Polymerization-Enhanced Optical Anisotropy in Selenites: A Trimeric [H2Se3O7] Motif Delivering Large UV Birefringence

Abstract

Birefringent materials are crucial for polarization and phase control in photonics, but achieving both large birefringence and ultraviolet transparency remains challenging. Herein, we report two ionic selenites, CsCl(H2SeO3)2 and (CsCl)2(H2SeO3)(H2Se3O7). Notably, (CsCl)2(H2SeO3)(H2Se3O7) contains an unprecedented trimeric selenite unit, [H2Se3O7], formed from CsCl(H2SeO3)2 through functional-group polymerization under concentrated-acid hydrothermal conditions. This trimerization endows the crystal with a well-balanced optical profile, combining a wide band gap of 4.2 eV (UV cutoff at 282 nm) with a markedly enhanced birefringence of 0.188 at 546 nm, nearly double that of CsCl(H2SeO3)2 (0.095). DFT calculations further reveal that this performance enhancement arises from the higher density and more effective alignment of the polymerized selenite groups, with the [H2Se3O7] trimer serving as the dominant contributor to the optical anisotropy. These findings establish functional-group polymerization as a viable strategy for enhancing birefringence in UV-transparent selenites.