K ₂ ZrSi ₂ O ₇ with a Wide Transmission Range and Enhanced Birefringence Induced by the d ⁰ -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.