Energetics of T, T′, and T* Phases in Some Rare Earth Cuprates

Abstract

The enthalpies of solution in molten 2PbO · B 2O 3 at 704°C of Nd 2CuO 4 (T′), (La 1- x Nd x ) 2CuO 4 (T or T′), (La 0.60Dy 0.35Ce 0.05) 2CuO 4 (T′ and T*), and (La 0.60Dy 0.35Th 0.05) 2CuO 4 (T′- and T*) have been measured. The enthalpy of formation (from oxides) at 700°C of (La 1- x Nd x ) 2CuO 4 (0 ≤ x ≤ 1) varies from -20 to -7 kJ mol -1, depending on composition. The enthalpy of the T′ to T transition in (La 0.75 Nd 0.25) 2CuO 4 is +6.3 ± 2.8 kJ mol -1. Extrapolated enthalpies of transition from T′ to T are +3 ± 8 kJ mol -1 for La 2CuO 4 and +28 ± 10 kJ mol -1 for Nd 2CuO 4. The enthalpy of mixing in the T′ phase of (La 1- x Nd x) 2 CuO 4 is positive, with a regular enthalpy interaction parameter of + 51.8 kJ mol -1. The positive enthalpy of mixing argues against cation ordering being important in this system. The enthalpies of transition of T* to T′ of (La 0.6Dy 0.35Th 0.05) 2CuO 4 and (La 0.6Dy 0.35Ce 0.05) 2CuO 4 are +38 ± 5 and +51 ± 6 kJ mol -1, respectively. There is a linear dependence of the enthalpy of transition (Δ H) between T′ and T (or T* and T′) phases on the volume of transition (Δ V), Δ H (kJ mol -1) = 37.4 + 9.27 Δ V (cm 3 mol -1).