Charge Compensation via Heterovalent Cation Disorder for Birefringence Modulation to Design UV Nonlinear Optical Crystal

ABSTRACT Nonlinear optical (NLO) crystals operating in the short‑wavelength ultraviolet (UV) region are essential for all‑solid‑state lasers. For such materials, a moderate birefringence represents a key requirement. Borates with planar π‑conjugated [B 3 O 6 ] rings are promising candidates for short‑wavelength UV NLO crystals, yet their tendency to adopt parallel alignment often leads to excessive birefringence and detrimental beam walk‑off. Herein, we propose a charge‑compensation strategy mediated by heterovalent cation occupation to rationally tailor the arrangement of [B 3 O 6 ] units. Finally, a new noncentrosymmetric crystal that adopts a unique zigzag packing of [B 3 O 6 ] groups distinct from previously reported structural types, RbBaB 3 O 6 , was designed and synthesized. This configuration yields a balanced set of optical properties: a birefringence of 0.075@532 nm, a moderate phase‑matching SHG response, a short UV cutoff edge below 196 nm, and the shortest type‑I phase‑matching wavelength of 236 nm, confirming its potential as a viable UV NLO crystal. This work demonstrates a practical route for fine‑tuning birefringence in borate‑based UV NLO materials, offering a viable candidate for all‑solid‑state laser systems operating in the UV region.