Raman-Scattering Study of Doping Effects on the Structural Phase Transitions under Hydrostatic Pressures in CuGeO₃

Abstract

Raman-scattering and optical-microscopy studies have been carried out in pure and doped CuGeO₃ crystals under strictly hydrostatic pressure up to about 10 GPa at room temperature by using diamond anvil cell. Anomalies of the mode Gruneisen parameters of the low-frequency phonons are found. In heavily Mg- and Ni-doped crystals successive first-order structural phase transitions of the normal phase (phase I)→the pale-green phase (phase II)→the green phase (phase IV) are observed as the hydrostatic pressure is increased in the first run. The critical pressure of the second phase transition between phases II and IV strongly depends on the dopant concentration, while it does not occur in Si-doped crystal. Upon releasing pressure the samples 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 the phases which probably possess pyroxene structures and have already been reported to emerge in pure crystal only under quasihydrostatic pressure.