Comprehensive DFT Analysis of FeSi and RhSi: Structural Stability, Electronic Contrast and Transport-Optical Responses

Abstract

Transition metal monosilicides, notably FeSi, the cubic (B20) and the orthorhombic (B31) phases of RhSi, have been widely studied due to their unconventional electronic properties and promising applications. Using the first-principles density functional theory in combination with the Boltzmann transport formalism, we systematically investigate their structural, mechanical, thermodynamic, thermoelectric, and optical properties. Elastic constants and polycrystalline moduli confirm the mechanical stability of these compounds. The relative softness and greater machinability of c-RhSi and o-RhSi compared to FeSi are attributed to their lower Debye temperature. We analyze the electronic character to be semiconducting for FeSi and semimetallic and metallic for c-RhSi and o-RhSi, respectively. FeSi shows superior mid-temperature power factor with a large Seebeck coefficient and peak power factor, whereas c-RhSi maintains robust electronic transport properties at higher temperatures. Both FeSi and c-RhSi are optically isotropic. c-RhSi features a high reflectivity and refractive index for the purpose of photonic applications, while FeSi offers a balanced optical response, which is suitable for cost-effective device integration.