The Influence of Different Group Distortions on the Structure–Property Relationship of Alkali Metal Thiophosphate

The family of thiophosphates has garnered significant interest in recent years due to their potential application value in infrared (IR) nonlinear optical (NLO) materials. To evaluate the optical properties of the metal thiophosphates A3PS4 (A = Li, Na, K, Rb), first-principles calculations based on density functional theory (DFT) were implemented to analyze and study their electronic structures and optical properties. The results indicate that the bandgaps vary from 3.24 to 4.09 eV, with the [PS4] groups primarily contributing. Among them, Na3PS4 has a large bandgap and a moderate NLO effect (3.24 eV, 0.65 × AGS), fitting the promising IR NLO material requirements (>3.0 eV, > 0.5 × AGS). Computational analysis of the mechanical properties of the title compounds reveals that as the cation radius increases, the values of Young’s modulus E, bulk modulus B, and shear modulus G gradually decrease. The distortion of the cationic groups leads to K3PS4 having brittleness properties different from those of the same crystal system.