Influence of Cation-Size Effects of Alkaline Earth (Ae) Metals on Dimensionality and Optical Anisotropy Regulation in KAePS Thiophosphates.

Cationic substitution demonstrates significant potential for regulating structural dimensionality and physicochemical performance owing to the cation-size effect. Leveraging this characteristic, this study synthesized a new family of KAePS (Ae = alkaline earth elements: Mg, Ca, Sr, and Ba) thiophosphates, involving the substitution of Ae cations. The synthesized compounds crystallized in distinct space groups, monoclinic 2/ (Ae = Mg) versus orthorhombic (Ae = Ca, Sr, and Ba), exhibiting intriguing dimensionality transformations from zero-dimensional (0D) [MgPS] clusters in KMgPS to 1D [AePS] chains in other KAePS thiophosphates owing to the varying ionic radii of Ae cations, Ae-S bond lengths, and coordination numbers of AeS (Mg: = 6 versus other: = 8). Experimental investigations revealed that KAePS thiophosphates featured wide optical bandgaps (3.37-3.64 eV), and their optical absorptions were predominantly influenced by the 3p and 3s orbitals, with negligible contributions from the K and Ae cations. Notably, within the KAePS series, birefringence (Δ) increased from KMgPS (Δ = 0.034) to other KAePS (Δ = 0.050-0.079) compounds, suggesting that infinite 1D chains more significantly influence Δ origins than 0D clusters, thus offering a feasible approach for enhancing optical anisotropy and exploring potential new birefringent materials.