AN UNQUENCHABLE PEROVSKITE PHASE OF MGGEO3 AND COMPARISON WITH MGSIO3PEROVSKITE

An unquenchable polymorph of MgGeO3, having a perovskite structure, ha s been observed using in situ X-ray diffraction in a diamond-anvil cel l. The perovskite polymorph formed at room temperature and 12-14 GPa b y pressure-induced phase transition from the lithium niobate phase of MgGeO3. It is indexed on an orthorhombic lattice, with unit-cell param eters a = 4.832(11), b = 5.031(13), c = 7.022(16) angstrom, and V = 17 0.7(5) angstrom3 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.