VAPORIZATION OF LACRO3 - PARTIAL AND INTEGRAL THERMODYNAMIC PROPERTIES

The vaporization of LaCrO3(s) and samples of the composition LaCrO3 La2O3 was investigated in the temperature range of 1887-2333 K by Knud sen effusion mass spectrometry using Knudsen cells made of tungsten li ned completely with iridium, The species Cr(g), CrO(g), CrO2(g), and L aO(g) were identified in the vapor, Their partial pressures were deter mined by calibration with pure platinum solid. The thermodynamic activ ity of Cr2O3, a(Cr2O3), in LaCrO3 for the Cr2O3-poor phase boundary of this phase was ln a(Cr2O3) = -(17953/T) - 0.485 (temperature T given in K) for the temperature range of the measurements with a probable ov erall error of +/-13%. The following values and temperature dependence of Delta G(f,T)degrees, resulted for the formation of LaCrO3(s) accor ding to the reaction 0.5Cr(2)O(3)(s) + 0.5La(2)O(3)(s) --> LaCrO3(s): Delta G(f,2100)degrees = -78.9 +/- 1.1 kJ/mol, Delta H(f,298)degrees = -76.8 +/- 5.2 kJ/mol, and Delta G(f,T)degrees(kJ/mol) = -74.7 - 0.002 02T. Computations for the vaporization of LaCrO3 were conducted to sho w the volatility of this material in different atmospheres at high tem peratures.