Thermodynamics of the volatilization of thallium(I) oxide from Tl2O, Tl4O3, and Tl2O3

Abstract

Volatile thallous oxide, Tl[sub 2]O, 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 Tl[sub 2]O, liquid and solid Tl[sub 4]O[sub 3], and solid Tl[sub 2]O[sub 3]. At low oxygen partial pressure, condensed Tl[sub 2]O is stable with respect to Tl[sub 4]O[sub 3] and Tl[sub 2]O[sub 3]. It melts at 852 [+-] 10 K and is very volatile, with a vapor pressure of about 1.0 [times] 10[sup [minus]2] atm at 900 K. At high oxygen partial pressure, Tl[sub 2]O[sub 3] is stable with respect to Tl[sub 2]O and Tl[sub 4]O[sub 3]. It does not melt congruently below 1,067 K. It volatilizes congruently through the reaction Tl[sub 2]O[sub 3](c) = Tl[sub 2]O(g) + O[sub 2](g), and the equilibrium vapor pressure of Tl[sub 2]O in oxygen at atmospheric pressure reaches 1.0 [times] 10[sup [minus]2] atm at about 1,050 K. Tl[sub 4]O[sub 3] exists as a thermodynamically stable phase at intermediate oxygen partial pressure. It melts at 990 [+-] 2 K, and it volatilizes congruently through the reaction Tl[sub 4]O[sub 3](c) = 2 Tl[sub 2]O(g)more » + 1/2O[sub 2](g). Thermodynamic properties for Tl[sub 4]O[sub 3](c) are estimated to be [Delta][sub f]H[degrees][sub 298] = [minus]520.7 kJ/mol and S[degrees][sub 298] = 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 Tl-O system and to determine the equilibrium vapor pressure of thallous oxide over the condensed phases as a function of temperature and oxygen partial pressure. 30 refs., 10 figs., 3 tabs.« less