A DFT study of optoelectronic, elastic and thermo-electric properties of the double perovskites Rb2SeX6 (X=Br,Cl)

Abstract

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.

<i>Ab Initio</i> Study of the Physical Properties of Cs-Based Double Perovskites Cs2AX6 (A = Ge, Mn; X = Cl, I)

“…The values of shear anisotropy (A), Cauchy's pressure (P C ), Young's modulus (Y ), Poisson's ratio (ν), Pugh's ratio (R P ), and Frantsevich's ratio (R F ) were calculated using the following equations [36]…”

“…A good crystal deformation of a material is denoted by its higher bulk modulus (B) values [36]. Bulk moduli for Cs 2 AX 6 (A = Ge, Mn; X = Cl, I) (GGA-PBE and PBESol) were computed using (9), and the results are shown in Table IV.…”

“…Materials with good thermoelectric properties can be used to convert thermal energy (heat) to electrical energy [36]. The movement of charges for energy transfer produces a heat gradient, which results in a potential difference and a thermoelectric effect [57].…”

“…Thermoelectric refrigerators, detectors, and cooling systems are some of the practical applications of thermoelectric materials [57]. Due to the low thermal and high electronic conductivity of double perovskite materials, they are widely used for this purpose [36]. Semiconductor materials that have narrow band gap can also be used in TE applications [36].…”

“…Due to the low thermal and high electronic conductivity of double perovskite materials, they are widely used for this purpose [36]. Semiconductor materials that have narrow band gap can also be used in TE applications [36]. Sr 3 SbN is an antiperovskite with a 1.15 eV bandgap, high thermopower and power factor, but generally, semiconductors have high S values and low thermal conductivity [36,58].…”