RbPbPS4: a promising IR nonlinear optical material achieved by lone-pair-cation-substitution-induced structure transformation

Abstract

When stereochemically-active-lone-pair (SCALP) cations are introduced into chalcogenide systems, it is beneficial to produce a remarkable second-harmonic generation (SHG) response (d_eff), but it also causes a narrow band gap (E_g), which ultimately results in negative two-photon and free-carrier absorption. Hence, how to obtain a wide E_g (>2.33 eV) while maintaining a strong d_eff remains a significant challenge in this domain. In this work, Rb is partially replaced by SCALP Pb in the known ternary centrosymmetric (CS) Rb₃PS₄ (space group: Pnma). This results in a quaternary non-centrosymmetric (NCS) RbPbPS₄ (space group: P2₁2₁2₁), which possesses distinct 2D [PbPS₄]⁻ layers and Rb⁺ occupies the interlayer spaces as the counter cations. Notably, RbPbPS₄ exhibits a promising overall performance, including strong d_eff (2.5 × AgGaS₂), wide IR transmittance cutoff edge (up to 18.1 μm) along with the large E_g (2.75 eV), resulting in an improved laser-induced damage threshold (7.5 × AgGaS₂). Theoretical calculations further indicated that the favorable balance between strong d_eff and large E_g in RbPbPS₄ can be attributed to the synergies of the [PbS₇] and [PS₄] nonlinear optical (NLO)-active units. This study not only presents a high-performance Pb-based IR-NLO candidate but also underscores the effectiveness of partial substitution of SCALP cations in inducing a CS-to-NCS structural transformation.