Thermodynamic Functions of Complex Zirconia Based Lanthanide Oxides—Pyrochlores Ln2Zr2O7 (Ln = La, Pr, Sm, Eu, Gd) and Fluorites Ln2O3 · 2ZrO2 (Ln = Tb, Ho, Er, Tm)

Abstract

The review of the thermodynamic properties of complex lanthanide–zirconium oxides, namely, Ln₂Zr₂O₇ (Ln = La, Pr, Sm, Eu, Gd) with a pyrochlore structure and Ln₂O₃ · 2ZrO₂ solid solutions (Ln = Tb, Ho, Er, Tm) with a fluorite structure, is presented on the basis of our and literature data. The heat capacity was experimentally studied for Ln₂Zr₂O₇ (for the cerium subgroup lanthanides) with a pyrochlore structure (Fd3m) and Ln₂O₃ · 2ZrO₂ (for the yttrium subgroup lanthanides) with a fluorite structure (Fm3m), which were characterized by X-ray diffraction and elemental analyses, scanning electron microscopy, and adiabatic and differential scanning calorimetry. The temperature dependences of the thermodynamic functions (entropy, enthalpy change, and reduced Gibbs energy) were calculated within a broad range of temperatures from smoothed heat capacity values. The available thermodynamic data were compared, and some recommended values were given. The contribution of low-temperature magnetic transitions was taken into account for Ln₂Zr₂O₇ (Ln = Pr, Nd, Sm, Gd). The effect of the Schottky anomaly on the heat capacity of lanthanide compounds was analyzed.