First-principles calculations to investigate structural, magnetic, optical, electronic and thermoelectric properties of X2MgS4(X=Gd, Tm) spinel sulfides

Abstract

Full potential linearized augmented plan wave (FP-LAPW) method based on density functional theory (DFT) is used to explore the optoelectronic, structural, thermo-electric and magnetic characteristics of cubic X2MgS4 (X = Gd, Tm) spinel sulfides. Stability of ferromagnetic (FM) state is confirmed by optimizing the unit cells and the stability of the compound is confirmed by negative values (−8.381 eV for Gd2MgS4 and -7.448 eVfor Tm2MgS4) of formation enthalpy (Δ Hf). Spin dependent band structures (BS) and density of states (DOS) show that Tm2MgS4 is the half-metallic (HM) whereas Gd2MgS4 is semiconductor and has a band-gap (Eg) of 1.67 eV in spin up and 1.34 eV in spin down state. Computed magnetic-moments (μB) of cubic Gd2MgS4 and Tm2MgS4 are 28.000 μB and 8.003 μB, correspondingly, which is further confirmed by spin-polarized density. From thermoelectric (TE) features, Tm2MgS4 has maximum value of figure of merit (ZT) 0.782 and Gd2MgS4 has peak value 0.753 at 800 K and seebeck coefficient (S) shows maximum value of 242 μV/k for Gd2MgS4 and 254 μV/k for Tm2MgS4 at 750 K, which makes Tm2MgS4 more efficient material than Gd2MgS4. Moreover, optical parameters such as absorption coefficient α(ω), optical conductivity σ(ω), dielectric constant ε(ω), refractive index n(ω), reflectivity R(ω) and extinction coefficient k(ω) are also explored. Optical properties of the X2MgS4 (X = Gd, Tm) show that both materials are transparent in ultraviolet (UV) region, indicating that both are suitable for use in optical filters and sensors. According to findings studied compounds might be useful for spintronics, thermoelectric and optoelectronic applications.