Magnetic, optical, electronic, structural and thermoelectric features of A2MgS4 (A=Sm, Tb): A DFT study

Abstract

The spinel A2MgS4 (A = Sm,Tb) sulfides were analyzed by first-principle using the WIEN2k software based on density functional theory (DFT). Dynamic stability has been adequately explained using enthalpy of formation values (ΔHf) (−3.50 for Sm2MgS4 and -3.57 eV for Tb2MgS4). By examining band structure (BS) with spin polarized density of states (DOS), it is confirmed that Sm2MgS4 exhibits half-metallic ferromagnetic (HMF) behavior whereas Tb2MgS4 has semiconducting nature with a bandgap (Eg) of 1.56 in spin (↑) up and 0.94 eV in spin (↓) down channel. The estimated magnetic moments (μB) of sulfo-spinel Sm2MgS4 and Tb2MgS4 are 19.99931 and 24 μB, respectively. Transport properties such as thermal (k/τ) and electrical (σ/τ) conductivities, Power factor (PF), Seebeck coefficient (S), and figure of merit (ZT) are computed to determine the thermal behaviour of both spinels. From thermoelectric (TE) analysis, Tb2MgS4 maximum value of figure of merit (ZT) is 0.731 at 800 K, whereas the maximum S is 234 for Sm2MgS4 at 500 K and 267 μV/K for Tb2MgS4 at 350 K, hence, Tb2MgS4 is more efficient TE material than Sm2MgS4. Additionally, optical characteristics including absorption coefficient α(ω), extinction coefficient k(ω), dielectric constants ε(ω), reflectivity R(ω), refractive index n(ω), and optical conductivity σ(ω) are investigated. Computational results show that A2MgS4 (A = Sm, Tb) materials are potential candidates for spintronics, optoelectronics, and TE devices.