X-ray diffraction and Raman studies of the CuGeO3(III)-(IV) transformation under high pressures

Both X-ray diffraction and Raman spectroscopy measurement were carried out on the same powder sample of CuGeO3(III) in a diamond anvil cell to high pressures at room temperature. The phase transformation of (III)-(IV) phase was observed at about 7 GPa with both methods and the results were also in accord with previous powder diffraction and Raman measurements, respectively. However, the powder diffraction data were strikingly different from those reported in a recent single-crystal study on the phase (III). It is, therefore, evident that the phase transformations in CuGeO3(III) would be as complicated as those in CuGeO3(I) and that the monoclinic phase obtained from single-crystal phase (III) at approximately 7 GPa [P. Dera, A. Jayaraman, C.T. Prewitt, S.A. Gramsch, Phys. Rev. B 65 (2002) 13415-1-13415-10] is not the phase (IV) previously observed but rather a new phase (IVa) in CuGeO3.