Ultraviolet nonlinear optical crystals A3SrBi(P2O7)2 (A = K, Rb) with large second harmonic generation responses

Abstract

Ultraviolet nonlinear optical (UV NLO) crystals are the key and basic materials for solid-state lasers. Phosphates as UV NLO crystals are limited in practical applications due to their weak second-harmonic generation (SHG) response. In this work, two non-centrosymmetric pyro-phosphates of K₃SrBi(P₂O₇)₂ and Rb₃SrBi(P₂O₇)₂ have been synthesized by a high-temperature solution method. Systematic property measurements show that they both have short UV cut-off edges (∼240 nm) and large SHG responses (4 × KDP for K₃SrBi(P₂O₇)₂ and 2.1 × KDP for Rb₃SrBi(P₂O₇)₂). Thus, they are both promising UV NLO candidates. In particular, K₃SrBi(P₂O₇)₂ shows the strongest SHG response in the family of A₃BBi(P₂O₇)₂ (A = K, Rb, B = Sr; A = Rb, Cs, B = Ba, Pb), which is about twice higher than that of isomorphic Rb₃SrBi(P₂O₇)₂. The study of the structure–property relationship shows that the synergistic effect of P₂O₇ dimers and BiO₆ polyhedra is the main contribution to their excellent NLO properties. And better orientations of P₂O₇ dimers and larger distortion of BiO₆ units play important roles in enhancing the SHG response of K₃SrBi(P₂O₇)₂.