Aliovalent-cation-substitution-induced structure transformation: a new path toward high-performance IR nonlinear optical materials

Abstract

Non-centrosymmetric (NCS) structure is the precondition and foundation for the development of high-performance nonlinear optical (NLO) crystals. In this research, a quaternary NCS thioantimonate, K₂Ag₃Sb₃S₇, is designed through an aliovalent-cation-substitution-induced strategy based on the centrosymmetric (CS) parent compound K₂Sb₄S₇ and successfully obtained through a facile surfactant-thermal technology. The unique crystal structure of K₂Ag₃Sb₃S₇ belongs to the orthorhombic space group Cmc2₁ (no. 36) and is made of charge-balanced K⁺ cations and two-dimensional (2D) anionic layers [Ag₃Sb₃S₇]²⁻ constructed by the vertex-sharing of _∞[Ag₃SbS₅]⁴⁻ and _∞[SbS₂]⁻ chains. Remarkably, K₂Ag₃Sb₃S₇ not only possesses a phase-matching (PM) ability in the IR region, but also displays a perfect balance between a high second-harmonic-generation (SHG) coefficient (1.5 × commercial AgGaS₂) and a large laser-induced damage threshold (3.2 × commercial AgGaS₂). Moreover, theoretical studies uncover that the synergistic effects of three types of asymmetric building motifs (i.e., [SbS₃], [Sb₂S₅] and [Ag₃S₆]) contribute to the large SHG effect and birefringence (Δn). This study not only demonstrates that aliovalent cation substitution can be a robust means of obtaining the NCS thioantimonates but also provides a deeper insight into the structure–property relationship of IR-NLO chalcogenides.