Growth, characterization, and high-pressure optical studies of CuWO4

Copper tungstate (CuWO4) crystals grown by the top-seeded solution growth method were characterized by X-ray diffraction, Raman scattering, and optical measurements. CuWO4 has a triclinic structure (P 1) with a = 4.709, b = 5.845, c = 4.884, = 88.3, = 92.5, and = 97.2. It consists of corner-linked CuO6 and WO6 octahedra, the former having a pseudo-tetragonally elongated geometry caused by the Cu2+ Jahn-Teller effect. Fifteen out of the eighteen Raman modes of CuWO4 are reported, discussed, and compared with those of other tungstates. We also determined the indirect band-gap energy of CuWO4 (2.3eV) and its negative pressure coefficient up to 25GPa. The pressure evolution of the band-gap is discussed in terms of the electronic structure of CuWO4. Finally, no clear evidence of structural changes were found in our high-pressure experiments, but large amounts of defects are apparent beyond 18GPa.