Pressure-dependence on the absorption spectrum of CuMoO4: study of the green -> brownish-red piezochromic phase transition at 2.5 kbar

The optical properties of CuMoO4 are investigated by means of hydrostatic p ressure techniques in the 0-60 kbar range. Attention is paid to the piezoch romism exhibited by this crystal at the alpha (green) --> gamma (brownish r ed) structural phase transition. The variation of the absorption spectra wi th pressure indicates that the piezochromic transition takes place at 2.5 k bar. The associated change of colour is due to the broadening of the first O2- --> Cu2+ charge transfer band and the disappearance of an intense band at 12000 cm(-1), related to the presence of pyramidal CuO5 complexes in alp ha-CuMoO4. The measured oscillator strength suggests that this band corresp onds to the e --> b(1) crystal field transition rather than to a charge tra nsfer within CuO5. The structural correlation with the absorption spectra p erformed in this work explains the strong dichroism exhibited by the crysta l in the high-pressure gamma-CuMoO4. (C) 1999 Elsevier Science B.V. All rig hts reserved.