Centric-to-acentric structure transformation induced by a stereochemically active lone pair: a new insight for design of IR nonlinear optical materials

Abstract

For second-order nonlinear optical (NLO) responses, such as second-harmonic-generation (SHG), a strict non-centrosymmetric (NCS) structure is the paramount precondition. In this work, a quaternary IR–NLO semiconductor, La₂CuSbS₅, has been designed by a stereochemically active lone pair induction (SALPI) strategy, with inspiration from the known centrosymmetric (CS) La₂CuInS₅ and was successfully obtained by a solid-state reaction using CsBr–BaCl₂ as the flux. It adopts the orthorhombic Ima2 space group (no. 46), and the remarkable structure features a 3D NCS network built by a unique teeter-totter (SbS₄)_n chain motif and isolated [CuS₄] tetrahedra. The NLO experiments indicated that La₂CuSbS₅ possesses a phase-matching (PM) behavior with relatively strong SHG (0.5 × commercial AgGaS₂) and a large laser-induced damage threshold (LIDT = 6.7 × commercial AgGaS₂). In addition, theoretical studies and analyses of the cutoff-energy-dependent d_ij coefficient were conducted and discussed to further understand the structure–property relationship. Moreover, the [SbS₄] tetrahedral building units play a pivotal role in the polarity of the crystal, based on the results of an analysis of the local dipole moment. All these results offer a new route to designing and discovering new IR–NLO materials.