Pressure-induced phases in the spin-Peierls compound CuGeO₃

Abstract

The high-pressure behavior of the spin-Peierls compound CuGeO₃ has been studied at room temperature using high-pressure Raman spectroscopy and optical microscopy in the diamond anvil cell. A number of pressure media, including helium, were used to study the behavior. Three pressure-induced phase transitions were found in the 7-10 GPa region: a pale-green phase (phase II) reported earlier, and two other phases, a blue phase (phase III), and a green phase (phase IV). Phase II is obtained only under strictly hydrostatic conditions (viz., helium, methanol, and methanol/ethanol mixture as pressure media) near 7 GPa. Phases III and IV are obtained near 7 GPa when phase I is pressurized in quasihydrostatic or solid pressure media, viz., argon, KBr, etc. Phase IV reverts to phase III, which can be quenched down to ambient pressure and retained. Using the known crystal chemistry of metasilicates and germanates, the pyroxene structure type arrangements are suggested for phases III and IV.