An unquenchable perovskite phase of MgGeO₃ and comparison with MgSiO₃ perovskite

An unquenchable polymorph of MgGeO₃, having a perovskite structure, has been observed using in situ X-ray diffraction in a diamond-anvil cell. The perovskite polymorph formed at room temperature and 12-14 GPa by pressure-induced phase transition from the lithium niobate phase of MgGeO₃. It is indexed on an orthorhombic lattice, with unit-cell parameters a = 4.832(11), b = 5.03(13), c = 7.022(16) Å, and V = 170.7(5) ų at 17.9 GPa. The transition is fully reversed during decompression. Like MgSiO₃ perovskite, MgGeO₃ perovskite only becomes thermodynamically stable at very high pressures (over 23 GPa). Unlike MgSiO₃, it also becomes unstable relative to the lithium niobate phase during pressure release. The latter property accounts for the lack of observation of the perovskite phase in previous studies of MgGeO₃ as an analogue material. -Authors