Enthalpies of formation of pyrochlore Tb ₂ Hf ₂ O ₇ and fluorite RE ₂ O ₃ ·2HfO ₂ (RE = Dy‐Lu) rare‐earth hafnates

Abstract The enthalpies of formation of heavy rare‐earth hafnates Tb 2 Hf 2 O 7 (pyrochlore structure) and RE 2 O 3 . 2HfO 2 (RE = Dy‐Lu) (defect fluorite structure) have been determined by the method of oxide melt drop‐solution calorimetry. The data obtained for Tb 2 Hf 2 O 7 revealed that they are in trend with previously experimentally determined values for “light” RE hafnates. The formation enthalpies of fluorite structured compounds in general become more positive with the RE ionic radius contraction. The values of enthalpy of formation from oxides for the fluorite solid solutions obtained in the current study are sufficiently more negative than was predicted by the ab initio calculations. The calculation of the Gibbs energy dependencies as well as the contribution of both entropy and enthalpy factors allowed us to estimate the stability of the compounds in the high‐temperature range.