Pressure-induced phase transitions in CuGeO₃ from Raman spectroscopic studies

Abstract

Pressure-induced phase transitions in the normal phase (I) and metastably retained phase III of CuGeO₃ were investigated up to 30 and 36 GPa, respectively, by Raman spectroscopy, using single-crystal samples in helium and methanol-ethanol mixture (M/E) as pressure medium. Phase I undergoes two pressure-induced phase transitions; the previously reported I-II near 7 GPa and another near 18 GPa to a phase designated VI. With phase III as the starting material three reversible pressure-induced phase transitions occur: IV, IV', V at 8, 12.5 and 18 GPa. The above sequence of phase transitions is observed in helium and in M/E. A highly disordered phase occurs only when phase II is pressurized in M/E but not in helium. The Raman data are discussed in terms of plausible structure models for the high-pressure phases of CuGeO₃, indicated by the x-ray diffraction data, as well as the high-pressure crystal chemistry of other divalent metagermanates. The need for further detailed x-ray diffraction studies is stressed, to gain a deeper understanding of the two different pathways that the pressure-induced transitions follow, depending on the nature of the pressure medium.