Insight into the structural, elastic, optoelectronic, magnetic and thermodynamic properties of Sr₂TbXO₆ (X = Bi, Sb) double perovskites employing DFT approach

Abstract

First-principles DFT calculations on Sr₂TbXO₆ (X = Bi, Sb) double perovskites were performed using the WIEN2K code, with PBE-GGA for electronic structure optimization and the LAPW method for valence and core electrons. Structural optimization of Sr₂TbXO₆ (X = Bi, Sb) using PBE-GGA revealed the most stable structure, with the Birch-Murnaghan EOS used to calculate key ground state parameters. Substituting Bi with Sb reduced the lattice parameter, and phonon dispersion confirmed dynamic stability, highlighting potential for thermoelectric applications. The elastic properties of Sr₂TbXO₆ (X = Bi, Sb) confirm mechanical stability and brittle behavior, with Sr₂TbSbO₆ showing higher stiffness due to a greater Young's modulus. Both compounds exhibit elastic anisotropy and ionic bonding, as indicated by positive Cauchy pressures. The electronic band structures of Sr₂TbBiO₆ and Sr₂TbSbO₆ exhibit similar band gaps in both spin configurations, indicating comparable semiconducting behavior. However, flat states near the Fermi level in the spin-down channel, due to localized 4f electrons from Tb, enhance electron–electron interactions and suggest potential applications in spintronics and correlated electron systems. The magnetic moments of Sr₂TbBiO₆ and Sr₂TbSbO₆ are dominated by Tb, contributing 6.06 μB and 5.85 μB, respectively, with minimal contributions from other atoms and interstitial regions. Both compounds have total magnetic moments of 6.00 μB. Sr₂TbBiO₆ and Sr₂TbSbO₆ exhibit maximum thermal expansion at 0 GPa, decreasing with increasing pressure as atomic mobility becomes restricted. Heat capacity and volume increase with pressure and temperature, while the Debye temperature decreases due to softer phonon modes at higher temperatures.