Phase transition engineering to break the symmetry of diamond-like chalcogenide for second-order nonlinear optics†

Temperature-induced phase transitions offer a promising route to engineer nonlinear optical materials, particularly for infrared applications where conventional design approaches face fundamental limitations. Herein, a temperature-induced centrosymmetric (CS) to noncentrosymmetric (NCS) irreversible phase transition strategy was employed to successfully prepare a novel NCS diamond-like (DL) chalcogenide, β-Ag₄P₂S₇, which was derived from the CS phase α-Ag₄P₂S₇ transformation. Structural analysis reveals that this transformation involves a reorganization of [Ag2PS₄]²⁻ layers from an AA′AA′ to AA′A′′AA′A′′ stacking pattern, facilitated by bridging [Ag1S₄]⁷⁻ tetrahedra. β-Ag₄P₂S₇ has exceptional IR NLO properties, including a strong phase-matchable second-harmonic generation (SHG) response (1.02 × AgGaS₂) and a wide band gap of 2.90 eV (the largest one in ternary Ag-based DL chalcogenides), which balances excellent NLO response with a wide band gap. Further structure–property relationship analyses show that superior NLO properties and band gap broadening of β-Ag₄P₂S₇ mainly originate from the alteration of the [Ag2PS₄]²⁻ layer stacking configuration, which is driven by a temperature-induced irreversible phase transition. This work not only presents a new paradigm for designing high-performance NLO materials through phase transitions but also significantly advances the potential of temperature-mediated crystal engineering for optical applications.