Pressure-Temperature Phase Diagram of the Spin-Peierls Compound ${\mathrm{CuGeO}}_3$

Using Raman scattering we have mapped out the phase diagram of ${\mathrm{CuGeO}}_3$ at temperatures from 3 to 300 K and pressures up to 11 GPa. At 300 K and 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 the recent claim that the structural behavior is affected by intercalation. Low temperature (LT) Raman spectra show that the spin-Peierls transition temperature (14.3 K at $P=0\mathrm{}$) 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 Raman feature near $46{\mathrm{cm}}^{-1\mathrm{}}$, which we tentatively assign to a singlet-triplet spin excitation.