High temperature solution calorimetry using a 2PbO · B2O3 solvent at 977 K was applied to the high Tc -related compounds, Ln2O3 (Ln = Nd-Lu), T′-Ln2CuO4 (Ln = Nd-Gd), and Ln2Cu2O5 (Ln = Dy-Lu). The heat of solution, ΔHs, 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 ΔHs 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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry