Nonlinear Optical-Active NaAlP₂S₆ Synthesized by the MOBQ Method: Synthesis, Structure, and Optical Properties

Group 13 metal chalcogenides have been extensively studied as infrared nonlinear optical (NLO) crystals and are considered promising candidates for well-balanced overall performances; however, the subgroup of group 13 metal chalcophosphates remains largely unexplored, particularly Al-based ones. Here, a quaternary thiophosphate NaAlP₂S₆ was synthesized via reactive flux NaCl-assisted metal oxide-boron-chalcogen (MOBQ) solid-state method. It crystallizes in the orthorhombic noncentrosymmetric Fdd2 structure, and the structure features {[AlP₂S₆]^-}_∞ three-dimentional framework constructed by AlS₆ octahedra and ethane-like P₂S₆ dimers. Notably, because of the strong ionicity and high bond energy of Al-S bonds in AlS₆ octahedra, NaAlP₂S₆ exhibits a wide bandgap (3.72 eV), a moderate SHG response (0.25 × AgGaS₂), and an ultrahigh LIDT (14.7 × AgGaS₂). Theoretical calculations reveal that the SHG response is mainly contributed by the P₂S₆ dimers. This work suggests the potential of Al-based functional motifs for NLO applications.