4224
J. Phys. Chem. 1993,97, 4224-4230
Thermodynamics of the Volatilization of TlzO from TlzO, T1403,and Tl2O3 W. L. Holstein Du Pont Central Research & Development, Experimental Station, P.O. Box 80304, Wilmington, Delaware 19880-0304 Received: November 5, 1992; In Final Form: January 26, 1993
Volatile thallous oxide, Tl20, is used in the fabrication of thallium-containing high-temperature superconducting powders and films. The partial pressure of thallous oxide was measured as a function of temperature and oxygen partial pressure by the transportation method over liquid and solid T120, liquid and solid T1403, and solid Tl2O3. At low oxygen partial pressure, condensed T120 is stable with respect to T1403 and Tl2O3. It melts at 852 f 10 K and is very volatile, with a vapor pressure of about 1.0 X le2atm at 900 K. At high oxygen partial pressure, Tl2O3 is stable with respect to T120 and T1403. It does not melt congruently below 1067 K. It volatilizes congruently through the reaction Tl203(c) = TlzO(g) 02(g), and the equilibrium vapor pressure of T120 in oxygen at atmospheric pressure reaches 1.0 X atm at about 1050 K. T1403 exists as a thermodynamically stable phase at intermediate oxygen partial pressure. It melts at 990 f 2 K,and it volatilizes congruently through the reaction T1403(c) = 2 T12O(g) l/202(g). Thermodynamic properties for T1403(c) are estimated to be Afkf0298 = -520.7 kJ/mol and S O 2 9 8 = 259.0 J/(mol K). The experimental results of this study were combined with data in previous studies to estimate the high temperature thermodynamic properties of the thallium oxide phases. These properties were used to generate a phase diagram for the T1-0 system and to determine the equilibrium vapor pressure of thallous oxide over the condensed phases as a function of temperature and oxygen partial pressure.
+
+
I. Introduction
to write each reaction on the basis of two T1 atoms
An understandingof the thermodynamicsof the thallium oxides has taken on new importance with the discovery of superconductivity in T1-Ba-Ca-Cu-0 compounds by Sheng and Hermannl and in T1-Pb-Sr-Ca-Cu-0 compounds by Subramanian et ale2 These materials possess the highest superconducting transition temperatures of any compounds, up to 127 K for some of the phases. The materials show promise for several applications, including high current-carrying conductors for high field magn e t ~and ~ , low ~ surface resistance films for thin film microwave ele~tronics.5~~ In the processing of these materials, the phases that form and their properties are strongly dependent on the thallous oxide and oxygen partial pressures. Three condensed thallium oxide phases are known to exist: thallous(1) oxide (T120), thallic(II1) oxide (T1203),and the mixed valence compound T1403.7 Other mixed valence compounds have also been proposed,*but they have not been positively identified. Theonly gaseous thallium oxide compound that has been positively identified is T120.' Previous studies of Tl2O and T1203 have led to strong disagreements concerningthe thermodynamic properties of these compounds, particularly their ~ o l a t i l i t y . ~ ,As ~ - ~a ~result, the calculation of TlzO vapor pressure from literature compilations of thermodynamic data can lead tovalues differing by more than an order of magnitude, depending on which data the calculations are based. No thermodynamic data have been reported for TI4O3. These facts, together with the renewed interest in the vapor processing of thallium oxide-containingcompounds, indicates the need to investigate more comprehensively the volatilization of these materials. The goal of this study was to develop a phase diagram for the T1-0 system and to determine thermodynamic properties that would allow for the accurate calculation of the equilibrium thallous oxide partial pressure as a function of temperature and oxygen partial pressure. 11. Chemical Reactions in the T1-0 System
The T1-0 system is defined by five chemical reactions, only three of which are linearly independent. It is most convenient 0022-3654193 l2097-4224$04.00/0
KI
T120(c,l) = Tl,O(g)
K~ = ( P ~ , ) ~ / ~
As we will discuss further below, a liquid phase for T1203has not yet been demonstrated, and we thus consider only the solid phase for this material. In the above equations, we have considered liquid T120 and T1403as distinct independent phases. However, these compounds are most likely completely miscible in the liquid state, forming a phase of composition TlO,, where x is a continuous function of oxygen partial pressure and temperature, varying from 0.5 to 0.75. In addition, Sabrokowski and MirzaZohave measured shifts in lattice constants by X-ray diffraction for both solid T1203 and T1403,and they suggest that both phases can exist as the nonstoichiometric compounds T1203-6 (6 = 00.05) and T1403+a.(6' = 04.05). In this work, we will treat all solid phases as stoichiometric compounds. 0 1993 American Chemical Societv
The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 4225
Volatile T120 111. Experimental Section
TOVENT
Materials. T1203(Aldrich, stated purity of 99.99%) was first heated briefly to 980 Kin oxygen to remove any adsorbed water. T1403was prepared by the thermal decomposition of T1203 in flowing 0.1% Oz/Ar at 960 K, followed by rapid cooling in a stagnant gas atmosphere of the same composition. The phase was confirmed by X-ray diffraction.21,22Produced in this manner, the T1403was a gray powder with a metallic luster. T120 was prepared by a previously reported method, the thermal decomposition of T12C03(Mackay, unstated purity) in argon at 900 K.23 The phase was confirmed by X-ray diffracti0n.~3Since the material was produced above the melting point of T120of about 852 K, it was present as a slab with very Iarge grains rather than as a powder. Ultrahigh purity argon (stated purity of 99.9995%,
