PRESSURE-TEMPERATURE PHASE-DIAGRAM OF THE SPIN-PEIERLS COMPOUND CUGEO3

Using Raman scattering we have mapped out the phase diagram of CuGeO3 at temperatures from 3 to 300 K and pressures up to 11 GPa. At 300 K a nd above 6 GPa we identify three high-pressure phases. Details of the phase transition behavior including reversibility depend on the shear strength of the pressure medium. The present results do not support th e recent claim that the structural behavior is affected by intercalati on. Low temperature (LT) Raman spectra show that the spin-Peierls tran sition temperature (14.3 K at P = 0) increases to 25 K at 3 GPa. Above this pressure a new LT phase is observed. The phase stable above 6.6 GPa (hydrostatic conditions) exhibits, upon cooling, an additional Ram an feature near 46 cm(-1), which we tentatively assign to a singlet-tr ipler spin excitation.