First-principles investigation of half-metallic, optical and thermoelectric properties in CaX₂Se₄ (X = Mn, V) spinels

Spinel chalcogenides of the type CaX₂Se₄ (X = Mn, V) represent a class of transition-metal compounds in which magnetic ordering, electronic structure, and lattice dynamics are strongly interrelated, making them attractive for spin-dependent transport and thermoelectric applications. In particular, the coexistence of partially filled transition-metal 3d states and chalcogen p states provides a favorable platform for exchange-driven spin polarization and tunable carrier transport. In this study, a comprehensive first-principles investigation based on density functional theory is carried out to examine the structural stability and magnetic ground state along with the electronic structure elastic response lattice vibrations optical characteristics and thermoelectric behavior of CaMn₂Se₄ and CaV₂Se₄. The calculated negative formation enthalpies together with the absence of imaginary phonon modes confirm both thermodynamic and dynamical stability. Total-energy analysis identifies the ferromagnetic phase as the ground state for both systems. The spin-resolved electronic band structures indicate half-metallic behavior, characterized by a metallic majority-spin channel and minority-spin band gaps of 2.44 eV for CaMn₂Se₄ and 2.05 eV for CaV₂Se₄. The computed elastic constants satisfy the mechanical stability criteria for cubic crystals and indicate a ductile mechanical response. Within the constant relaxation time approximation, n-type transport calculations predict large Seebeck coefficients and enhanced thermoelectric performance at elevated temperatures up to 800 K. Optical analysis further reveals strong dielectric polarization and pronounced absorption extending from the visible to the ultraviolet region. Collectively, these results establish CaMn₂Se₄ and CaV₂Se₄ as stable, spin-polarized chalcogenide spinels with coupled magnetic, transport, and optical functionalities.

All calculations are performed within the framework of density functional theory using the WIEN2k package, which implements the full-potential linearized augmented plane-wave (FP-LAPW) method. Structural optimization is carried out using the generalized gradient approximation in the Perdew-Burke-Ernzerhof form for the exchange-correlation functional. To achieve an improved description of the electronic structure and band gaps, the modified Becke-Johnson exchange potential is employed. The valence states are treated semi-relativistically, while the core states are treated fully relativistically. Spin-orbit coupling is neglected after test calculations confirm its negligible influence on the electronic structure near the Fermi level. The plane-wave cutoff parameter $R_{\mathrm{MT}}{K}_{\max }$ is set to 8.0, and appropriate muffin-tin radii are chosen for Ca, Mn/V, and Se atoms. Brillouin-zone integrations are performed using a Monkhorst-Pack k-point mesh corresponding to a 10 × 10 × 10 grid for self-consistent calculations, and the total energy is converged to 1 × 10⁻⁵ Ry. Spin-dependent thermoelectric transport coefficients are calculated using the BoltzTraP code within the semi-classical Boltzmann transport formalism under the constant relaxation time approximation. Dense k-point sampling is employed to ensure convergence of the Seebeck coefficient, electrical conductivity, and the electronic contribution to thermal conductivity. Phonon dispersion relations are computed using density functional perturbation theory as implemented in the Quantum ESPRESSO package. The exchange-correlation effects in the lattice-dynamical calculations are treated within the generalized gradient approximation to maintain methodological consistency. Interatomic force constants are obtained using a 2 × 2 × 2 supercell in combination with a 3 × 3 × 3 q-point mesh to accurately describe lattice vibrations and assess dynamical stability.