First‐Principles Investigation of the Structural, Electronic, Optical, Elastic, and Thermal Properties of Defect Diamond‐Like Hg ₃ P ₂ S ₈

ABSTRACT Defect diamond‐like chalcogenides have emerged as an important class of infrared nonlinear optical (IRNLO) materials because they can combine strong second‐harmonic generation, structural flexibility, and useful optical transparency. In this work, the structural, electronic, vibrational, optical, elastic, and thermal transport properties of Hg 3 P 2 S 8 (HPS) were systematically investigated using first‐principles calculations based on density functional theory and density functional perturbation theory. The results show that HPS is a direct‐band‐gap semiconductor with a bandgap of 1.906 eV. Phonon‐dispersion calculations reveal the absence of imaginary modes, confirming the dynamical stability of the crystal. Raman analysis identifies the characteristic vibrations of the distorted HgS 4 and rigid PS 4 tetrahedral units, which are key to the defective diamond‐like framework. Optical calculations indicate a strong ultraviolet response, with pronounced reflectivity, absorption, dielectric activity, optical conductivity, and energy‐loss features concentrated in the short‐wavelength region. HPS is a relatively soft, anisotropic material (B H = 16.067 GPa, G H = 10.279 GPa, E = 25.416 GPa) with a low acoustic Debye temperature (170 K) and intrinsically low lattice thermal conductivity (0.993–0.298 W m − 1 K − 1 from 300–1000 K). These findings provide a comprehensive structure–property understanding of HPS and highlight promise for IR‐NLO, optoelectronic, and photonic applications.