Pressure-induced phase transition of La 2 Zr 2 O 7 and La 0.5 Gd 1.5 Zr 2 O 7 pyrochlore.

In situ high-pressure experiments on La 2 Zr 2 O 7 and La 0.5 Gd 1.5 Zr 2 O 7 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 La 2 Zr 2 O 7 and La 0.5 Gd 1.5 Zr 2 O 7 undergo a phase transition from a pyrochlore phase ( Fd 3̄ m ) 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 Gd 3+ substitution of La 3+ reduces the stability of zirconate pyrochlore. However, abnormal changes to the unit-cell volumes and vibrational modes observed at 5.5 GPa in La 2 Zr 2 O 7 and 6.5 GPa in La 0.5 Gd 1.5 Zr 2 O 7 are attributed to an anion disorder in the pyrochlore.