Thermochemical Studies of LnBa₂CU₃O_7-δ (Ln = Pr, Nd, Eu, Gd, Dy, Ho, Tm), LnBa₂Cu₄O₈ (Ln = Sm, Eu, Gd, Dy, Ho), and Y_1-xPr_xBa₂Cu₃O_7-δ

Employing oxide-melt drop-solution calorimetry, we have performed the first comparative thermochemical investigations of the title compounds. The enthalpies of formation of the Ln123 derivatives display an essentially linear trend toward less exothermic values with increasing lanthanide atomic number, implying that these phases become more stable with increasing lanthanide radius. In contrast, the enthalpies of oxidation vary to only a minor extent with the choice of lanthanide and suggest that the chain-site charge distributions of the Ln123 are comparable. The thermodynamic characteristics of Pr123 generally follow the trends established by the other Ln123 and reveal no gross energetic anomaly associated with the suppression of superconductivity. The enthalpies of formation of the Ln124 derivatives manifest a maximum at Gd124, while the unit-cell volumes and enthalpies of drop-solution and formation of these compounds are nearly constant multiples (2.3, 1.6-1.7, and 1.3-1.4, respectively) of the corresponding parameters of the Ln123 derivatives. The enthalpies accompanying the reactions Ln123(s) + CuO(s) → Ln124(s) imply that the Ln124, like Y124, are stable with respect to an assemblage of the fully oxygenated 123 phase and CuO. The data for the (Y,Pr)123 solid solutions are continuous across the join, arguing against any sudden x-dependent valence instability, but display asymmetric deviations from ideal solution behavior that can be modeled satisfactorily within the subregular approximation. Perhaps most intriguingly, the superconducting critical temperatures and the enthalpies of oxidation of the (Y,Pr)123 solid solutions are linearly related.