Diamond-like AgxCu3-xPS4 (x = 1, 2) Synthesized by Metal Oxide-Boron-Chalcogen (MOBQ) Routine: Local High-Entropy Engineering for Balanced Infrared Nonlinear Optical Performance

Abstract

Infrared nonlinear optical (NLO) crystals play important roles in modern IR laser techniques and devices. For diverse candidates, thiophosphates receive increasing interest in view of their rich NLO-functional motifs. Here, by using Cu₃PS₄ (0) as the template, AgCu₂PS₄ (1) and Ag₂CuPS₄ (2) were successfully synthesized via the facile flux-assisted metal oxide-boron-chalcogen (MOBQ) solid-state method based on the cation partial substitution strategy and local high-entropy engineering. Their three-dimensional anionic frameworks are constructed by T2-supertetrahedra, and Ag⁺/Cu⁺ cations occupy the tetrahedral cavities. By adjusting the Ag/Cu ratio, and introducing controllable structural disorder and elevated configurational entropy via Ag/Cu mixed-occupation, the polar tetrahedral motifs can be effectively modulated to facilitate the positive superposition of microscopic second-order polarizability, which consequently enhances the structural anisotropy and hyperpolarizability, leading to increased birefringence and strong NLO response transformation from non-phase-matching for 0 to phase-matching for 1 and 2. Here, their structural chemistry and NLO properties are systematically investigated experimentally and theoretically, and the improvement strategy of NLO properties can be extended to many promising systems for aiding discovery of high-performance NLO materials.