Thermodynamic Properties of LaCrO 4 , La 2 CrO 6 , and La 2 Cr 3 O 12 , and Subsolidus Phase Relations in the System Lanthanum–Chromium–Oxygen

In the system La – Cr – O , there are three ternary oxides ( LaCrO 4 , La 2 Cr 3 O 12 , and La 2 CrO 6 ) that contain Cr in higher valence states (V or VI). On heating, LaCrO 4 decomposes to LaCrO 3 , La 2 Cr 3 O 12 to a mixture of LaCrO 4 and Cr 2 O 3 , and La 2 CrO 6 to LaCrO 3 and La 2 O 3 with loss of oxygen. The oxygen potentials corresponding to these decomposition reactions are determined as a function of temperature using solid‐state cells incorporating yttria‐stabilized zirconia as the electrolyte. Measurements are made from 840 K to the decomposition temperature of the ternary oxides in pure oxygen. The standard Gibbs energies of formation of the three ternary oxides are derived from the reversible electromotive force ( EMF ) of the three cells. The standard enthalpy of formation and standard entropy of the three ternary oxides at 298.15 K are estimated. Subsolidus phase relations in the system La – Cr – O are computed from thermodynamic data and displayed as isothermal sections at several temperature intervals. The decomposition temperatures in air are 880 (±3) K for La 2 Cr 3 O 12 , 936 (±3) K for LaCrO 4 , and 1056 (±4) K for La 2 CrO 6 .