Deep ultraviolet transparent borophosphates LiAB2P2O9 (A = NH4, K) and LiNaB2P2O8(OH)2·H2O exhibiting a moderate second-harmonic generation response

Abstract

There is still a high demand for new deep ultraviolet (deep-UV) nonlinear optical (NLO) materials, but synthesizing an ideal deep-UV NLO crystal remains a significant challenge. In this work, three borophosphate crystals, LiAB₂P₂O₉ (A = NH₄, K) and LiNaB₂P₂O₈(OH)₂·H₂O, have been obtained via a solid-state reaction in a closed system. LiAB₂P₂O₉ (A = NH₄, K) crystallize in the acentric P2₁2₁2₁ space group with a three-dimensional (3D) anion framework, while LiNaB₂P₂O₈(OH)₂·H₂O belongs to the C2 space group with a two-dimensional (2D) anion framework. The mechanism of the nonpolar to polar space group transition of LiAB₂P₂O₉ (A = NH₄, K) and LiNaB₂P₂O₈(OH)₂·H₂O is investigated. Furthermore, the structure of borophosphates with a B/P ratio of 1 : 1 is discussed, which possess a variety of functional building blocks (FBBs) and connection modes. LiAB₂P₂O₉ (A = NH₄, K) exhibit short cutoff edges (< 200 nm), and second harmonic generation (SHG) responses of 0.4, and 0.9 × KH₂PO₄ (KDP), respectively. Additionally, structure–function correlation was investigated through theoretical calculations. This work offers valuable insight for the subsequent exploration of borophosphate deep-UV NLO materials.