Exploring the structural, elastic, electronic, optical properties and thermoelectric properties of Na2XGaF6 (X = In, or Tl) double perovskite: DFT study

Double perovskites exhibit considerable potential for optoelectronic and thermoelectric applications, positioning them as strong contenders for efficient and reliable renewable energy systems. Our study focuses on the cubic Na2XGaF6 (X = In or Tl) double perovskite using density functional theory (DFT). We utilize the GGA and mBJ methods to account for exchange-correlation effects, with GGA employed to determine the ground state energy and optimal structural parameters, highlighting the stability of Na2XGaF6 (X = In or Tl) through its formation energy, tolerance factor, and octahedral tilting. Mechanical property evaluations indicate the brittle nature of the material. The bandgaps of Na2InGaF6 and Na2TlGaF6 double perovskites are found to be 2.6 eV and 5.84 eV, respectively, using the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential for band structure and optical property analysis. Optical property assessments reveal significant polarization in the UV and visible spectrum, suggesting Na2XGaF6 as a promising candidate for photovoltaic applications. The thermoelectric performance of Na2XGaF6 (X = In, Tl) was evaluated using Boltzmann transport theory. Both compounds showed increasing electrical conductivity and power factor with temperature, while ZT slightly decreased. Na2TlGaF6 exhibited better thermal stability, making it more suitable for high-temperature thermoelectric applications.