Phase Transition and Second Harmonic Generation in Thiophosphates Ag2Cd(P2S6) and AgCd3(PS4)S2 Containing Two Second-Order Jahn-Teller Distorted Cations

Two new phases in the Ag-Cd-P-S system containing two second-order Jahn-Teller (SOJT) distorted d(10) cations (Cd2+ and Ag+), namely, Ag2Cd(P2S6) (1) and AgCd3(PS4)S-2 (2), are obtained via medium temperature solid-state synthesis. Compound I exhibits a two-dimensional layered structure and undergoes a first-order structural phase transition at approximately 280 degrees C. This outcome can be ascribed to the significant mismatch in the expansion coefficients between Cd-S (Ag-S) and P-P (PS) bonds evaluated through bond valence theory. The three-dimensional noncentrosymmetric (NCS) framework of 2 is constructed by two types of tetrahedral layers consisting of corner-shared CdS4, AgS4, and PS4 tetrahedra. Compound 2 exhibits second harmonic generation (SHG) intensity of 0.45 times that of commercial AgGaS2 (AGS) at a laser irradiation of 1.85 mu m and an optical band gap of 2.56 eV, and no intrinsic vibrational absorption of chemical bonds is observed in the range of 2.5-18.2 mu m. Both phase transition in 1 and SHG properties in 2 are closely related to the SOJT distorted d(10) cations and diverse phosphorus-sulfur polyanions (PaSb)(n-), which together can easily result in NCS distorted structures and interesting properties.