THERMODYNAMICS OF THE VOLATILIZATION OF TL2O FROM TL2O, TL4O3, AND TL2O3

Volatile thallous oxide, Tl2O, 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 Tl2O, liquid and solid Tl4O3, and solid Tl2O3. At low oxygen partial pressure, condensed Tl2O is stable with respect to Tl4O3 and Tl2O3. It melts at 852 +/-10 K and is very volatile, with a vapor pressure of about 1.0 x 10(-2) atm at 900 K. At high oxygen partial pressure, Tl2O3 is stable with respect to Tl2O and Tl4O3. It does not melt congruently below 1067 K. It volatilizes congruently through the reaction Tl2O3(c) = Tl2O(g) + 02(g), and the equilibrium vapor pressure of Tl2O in oxygen at atmospheric pressure reaches 1.0 X 10(-2) atm at about 1050 K. Tl4O3 exists as a thermodynamically stable phase at intermediate oxygen partial pressure. It melts at 990 +/- 2 K, and it volatilizes congruently through the reaction Tl4O3(C) = 2 Tl2O(g) + 1/2O2(g). Thermodynamic properties for Tl4O3(C) are estimated to be DELTA(f)H-degrees 298 = -520.7 kJ/mol and S-degrees298 = 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.