Nonlinear Optical-Active NaAlP ₂ S ₆ 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 NaAlP 2 S 6 was synthesized via reactive flux NaCl-assisted metal oxide–boron–chalcogen (MOBQ) solid-state method. It crystallizes in the orthorhombic noncentrosymmetric Fdd 2 structure, and the structure features {[AlP 2 S 6 ] − } ∞ three-dimentional framework constructed by AlS 6 octahedra and ethane-like P 2 S 6 dimers. Notably, because of the strong ionicity and high bond energy of Al–S bonds in AlS 6 octahedra, NaAlP 2 S 6 exhibits a wide bandgap (3.72 eV), a moderate SHG response (0.25 × AgGaS 2 ), and an ultrahigh LIDT (14.7 × AgGaS 2 ). Theoretical calculations reveal that the SHG response is mainly contributed by the P 2 S 6 dimers. This work suggests the potential of Al-based functional motifs for NLO applications.