Enhanced Birefringence and Excellent Thermal Stability in Two Tellurium(IV) Vanadates: KTeOVO4 and Ca2TeV2O9

Two tellurium(IV) vanadates, KTeOVO4 and Ca2TeV2O9, were synthesized by integrating Te4+ with lone-pair of electrons and V5+ exhibiting the second-order Jahn–Teller effect (SOJT) active centers. Both compounds feature unique one-dimensional chain structures formed by [TeO4] and [VOx] polyhedra, resulting in notable birefringence values of 0.162 and 0.128 at 546 nm, respectively, higher than those of commonly used materials like α-BaB2O4 (0.116@1064 nm) and LiNbO3 (0.074@1440 nm). Additionally, they demonstrate excellent thermal stability (up to 500 and 700 °C) and broad transparency ranges (0.39–10.00 μm and 0.53–10.68 μm), enabling potential applications in near- and mid-infrared optical devices. Structural and theoretical analyses reveal that the enhanced birefringence arises from the synergistic effects of Te4+ lone-pair electrons and the anisotropic polarizability of V5+ centers.