is in progress. Acknowledgments.-The cooperation of Dr. D. F. Smith, Mr. P. A. Agron, and Mr. J. E. Eorgan in preparation and analysis of samples and helpful discussions with Dr. H. A. Levy and Dr. J. A. Ibers are gratefully acknowledged. (1) H. H. Claassen, H. Selig, and J. G. Malm, J. Am. Chem. Soc., 84, 3593 (1962); C. L. Cherniok. et al., Sczence, 1S8, 136 (1962). (2) S. Siegel and E. Gebert, J. A m . Chem. Soc.. 86, 240 (1963). (3) D. F. Smith, J . Chem. Phys., in press. (4) The other possibility, not requiring molecular centrosymmetry, is for X e t o OCOUPY 4(e) wlth z = 0.25, y = 0, z = 0.25. (5) Operated for the U. S. Atomic Energy Commission by Union Carbide Corporation.
JOHN H. BURNS REACTOR CHEMISTRY DIVISION OAKRIDGENATIONAL LABORATORY OAKRIDGE,TENNESSEE RECEIVED DECEMBER 17, 1962
