Pressure-induced phase transitions in CuGeO3 from Raman spectroscopic studies

Pressure-induced phase transitions in the normal phase II) and metastably r etained phase III of CuGeO3 were investigated up to 30 and 36 GPa, respecti vely, by Raman spectroscopy, using single-crystal samples in helium and met hanol-ethanol mixture (M/E) as pressure medium. Phase I undergoes two press ure-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 startin g material three reversible pressure-induced phase transitions occur: IV, I V ', V at 8, 12.5 and 18 GPa. The above sequence of phase transitions is ob served in helium and in MIE. A highly disordered phase occurs only when pha se II is pressurized in MIE but not in helium. The Raman data are discussed in terms of plausible structure models for the high-pressure phases of CuG eO3, indicated by the x-ray diffraction data, as well as the high-pressure crystal chemistry of other divalent metagermanates. The need for further de tailed x-ray diffraction studies is stressed, to gain a deeper understandin g of the two different pathways that the pressure-induced transitions follo w, depending on the nature of the pressure medium. Copyright (C) 2001 John Wiley & Sons, Ltd.