Fluorite-pyrochlore transformation in Eu ₂Zr ₂O ₇ - Direct calorimetric measurement of phase transition, formation and surface enthalpies

The energetics of the order-disorder phase transformation in the binary oxide system, Eu ₂O ₃-ZrO ₂, is studied by powder X-ray diffraction and high temperature drop solution calorimetry. The nanocrystalline defect fluorite phase of Eu ₂Zr ₂O ₇ 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 Eu ₂Zr ₂O ₇ is -75 ± 2.5 kJ mol ^-1 H ₂O for coverage of 9.5 ± 0.8 H ₂O/nm ². The calculated surface enthalpies for the anhydrous and hydrous surfaces of defect fluorite Eu ₂Zr ₂O ₇ are 1.47 ± 0.13 and 1.01 ± 0.15 J m ^-2, respectively.