Optical absorption and Raman spectroscopy of CuWO4

The electronic absorption and Raman spectra of CuWO4 are studied as a function of pressure in the 0 - 20 GPa range. The below-gap absorption bands at 1.15, 1.38 and 1.56 eV correspond to Cu2+ d-transitions split by the Jahn-Teller distortion of CuO6 (Req = 1.98 A; Rax = 2.39 A; Qθ 0.47 A). Pressure induces a strong reduction of the JT distortion up to 10 GPa. Above this pressure we observe, by optical absorption and Raman spectroscopy, a first-order phase transition at 11 GPa with phase coexistence in the 10-12 GPa range, as it is confirmed by Raman spectroscopy. The absorption spectra suggest that two different Cu2+ sites are formed in the high pressure phase, each having rather different CuO6 distortion. The more JT distorted CuO6 centre is stable up to 20 GPa. Rather than JT reduction, pressure induces reorientations of the CuO6 octahedra in the high-pressure phase.