Energy-Driven Optoelectronic and Mechanical Insights into X₃GeC (X = Mn, Ni) Antiperovskites

We present a comprehensive first-principles comparative study of the antiperovskite carbides X 3 GeC (X = Mn, Ni) using density functional theory within the all-electron WIEN2k and planewave CASTEP frameworks. Both compounds adopt a cubic Pm3.m structure and satisfy mechanical stability criteria. Optimized lattice parameters are 3.62 Å (Mn 3 GeC) and 3.79 Å (Ni 3 GeC), with corresponding densities of 8.70 and 7.95 g cm −3 . Elastic constant analysis reveals high stiffness for Mn 3 GeC (C 11 = 436.87 GPa, bulk modulus = 235.82 GPa) and pronounced ductility in Ni 3 GeC (C 11 = 304.76 GPa, C 44 = 235.31 GPa). Electronic structure calculations (TB-mBJ and HSE06) indicate metallic behavior with zero band gaps. Optical spectra show strong reflectivity (>45%) in the visible–UV range and distinct plasma resonance features, highlighting potential plasmonic applications. Thermoelectric evaluation reveals negative Seebeck coefficients (−3.0 × 10 −4 V K −1 for Mn 3 GeC; − 2.7 × 10 −4 V K −1 for Ni 3 GeC) with low ZT values. Overall, Mn 3 GeC is mechanically robust, while Ni 3 GeC offers promising optoelectronic and plasmonic functionality.