4086
SURESH K. GUPTA
Thermal Stabilities of Tungsten Oxyiodides by Suresh K. Gupta Lamp Research Laboratory, General Electric Co., Cleveland, Ohw Gii9 (Received February 94, 1969)
A mass spectrometric and Knudsen effusion investigation of the W-0-1 system at high temperatures has been conducted. Vaporization of solid WOZIZhas been observed to involve its partial dissociation to molecular
iodine. Iodine pressures over solid WOZIZin a Knudsen cell exhibit strong effusian orifice dependence. The enthalpy of WOJz vaporization = 45.2 f. 1.0 kcal/mol at 298°K has been derived. A decomposition reaction assumed as WOzIz(s)= WOzI(s) I/ZIz(g) has given -136 f 5 kcal/mol as AHf"of solid WOZIat 550°K. WO18has been observed as a minor gaseous species over solid WOZIZin an effusion cell. From our extensive study of the WOz-Izreaction, -102.8 f 2.0 kcal/mol and 90.1 rt: 2.0 eu have been evaluated as AHf"and So, respectively, of WO&(g) at 298°K.
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Introduction Thermodynamic studies of tungsten halogen chemistry are considered vital in view of the growing technological importance of tungsten as an important refractory metal. Such data are of significant theoretical value because of known similarities in the chemical beha,vior of group VIa elements of the periodic table. Among oxyhalides, WOX4 and WOZX2 have been estakdished as the prominent tungsten(V1) compounds.' Lower oxidation state oxyhalides, WOXS and WOX2, have also been rep0rted.l However, most of these investigations have been restricted to fluorides, chlorides, and bromides. Recently, WOzIz has been reported as a stable tungsten oxyiodide.2J The present investigation has been prompted with a view to establish the existence of tungsten oxyiodides and to obtain their thermochemical stabilities using mass spectrometric and Knudsen effusion techniques. During the course of the present work, a series of three papers on the W-0-1 system have been publ i ~ h e d . ~ -Comparisons ~ of the results have been made wherever appropriate. Experimental Section A . Vapor Pressure Determination. An Ainsworth automatic recording semimicrobalance Model AU-2 with a sensitivity of 0.02 mg was used in Knudsen effusion vapor pressure work. The apparatus included a specially designed 2 in. i.d. and 14 in, long resistance furnace. A 7 in. long copper liner in the middle of the furnace provided a 3 in. long central constant-temperature zone. Voltage regulation with a Sola transformer and minimization of air convections through the furnace provided temperature stabilities of about 1". A pressure of less than 10-5 Torr was maintained during the course of a run. The effusion cell was supported on a molybdenum pan rest suspended by a 0.015-in. diameter molybdenum wire. Temperature of the cell was continuously measured on a dual pen strip-chart recorder using a Calibrated Pt-P t-Rh thermocouple. The thermocouple The Journal of Physical Chemistry
bead was positioned as near as possible without contact to the cell. In separate experiments, the reliability of this technique was verified by comparing temperatures from the thermocouple in contact with the cell and the one positioned near it. I n a typical experiment, the sample was outgassed at an average temperature of the entire range of investigation. Both the weight of the cell and the temperature were continuously monitored. A few initial measurements were always discarded to eliminate outgassing contributions to the weight loss and only data corresponding to constant temperature periods were used. The vapor pressure of zinc was measured to establish the reliability of the experimental setup. Graphite effusion ovens with screw-type covers containing 0.025- and 0.070-in. orifices were employed after outgassing at 1100°K under vacuum (-lom6 Torr). Zinc shot of 99.999% purityT were used in these measurements in the temperature range 530630°K. Diameter and length of orifices were obtained through their highly magnified photographs in a horizontal plane and at 45" inclination. Orifice areas were corrected for expansion due to heating in the evaluation of vapor pressures using the Knudsen effusion equation* dm rate of effusion -- = aWaP, X dt where M , P,, and a are the molecular weight, equilibrium pressure, and condensation coefficient, respec(1) J. E. Ferguson in "Halogen Chemistry," Vol. 111, V. Gutman, Ed., Academic Press, New York, N. Y.,1967,p 227. (2) J. Tillack, P. Eckerlin, and J. H. Dettingmeijer, Angew. Chem., 78, 451 (1966). (3) B. McCarroll, J. Chem. Phys., 47, 5077 (1967). (4) J. H.Dettingmeijer and B. Meinders, 2.Anorg. Allg. Chem., 357, 1 (1968). (5) J. Tillack, ibid., 357, 11 (1968). (6) H. SchBfer, D.Geigling, and K. Rinke, ibid., 357, 25 (1968). (7) Supplied by Alfa Inorganics, Inc., Beverly, Mass. (8) M. Knudsen, Ann. Phys., 29, 179 (1909).
4087
THERMAL STABILITIES OF TUNGSTEN OXYIODIDES tively, of the vapor molecule at T”I
