A2Bi2(SO4)2Cl4 (A = NH4, K, Rb): achieving a subtle balance of the large second harmonic generation effect and sufficient birefringence in sulfate nonlinear optical materials

Abstract

[SO₄]²⁻, a tetrahedral anionic group similar to [PO₄]³⁻, possesses small microscopic second order polarizability and polarizability anisotropy, which tends to result in a weak second harmonic generation effect and small birefringence. Here, three non-centrosymmetric A₂Bi₂(SO₄)₂Cl₄ (A = NH₄, K, Rb) compounds were obtained by an evaporation solution method, which exhibit strong SHG effects of about 4.8, 5.5, and 5.3 × KDP, as well as a large birefringence of 0.055, 0.056 and 0.047 at 1064 nm, respectively. The three compounds exhibit the largest SHG effects among the reported sulfate NLO materials. The first-principles calculations elucidated that the large SHG effects and birefringence were mainly attributed to the synergistic effect of the BiO₅Cl₂ and [SO₄]²⁻ groups. This work may give feasible guidance for the subsequent exploration of sulfate NLO crystals with a large SHG effect and birefringence.