Monoclinic zinc monotungstate Yb³⁺,Li⁺:ZnWO4: Part I. Czochralski growth, structure refinement and Raman spectra

We report on the growth of a co-doped divalent-metal monotungstate crystal, Yb3+,Li+:ZnWO4, its structure refinement, thermo-mechanical properties and polarized Raman spectra. Yb3+,Li+:ZnWO4 crystals are grown by the Czochralski method using Pt/Rh and Pt crucibles. The nature of crystal coloration is discussed and assigned to color centers based on oxygen vacancies as well as Fe2+(Fe3+) species and, possibly, other uncontrolled impurities of transition metal ions. Oxidizing annealing at 800 °C helps to improve the transparency of Yb3+,Li+:ZnWO4 leading to a weak residual pink coloration. Yb3+,Li+:ZnWO4 was confirmed to be monoclinic (sp. gr. P2/c – C42h, No. 13) with lattice constants a = 4.702(2) Å, b = 5.718(6) Å, c = 4.930(4) Å and β = 90.713(5)°. The coefficients of linear thermal expansion are α[100] = 11.71, α[010] = 9.90 and α[001] = 7.01 [10−6 K−1]. The melting point of the crystal is at 1166 °C and no thermal effects are found below the melting point. The Yb3+,Li+:ZnWO4 crystal shows intense and strongly polarized Raman spectra with the most intense mode at 906.0 cm−1 assigned to the W – O stretching vibrations in the [WO6] octahedra. It is thus a promising gain medium for laser emission at ~1 μm and for self-Raman frequency conversion.