Growth and ultraviolet optical properties of KGd_1–xRE_x(WO₄)₂ single crystals

The suitable conditions for growth of KGd(WO₄)₂ (KGW) and KGd_1–xRE_x(WO₄)₂— RE = Nd, Er, Yb, Ho, Tm, Pr—by the top-seeded-solution-growth method, using K₂W₂O₇ as solvent, are discussed. The relation between crystal size, mean growth rate, distribution coefficient of the substituting element, and the presence of macrodefects is analyzed. The optical absorption corresponding to the band-gap transition of KGW has been found to be temperature dependent; the absorption threshold energy changes from 34405 cm⁻¹ at 300 K to 35330 cm⁻¹ at 7 K. Narrow pre-edge absorption bands at about 32000 and 32600 cm⁻¹ have been ascribed to Gd³⁺ intraconfigurational transitions. The photoluminescence of most RE³⁺ ions has been observed under ultraviolet (UV) excitation close to the absorption threshold of KGW. This suggests the contribution of charge transfer bands. In Pr-doped samples the presence of a minor concentration of Pr⁴⁺ could also contribute in this region. The irradiation with UV light does not introduce any significant coloration of our samples.