Centrosymmetric Rb[Te2O4(OH)5] and noncentrosymmetric K2[Te3O8(OH)4]: metal tellurates with corner and edge-sharing (Te4O18)12− anion groups

Abstract

New nonlinear optical (NLO) crystals have become an urgent need for extending laser wavelengths. By cation substitution between alkali metal (K⁺, Rb⁺), anion group arrangement transition was realized, and centrosymmetric Rb[Te₂O₄(OH)₅] (RTOH) (monoclinic, P2₁/c) and noncentrosymmetric K₂[Te₃O₈(OH)₄] (KTOH) (orthorhombic, Fdd2) featuring corner/edge-sharing (Te₄O₁₈)¹²⁻ anion groups were successfully synthesized. These two compounds exhibit wide band gaps (>4.0 eV), high thermal stabilities (>290 °C) and suitable birefringences (Δn_exp > 0.04). In addition, KTOH exhibits an SHG response about 0.6 × KDP under 1064 nm fundamental light. First principles calculations were aslo performed, which demonstrated that their optical properties are mainly contributed by the TeO₆⁶⁻ octahedral units.