ANb2(PS4)(S2)3 (A = K, Rb, Cs): the first Nb-based NLO thiophosphates with [NbS2(S2)3] and [Nb2PS6(S2)4] units exhibiting tri-state structural transformation and high performance

Abstract

Chalcogenides stand as the most promising candidates for infrared nonlinear optical (NLO) materials, and the pursuit of high-performance ones remains both compelling and challenging. Hitherto, there are no NLO Nb-based thiophosphates experimentally studied. Here, a series of Nb-based thiophosphates ANb₂(PS₄)(S₂)₃ (A = K (1), Rb (2), Cs (3)) featuring unique functional motifs [NbS₂(S₂)₃]⁶⁻ and [Nb₂PS₆(S₂)₄]⁷⁻ were synthesized by using the reactive flux ACl assisted M_xO_y–B–S (M: metal) solid state method. The tri-state structural transformation among them under the influence of the A⁺ cation's size effect is well elucidated. Notably, as the first NLO-active Nb-based thiophosphates, they exhibit large NLO responses (0.9, 1, and 1.1 × AgGaS₂) and 3 exhibits the largest birefringence (0.31@2100 nm) among quaternary chalcophosphates. Theoretical calculation results suggest that the polar functional motif [Nb₂PS₆(S₂)₄]⁷⁻ assembled from two [NbS₂(S₂)₃]⁶⁻ and one PS₄ units contributes to the excellent NLO properties. In this work, we not only provide three high-performance infrared NLO materials, but also demonstrate the potential of Nb-based functional motifs for NLO applications.