Structure and Characterization of KNaBPO, a New Nonlinear Optical Borophosphate with One-Dimensional Chain Structure and Short Ultraviolet Cutoff Edge.

Nonlinear optical (NLO) crystals, being the primary medium for laser wavelength conversion, are crucial in all-solid-state lasers. Borophosphates offer more structural varieties than pure borates and phosphates, and they have become popular as NLO crystal candidates. Through spontaneous crystallization, we acquired a noncentrosymmetric alkali metal borophosphate crystal material, KNaBPO (KNBPO). KNBPO crystallizes in the orthorhombic 2 space group with the following unit cell parameters: = 13.9238(18) Å, = 6.7673(8) Å, = 12.1298(15) Å, and = 4, and its structure is characterized by a fundamental building unit 1 [BPO] chain structure made up of bridging oxygen linkages between BO and PO tetrahedra. KNBPO has a short ultraviolet (UV) cut-off edge (<186 nm), a congruent melting characteristic, good thermal stability, and a moderate second harmonic generation response roughly 0.42 times that of KHPO. Theoretical calculations reveal that the optical properties of the compound mainly originate from BO and PO units. Due to the short UV cut-off edge, KNBPO can be used as a potential NLO material in the UV and even deep UV regions, and it enhances the structural variety of borophosphates, which has a reference value for scholars investigating similar materials.