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 pressure techniques in the 0-60 kbar range. Attention is paid to the piezochromism exhibited by this crystal at the α (green) → γ (brownish red) structural phase transition. The variation of the absorption spectra with pressure indicates that the piezochromic transition takes place at 2.5 kbar. The associated change of colour is due to the broadening of the first O^2- → Cu²⁺ charge transfer band and the disappearance of an intense band at 12000 cm^-1, related to the presence of pyramidal CuO₅ complexes in α-CuMoO₄. The measured oscillator strength suggests that this band corresponds to the e→b₁ crystal field transition rather than to a charge transfer within CuO₅. The structural correlation with the absorption spectra performed in this work explains the strong dichroism exhibited by the crystal in the high-pressure γ-CuMoO₄.