Energetics of T, T′, and T* phases in some rare earth cuprates

The enthalpies of solution in molten 2PbO · B2O3 at 704°C of Nd2CuO4 (T′), (La1-x Ndx)2CuO4 (T or T′), (La0.60Dy0.35Ce0.05)2CuO 4 (T′ and T*), and (La0.60Dy0.35Th0.05)2CuO 4 (T′- and T*) have been measured. The enthalpy of formation (from oxides) at 700°C of (La1-x Ndx)2CuO4 (0 ≤ x ≤ 1) varies from -20 to -7 kJ mol-1, depending on composition. The enthalpy of the T′ to T transition in (La0.75 Nd0.25)2CuO4 is +6.3 ± 2.8 kJ mol-1. Extrapolated enthalpies of transition from T′ to T are +3 ± 8 kJ mol-1 for La2CuO4 and +28 ± 10 kJ mol-1 for Nd2CuO4. The enthalpy of mixing in the T′ phase of (La1-x Ndx)2 CuO4 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 (La0.6Dy0.35Th0.05)2CuO4 and (La0.6Dy0.35Ce0.05)2CuO4 are +38 ± 5 and +51 ± 6 kJ mol-1, respectively. There is a linear dependence of the enthalpy of transition (ΔH) between T′ and T (or T* and T′) phases on the volume of transition (ΔV), ΔH (kJ mol-1) = 37.4 + 9.27 ΔV (cm3 mol-1). © 1993 Academic Press, Inc.