Pressure-induced phase transition of La2Zr2O7 and La0.5Gd1.5Zr2O7 pyrochlore

In situ high-pressure experiments on La2Zr2O7 and La0.5Gd1.5Zr2O7 have been carried out at up to approximately 40 GPa using synchrotron X-ray diffraction and Raman spectroscopy combined with a diamond anvil cell technique. Both La2Zr2O7 and La0.5Gd1.5Zr2O7 undergo a phase transition from a pyrochlore phase (Fdm) into a cotunnite-like phase (Pnma) at 22.7 and 23.3 GPa, respectively. This type of phase transition is mainly controlled through the order-disorder occupancy of cations, and Gd3+ substitution of La3+ reduces the stability of zirconate pyrochlore. However, abnormal changes to the unit-cell volumes and vibrational modes observed at 5.5 GPa in La2Zr2O7 and 6.5 GPa in La0.5Gd1.5Zr2O7 are attributed to an anion disorder in the pyrochlore.