A high temperature study of the stoichiometry, phase behavior, vaportization characteristics, and thermodynamic properties of the lanthanum+oxygen system

Abstract

The lanthanum + oxygen system has been studied by investigating the vaporization behavior of the sesquioxide phase as a function of temperature and composition, employing mass effusion and mass-spectrometric techniques. The partial pressures of LaO(g) over the sesquioxide phase were measured in the presence of tungsten (1933 to 2408 K) and in the presence of rhenium (1778 to 2427 K). The former were greater by 40 to 80 per cent. The congruently vaporizing composition begins to depart from stoichiometry with increasing temperature and at 2400 K reaches values of La2O2.980 ± 0.002 in tungsten and La2O2.998 ± 0.002 in rhenium, demonstrating a slight reducing effect of rhenium and a measurably greater reducing effect of tungsten. The following thermochemical quantities were determined: ΔGfo(LaO,g,T)/calmol−1=−36940−11.73T/K,ΔHfo(LaO,g,0)=−(28.6±1.0)kcalmol−1,andD(LaO,g,0)=(189.9±1.0)kcalmol−1. The partial pressures of La(g) and LaO(g) over the liquid metal + sesquioxide were measured over the temperature range 1516 to 1904 K. The composition of the sesquioxide phase at the lower phase boundary was determined as a function of temperature from 1660 to 1925 K and appeared to depart from the stoichiometric value above 1400 K. At 1925 K the sesquioxide was reduced to La2O2.82. The standard Gibbs free energy of formation of the substichiometric oxide at the lower phase boundary was evaluated as a function of temperature; a calculated value of — 282.9 kcal mol−1, corresponding to the composition La2O2.83, is about 16 kcal mol−1 less negative than that of the stoichiometric sesquioxide. The standard Gibbs free energy change for the isomolecular reaction, La(g) + YO(g) = LaO(g) + Y(g), was redetermined and compared with the dissociation energies of the monoxides evaluated from the thermochemical data.