RbBiP₂S₆: A Promising IR Nonlinear Optical Material with a Giant Second-Harmonic Generation Response Designed by Aliovalent Substitution

Non-centrosymmetric (NCS) Bi-based oxides with giant second-harmonic generation (SHG) responses have been widely studied as UV–visible nonlinear optical (NLO) materials over the past few decades. However, high-performance Bi-based NLO chalcogenides in the infrared (IR) region have rarely been reported. Herein, a novel NCS Bi-based thiophosphate RbBiP₂S₆ (P2₁) has been successfully designed by the aliovalent substitution of centrosymmetric Rb₄P₂S₆ (P2₁/c). Notably, this compound affords a record-high SHG intensity (11.9 × AgGaS₂ @ 2.05 μm) ever reported for IR-NLO sulfides. Moreover, detailed structure analysis and theoretical studies indicate that the superior IR-NLO performance can be attributed to the highly polarizable [BiS₇] units together with the favorable alignment of ethane-like [P₂S₆] units. This discovery offers an excellent candidate for IR-NLO applications and suggests that aliovalent substitution is a promising way to search for unknown high-performance NLO materials.