July, 1963
CORIMUNICATIOW TO THE EDITOR
static charge on the sample tube due to the very intense radiation field (- 3 X l O I 4 disintegrations/min.), nonuniform packing, and, of course, possible ferromagnetic contamination of the sample are possible experimental errors. Since promethium oxide may not be magnetically dilute, there may be an appreciable TNeiss constant which makes the observed value lower than that calculated.
5 i=i
1569
Ei(ick,T) = constant (1 QAT)
+
and
HAXFORD LABORATORIES J. C. SHEPPARD The first equation specifies that the molar absorptivjCOMFANY E. J. WHEELWRIGHT ties of all absorbing species in the solution must have GENERAL ELECTRIC RICHLAND, WASHINGTON F. P. ROBERTS the same temperature dependence as the volume of the RECE~VED MARCH 4, 1963
solution a t all S isosbestic points. The probability of such an occurrence for n > 1and S > 1is nil.
ISOSBESTIC POINTS I N ABSORBANCE SPECTRA
Si?-: With reference to my recent article on the occurrence of isosbestic points1 It has been kindly drawn to my attention by Dr. K. Buijs that isosbestic points can conceivably occur in closed systems consisting of three variable absorbing species. It was argued in the paper that the occurrence of more than two absorbing species was highly unlikely sirice the equation
must hold a t each isogbestic point, requiring in general that all molar absorptivities be equal. Systems which provide constant values of dCi/dCj, or derivatives all of which show the same dependence on the variable j , were overlooked. Under these conditions closed systems containing not only three but several absorbing species may give rise to isosbestic points, providing the species can be grouped into not more than two groups wherein the ratios of concentrations of the species within the group are constant. Such systems fall into the category described by Cohen and Fischer2 wherein the deDonder-Van Rysselberghe parameter can be successfully defined as a system parameter (linearly related systems). My original conclusions that occurrence of isosbestic points in closed, temperature dependent systems indicates only one absorbing species are still valid, even for linearly related systems. This is directly shown by development of Cohen’s and Fischer’s equation in extended version with consideration of time, concentration of a j t h species, and temperature as independent variables. The equation is given as follows. The reader is referred to the papers cited for definition of symbols.
In a closed system with equilibrium a t each temperature, this equation is reduced to
For a system OS n absorbing species to produce spectra with S wave lengths (&) of temperature invariant absorbance. two conditions must be obeyed, namely (1) J. R. RIorrey, J . Phys. Chem., 66, 2169 (1962). ( 2 ) X D. Cohsn a n d E. Fischer, J . Chem. Soc., 3044 (1962).
HANFORD LABORATORIES GEXERAL ELIBCTRIC COMPANY RICHLAND, WASHIXGTON RECEIVED MAY2, 1963
J. R. MORRI~Y
THE CRYSTAL STRUCTURE OF THE MOLECULBR ADDITION COMPOUND XENOJS DIFLUORIDE-XENON TETRAFLUORIDE
Sir: The existence of the crystalline phase whose structure is reported here was noted in the earliest examinations1f2 of the xenon fluorides. Because it could be crystallized from vapor having primarily the infrared spectrum of XeF4, the substance was reported3 to be a polymorph of XeF4. From this assumed composition and the b = 7.33 8.) c = crystallographic data,3 a = 6.64 8., 6.40 A., p = 92” 40’, 2 = 4, it was deduced that the density was 10% higher than that of the other form; hence it has been referred to in the literature as “the highdensity form of XeF4.” We have shown, by crystal structure analysis, that it is in reality a distinct compound with the composition XeFz.XeF4. The true X-ray density is 4.02 g. ~ m . - - ~ . The preparation of this compound from the elements was described previously,3 but it should be added that the results of the crystal structure analysis indicate that some XeF2must have been present in the predominantly XeF4 preparation, either by incomplete reaction4 or by decomposition of XeF4. Further work is being carried out to prepare 1arge.r quantities of XeFz.XeF4by combining the components. The diffraction intensities from a single crystal of XeFz* XeF4 were measured by use of ill0 K a X-rays, ab goniostat,, and a scintillation counter detector. A total of 574 independent reflections was recorded, which included virtually all having detectable intensity. The specimen grew in size during the data collection, and a normalizat’ion factor, derived from repeated measurements of a reference reflection, was applied. The approximate shape O F the crystal was determined, making it possible t.0 calculat,e an absorption correction for each reflection.6 The mean diameter of the crystal was about 0.015 cm. ; the value of the absorption coefficient used was 119.5 cm.-l. (1) C. L. Chernick, et al., Science, 188, 136 (19132). (2) 8. Siege1 and E. Gebert, J . A m . Chem. SOC.,85, 240 (1963). (3) J. H. Burns, J . Phys. Chenz., 67, 536 (1963). (4) D. F. Smith, J . Chem. Phys., 38, 270 (1963). ( 5 ) D. J. Wehe, W. R. Busing, a n d H. A. Levy, “ORABS, A Fortran Program for Calculating Single Crystal Absorption Corrections,” ORNL TX-229, 1962.
