STANDARD MOLAR GIBBS FREE-ENERGIES OF FORMATION OF LICRO2 AND OF NACRO2

The standard molar Gibbs free energies of formation of lithium chromite, ΔfGmo(LiCrO2, s, T), and of sodium chromite, ΔfGmo(NaCrO2, s, T), have been determined by measuring Li(g) and Na(g) pressures over the equilibrium phase mixtures: (a), (LiCrO2 + Cr + Cr2O3); (b), (LiCrO2 + Cr + Li2O); and (c), (NaCrO2 + Cr + Cr2O3), in the respective temperature ranges 1251 to 1352 K, 1065 to 1206 K, and 930 to 1044 K, using the Knudsen-effusion mass-loss method. Pure LiCrO2 and NaCrO2 did not show any mass loss in the temperature ranges of the present investigation. The ΔfGmo(LiCrO2, s, T) and ΔfGmo(NaCrO2, s, T) were calculated by taking standard molar Gibbs free energies of formation of Cr2O3(s), Li2O(s), Li(g), and Na(g) from the literature. The standard molar Gibbs free energies for LiCrO2 and NaCrO2 can be represented by equations: ΔfGom(LiCrO2, s, T)/(kJ·mol-1) = -895±0.1953(T/K)±2.1ΔfGom(LiCrO2, s, T)/(kJ·mol-1) = -918.5+0.2002(T/K)±1.7ΔfGom(NaCrO2, s, T)/(kJ·mol-1) = -847.0±0.1772(T/K)±3.7. The ΔfHmo(LiCrO2, 298.15 K) and ΔfHmo(NaCrO2, 298.15 K) were obtained using the second-law method. The values for LiCrO2(s) are -917.8 and -934.7 kJ·mol-1 and for NaCrO2(s) -857.4 kJ·mol-1. Smo(298.15 K) for LiCrO2(s) obtained for two-phase fields are 36.3 and 36.3 J·K-1·mol-1 while for NaCrO2(s) the value is 95.9 J·K-1·mol-1. © 1990.