K₂ZrSi₂O₇ with a Wide Transmission Range and Enhanced Birefringence Induced by the d⁰-TM Zr-Based Units

Silicates are a highly attractive system for exploring ultraviolet (UV) optical crystals. However, silicates typically exhibit relatively small birefringence due to the weak polarizability anisotropy of tetrahedral units. 4d⁰ transition-metal Zr⁴⁺ not only helps to enhance structural anisotropy due to its second-order Jahn-Teller effect but also has relatively low electronegativity, and the binding of Zr to O with high electronegativity enhances the ionicity of Zr-O bonds, which helps reduce the UV cutoff edge. Herein, by introducing Zr⁴⁺ into the silicate system, K₂ZrSi₂O₇ with enhanced birefringence and a short cutoff edge was successfully synthesized by a high-temperature solid-state method. It exhibits an enhanced birefringence of 0.047 @ 550 nm, which is at least twice that of most known UV optical silicates. Combined with its wide transmission range, short UV cutoff edge, and high thermal stability, K₂ZrSi₂O₇ has become a promising candidate for a UV birefringent material. This work is anticipated to provide useful guidance for exploring short-UV birefringent crystals with enhanced optical properties.