K2Na(I3O8)3: a promising mid-infrared birefringent material with two different structures of octaoxotriiodate(V) groups

Abstract

An alkali metal polyiodate, K₂Na(I₃O₈)₃, has been synthesized via a hydrothermal method using phosphoric acid as the reaction medium. The compound features two distinct octaoxotriiodate(V) polyanions, which adopt divergent structural configurations due to the disparate arrangements of their lone electron pairs. The material exhibits noteworthy optical properties, including a considerable band gap (3.89 eV), a substantial transmission range (0.27–12.67 μm), and a remarkable birefringence (0.283 at 546 nm). Notably, this material exhibits the highest reported birefringence among inorganic polyiodates to date. These findings position K₂Na(I₃O₈)₃ as a promising candidate for mid-infrared birefringent applications.