Thermo-electric (TE) material applications reduce reliance on traditional energy resources by converting heat to electric energy. We have studied, for the first time, the thermo-electric properties of Rb2SeX6 (X=Br,Cl). Using norm-conserving pseudo potentials in a plane wave basis set of Quantum Espresso code, the optoelectronic, elastic and thermo-electric properties of Rb2SeX6 (X=Br,Cl) have been investigated using density functional theory. Generalized Gradient Approximation of Perdew Burke Ernzerhof (GGA-PBE) and Generalized Gradient Approximation of Perdew Burke Ernzerhof adapted for Solid (GGA-PBESol) exchange correlation functionals were employed in all calculations. The band structure plots suggest that the studied double perovskites have indirect band gaps. Rb2SeBr6 band gap values of 1.7574/ 1.569 eV (using GGA-PBE/PBEsol) are remarkably similar to that of two effective inorganic/organic perovskites FAPbI3 and MAPbI3 . Maximum peaks generated from refractive index results indicate possible solar cell uses of the materials because they are in the visible and ultraviolet ranges. The results of other optical properties such as absorption coefficients, electron energy loss, conductivity, and reflectivity concludes that Rb2SeX6 (X=Br,Cl) have good values for electron generation, high potential for applications in the optoelectronic industry and are semiconductor in nature. The calculated shear anisotropy values of Rb2SeBr6/Cl6 are 3.09/1.71, suggesting that they are isotropic materials. With calculated Poisson’s ratio of 0.32 and 0.26, the materials are predicted to be ductile in nature. The two materials are appropriate for thermo-electric applications since their thermal to electrical conductivity ratio are small (the order of 10-5). The calculated minimum values of Seebeck coefficient values of 0.198×103 / 0.166 ×103 (mV/K) at 750 K, for Rb2SeBr6/Cl6 are positive, indicating that they have p-type conduction. Figure of merit values at all temperature range considered are greater than one (ZT > 1) for both Rb2SeBr6 and Rb2SeCl6, suggesting that they are good thermo-electric materials. The results of the calculations provide the basis for the industrial application of Rb2SeBr6/Cl6 as solar cells.

Thermoelectric properties of SrTiO3doped with 8%Pm at Sr site were investigated using density functional theory and generalized gradient approximation. The transport properties were calculated based on BoltzTraP code at temperature range 300-1200K. In electronic properties study Fermi level were shifted to conduction band region due to high contribution 4f orbital in Pm. Present study thermoelectric figure of merit ZT result was 0.395 at 300K and 0.638 at 1200K. This shows a considerably good value of ZT for SrTiO3as n-type oxide. Compared to previous work, ZT were at the range of 0.21 - 0.37 for temperature of 300-1000K in Pr, La, Ta and Ho.

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The double perovskite Cs2AgInCl6is a potential material for the absorbing layer of a thin film solar cell due to its direct band gap. The only current limitation the material has is its wide band gap. A careful engineering of its structural, mechanical and opto-electronic properties with the aid of hydrostatic pressure ranging from 0 GPa – 16 GPa has been studied using density functional theory. The calculations were carried out using GGA-PBEsol exchange correlation functional. It is found that the lattice constant reduces as the pressure increases, while the bulk modulus increases as the exerted pressure increases. The bulk moduli calculated from the elastic constants are found to be in agreement with those obtained via Birch-Murnaghan equation of state. This indicates the accuracy of the calculations, and it is achieved at all pressure values. The mechanical properties of the material are investigated, and the material is found to be anisotropic and ductile at all pressure considered. Due to the underestimation of the energy band gap by GGA-PBEsol exchange correlation functional, TB-mBJ, a metaGGA functional, was used to calculate the electronic and optical properties. The energy band gap is found to reduce from 2.746 eV at ambient pressure to 2.482 eV at 6 GPa and momentarily increases until it reached 2.501 eV at 16 GPa. The optical properties of Cs2AgInCl6 revealed its absorption threshold is in the visible range, although a shift in the absorption threshold is observed as pressure is applied on it. An 8.7 % increase in the refractive index is observed as pressure increases. The calculated absorption coefficient corresponds reasonably with the calculated band gap. The electron energy loss function and reflectivity of the material have also been investigated.