Standard molar Gibbs free energies of formation of LiCrO 2 and of NaCrO 2

Abstract

The standard molar Gibbs free energies of formation of lithium chromite, Δ f G m o(LiCrO 2, s, T), and of sodium chromite, Δ f G m o(NaCrO 2, s, T), have been determined by measuring Li(g) and Na(g) pressures over the equilibrium phase mixtures: (a), (LiCrO 2 + Cr + Cr 2O 3); (b), (LiCrO 2 + Cr + Li 2O); and (c), (NaCrO 2 + Cr + Cr 2O 3), 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 LiCrO 2 and NaCrO 2 did not show any mass loss in the temperature ranges of the present investigation. The Δ f G m o(LiCrO 2, s, T) and Δ f G m o(NaCrO 2, s, T) were calculated by taking standard molar Gibbs free energies of formation of Cr 2O 3(s), Li 2O(s), Li(g), and Na(g) from the literature. The standard molar Gibbs free energies for LiCrO 2 and NaCrO 2 can be represented by equations: Δ fG o m( LiCrO 2, s, T)/( kJ·mol −1 ) = −895±0.1953(T/K)±2.1 Δ fG o m( LiCrO 2, s, T)/( kJ·mol −1 ) = −918.5+0.2002(T/K)±1.7 Δ fG o m( NaCrO 2, s, T)/( kJ·mol −1 ) = −847.0±0.1772(T/K)±3.7 The Δ f H m o(LiCrO 2, 298.15 K) and Δ f H m o(NaCrO 2, 298.15 K) were obtained using the second-law method. The values for LiCrO 2(s) are −917.8 and −934.7 kJ·mol −1 and for NaCrO 2(s) −857.4 kJ·mol −1. S m o(298.15 K) for LiCrO 2(s) obtained for two-phase fields are 36.3 and 36.3 J·K −1·mol −1 while for NaCrO 2(s) the value is 95.9 J·K −1·mol −1.