First-principles insights into half-metallic ferromagnetism, lattice dynamics, and thermoelectric performance of MgX2Se4 (X=Ti, Mn) spinel chalcogenides

Abstract

Half-metallic ferromagnets are central to spintronic and energy-conversion technologies due to their high spin polarization and multifunctional transport behavior. In this study, a first-principles investigation based on density functional theory within the GGA framework employing the modified Becke–Johnson potential is carried out to examine the structural, electronic, magnetic, optical and thermoelectric properties of the spinel chalcogenides MgTi₂Se₄ and MgMn₂Se₄. Total-energy calculations identify the ferromagnetic configuration as the ground state for both compounds. The negative formation enthalpies and phonon dispersion spectra free of imaginary frequencies confirm their thermodynamic and dynamical stability. Electronic band-structure analysis reveals half-metallicity arising from strong hybridization between transition-metal-d and Se-p orbitals, which is characterized by metallic behavior in the spin-up channel and a semiconducting gap in the spin-down channel. This electronic structure yields integer magnetic moments of 4 µ_B per formula unit for MgTi₂Se₄ and 16 µ_B per formula unit for MgMn₂Se₄ and is consistent with half-metallic ferromagnetism. The calculated elastic constants satisfy the Born stability criteria, confirming mechanical stability. The materials exhibit intrinsic ductility and pronounced elastic anisotropy. High elastic moduli, sound velocities and Debye temperatures further indicate enhanced lattice rigidity and high thermal stability. Optical properties derived from the complex dielectric function reveal large static dielectric constants. Strong interband transitions lead to intense ultraviolet absorption and high optical conductivity. These features indicate efficient electron–photon coupling. Spin-resolved transport analysis reveals pronounced Seebeck asymmetry, confirming dominant majority-spin carrier transport. The combined effects of finite Seebeck coefficients, relatively high electrical conductivity and suppressed lattice thermal conductivity lead to enhanced thermoelectric performance under n-type doping, with the dimensionless figure of merit reaching ZT ≈ 0.9 for MgTi₂Se₄ and ZT ≈ 0.99 for MgMn₂Se₄ at room temperature. These results establish MgTi₂Se₄ and MgMn₂Se₄ as promising multifunctional materials for spintronic, spin-caloritronic, and energy-harvesting applications.