KLi2CO3F: a beryllium-free KBBF-type deep-UV carbonate with an enhanced interlayer interaction and large birefringence

Abstract

KBe₂BO₃F₂ (KBBF) with a perfect honeycomb hexagonal layered structure is the sole practical deep-ultraviolet (DUV) nonlinear optical material which could generate DUV coherent light through direct frequency doubling. However, the layered growth habit and the toxicity of Be severely hinder its extensive applications. Herein, a non-centrosymmetric fluoride carbonate, KLi₂CO₃F, was successfully developed via chemical co-substitution with KBBF as the prototype. KLi₂CO₃F preserves the structural merits of KBBF, containing neatly arranged infinite planar [Li₂CO₃F₂]_∞ layers. In particular, compared with the weak interlayer K–F electrostatic attraction in KBBF, KLi₂CO₃F presents a stronger interlayer binding energy induced by the shorter and stronger Li–F bonds, which effectively overcomes the layered tendency of crystal growth. Simultaneously, KLi₂CO₃F exhibits a large birefringence of approximately 0.102@546 nm and a short ultraviolet absorption edge down to 190 nm, indicating that it is a promising DUV birefringent material.