Thermochemical study of Ln₂O₃, T′-Ln₂CuO₄, and Ln₂Cu₂O₅ (Ln = Rare Earth)

High temperature solution calorimetry using a 2PbO · B₂O₃ solvent at 977 K was applied to the high T_c -related compounds, Ln₂O₃ (Ln = Nd-Lu), T′-Ln₂CuO₄ (Ln = Nd-Gd), and Ln₂Cu₂O₅ (Ln = Dy-Lu). The heat of solution, ΔH_s, of Ln₂ O₃ becomes less exothermic with decreasing size of Ln³⁺ from ∼ -130 kJ/mole in La₂O₃ to ∼ -30 kJ/mole in Lu₂O₃. The heats of solution of rare earth oxides containing unevenly filled “4f” orbitals are slightly more endothermic than those predicted by a straight line relating ΔH_s to reciprocal ionic radius, 1/r, for La, Gd, Y, and Lu, presumably reflecting the crystal field stabilization energy of Ln³⁺ (CFSE) in the solid. The heat of solution of the T′ phase when plotted against 1/r shows a maximum value at Ln = Sm. This may be explained by the CFSE and/or by a nonlinear change in lattice energy of the T′ phase as a function of 1/r. The stability relations between T′ and Ln₂Cu₂O₅ structures are discussed using the thermochemical data obtained. The data generally confirm patterns of stability seen in synthesis experiments.