Synergistic Assembly of High-Symmetry Motifs Enhances Nonlinear Optical Response in the Chromium-Based Thiophosphates ACrP2S7 (A = K, Rb, Cs)

Transition-metal chalcogenides represent promising candidates for nonlinear optical (NLO) materials, yet Cr-based ones remain largely unexplored owing to the challenge of simultaneously achieving suitable electronic structures and noncentrosymmetric structures. Herein, we report the reactive flux assisted solid-state synthesis of a series of thiophosphates ACrP2S7 (A = K, Rb, Cs) and systematically investigate their structural chemistry and optical properties. All compounds crystallize in the polar monoclinic space group C2 and feature a layered structure composed of distorted [CrS6] octahedra and [P2S7] dimers, which cooperatively construct low-symmetric [CrP2S11] motifs. Structural analysis reveals that the polar structure can be derived from the centrosymmetric CrPS4 through symmetry breaking via insertion of A+ ions. The band gaps of ACrP2S7 are tunable in the range of 1.22–1.27 eV, and the second-harmonic generation measurements confirm that ACrP2S7 represents the first Cr-based chalcogenide exhibiting significant NLO activity (0.3–0.8 × AgGaS2). This work suggests that Cr-based thiophosphates can also be explored as promising infrared NLO materials.