State of Art in Lead Free Double Perovskite Ceramics, X₂MgTeO₆ (X = Sr, Ba): Structural Stability and their Potential Energy Harvesting Applications

The present first principles calculations regarding lead-free double perovskite ceramics X₂MgTeO₆ (X = Sr, Ba) have discovered their structural, thermal, and thermodynamic stability in the rock-salt phase. The lattice constants were calculated to be in close agreement with experimental values. The dynamical properties were explored in terms of phonon dispersion spectra, vibrational internal energy, Debye temperature, vibrational Helmholtz free energy, lattice thermal conductivity, specific heat, and vibrational entropy. It was found that both perovskites follow the Dulong-Petit law at high temperatures and Debye’s law at low temperatures. The electronic properties revealed the direct band gap nature of both double perovskites, with band gap values of 3.18 eV for Sr₂MgTeO₆ and 2.42 eV for Ba₂MgTeO₆. The optical properties predicted that the most active absorption occurs in the ultra violet (UV) region with an absorption coefficient greater than 10⁵ cm⁻¹. Therefore, the X₂MgTeO₆ double perovskites are proposed for optoelectronic devices operating in the UV region. The positive values of the Seebeck coefficients indicate the p-type conductivity in both ceramics. The values of the figure of merit (0.609 for Sr₂MgTeO₆ and 0.594 for Ba₂MgTeO₆ at 800 K) suggest that both ceramics are suitable for thermoelectric-based applications at high temperatures.