Influence of Cation-Size Effects of Alkaline Earth (Ae) Metals on Dimensionality and Optical Anisotropy Regulation in K4AeP2S8 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 K4AeP2S8 (Ae = alkaline earth elements: Mg, Ca, Sr, and Ba) thiophosphates, involving the substitution of Ae2+ cations. The synthesized compounds crystallized in distinct space groups, monoclinic P2/c (Ae = Mg) versus orthorhombic Ibam (Ae = Ca, Sr, and Ba), exhibiting intriguing dimensionality transformations from zero-dimensional (0D) [Mg2P4S16]8- clusters in K4MgP2S8 to 1D ∞[AeP2S8]4- chains in other K4AeP2S8 thiophosphates owing to the varying ionic radii of Ae2+ cations, Ae-S bond lengths, and coordination numbers of AeSn (Mg: n = 6 versus other: n = 8). Experimental investigations revealed that K4AeP2S8 thiophosphates featured wide optical bandgaps (3.37-3.64 eV), and their optical absorptions were predominantly influenced by the S 3p and P 3s orbitals, with negligible contributions from the K and Ae cations. Notably, within the K4AeP2S8 series, birefringence (Δn) increased from K4MgP2S8 (Δn = 0.034) to other K4AeP2S8 (Δn = 0.050-0.079) compounds, suggesting that infinite 1D chains more significantly influence Δn origins than 0D clusters, thus offering a feasible approach for enhancing optical anisotropy and exploring potential new birefringent materials.