KSrB ₂ O ₄ F: a beryllium-free zero-order wave plate material with remarkably small birefringence and a deep-ultraviolet cutoff edge

A new borate fluoride, KSrB ₂ O ₄ F, was synthesized. KSrB ₂ O ₄ F exhibits an ultralow birefringence of Δ n  = 0.0004 at 1064 nm and a deep-ultraviolet cutoff below 190 nm, highlighting its potential for zero-order waveplate applications.