Raman-scattering study of doping effects on the structural phase transitions under hydrostatic pressures in CuGeO3

Raman-scattering and optical-microscopy studies have been carried out in pu re and doped CuGeO3 crystals under strictly hydrostatic pressure up to abou t 10 GPa at room temperature by using diamond anvil cell. Anomalies of the mode Gruneisen parameters of the low-frequency phonons are found. In heavil y Mg- and Ni-doped crystals successive first-order structural phase transit ions of the normal phase (phase I)--> the pale-green phase (phase II)--> th e green phase (phase IV) are observed as the hydrostatic pressure is increa sed in the first run. The critical pressure of the second phase transition between phases II and IV strongly depends on the dopant concentration, whil e it does not occur in Si-doped crystal. Upon releasing pressure the sample s are transformed from phase IV to the deep blue phase (phase III) which is quenchable down to ambient pressure. Phases III and IV are identified as t he phases which probably possess pyroxene structures and have already been reported to emerge in pure crystal only under quasihydrostatic pressure.