Half-metallic ferromagnetism and thermoelectric-efficient behavior in chalcogenide spinels MgNi₂X₄ (X = S, Se): a...

We present a comprehensive first-principles investigation of the structural, electronic, magnetic, thermoelectric, and optical properties of MgNi₂X₄ (X = S, Se) spinels. Both compounds are confirmed to be mechanically and dynamically stable in the cubic Fd3̄m structure. Energy-volume calculations establish ferromagnetism as the ground state, and phonon dispersion curves obtained via density functional perturbation theory (DFPT) exhibit no imaginary frequencies, confirming dynamic stability. Electronic structure calculations using both GGA and TB-mBJ functionals reveal half-metallic ferromagnetism having total magnetic moments of approximately 4 μ_B per formula unit predominantly arising from Ni²⁺ ions. Thermoelectric properties, including Seebeck coefficient, electrical and thermal conductivities, and the dimensionless figure of merit (zT) has been evaluated in the temperature range of 100-800 K as a function of chemical potential. The maximum zT values of ∼1.00 for MgNi₂S₄ and ∼0.98 for MgNi₂Se₄ are attained near room temperature, indicating its efficient thermoelectric performance. High See beck coefficients in the spin-down channel 180 μV K^-1 for MgNi₂S₄ and 450 μV K^-1 for MgNi₂Se₄ reflect their spin-polarized electronic structures. Additionally, low lattice thermal conductivities derived from phonon-based calculations further enhance their thermoelectric potential. Optical analyses reveal strong absorption, high photoconductivity, and significant dielectric response in the visible to ultraviolet range, making these materials suitable for optoelectronic applications. These results establish both MgNi₂S₄ and MgNi₂Se₄ as promising multifunctional candidates for the use in next-generation spintronic, thermoelectric, and optoelectronic devices.