Pressure-induced phase transitions in CuGeO3 from Raman spectroscopic studies

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. Copyright © 2001 John Wiley & Sons, Ltd.