Fluorite-pyrochlore transformation in Eu2Zr2O7—direct calorimetric measurement of phase transition, formation and surface enthalpies

Abstract

The energetics of the order-disorder phase transformation in the binary oxide system, Eu₂O₃–ZrO₂, is studied by powder X-ray diffraction and high temperature drop solution calorimetry. The nanocrystalline defect fluorite phase of Eu₂Zr₂O₇ is synthesized on crystallization of an amorphous precursor from aqueous precipitation. The defect fluorite transforms to an ordered pyrochlore above 1200 °C. Aerodynamic levitation combined with laser heating is used to prepare coarse defect fluorite, which is otherwise impossible by conventional synthesis techniques. Formation enthalpies from oxides are −62.4 ± 2.6 and −24.6 ± 3.7 kJ mol⁻¹ for the pyrochlore and defect fluorite phase, respectively. The transformation enthalpy from pyrochlore to defect flourite in the coarse sample is 37.8 ± 3.1 kJ mol⁻¹ at 25 °C. The enthalpy of water vapor adsorption on the surface of the nanocrystalline defect fluorite Eu₂Zr₂O₇ is −75 ± 2.5 kJ mol⁻¹ H₂O for coverage of 9.5 ± 0.8 H₂O/nm². The calculated surface enthalpies for the anhydrous and hydrous surfaces of defect fluorite Eu₂Zr₂O₇ are 1.47 ± 0.13 and 1.01 ± 0.15 J m⁻², respectively.