Exploring the Li₂CuAlX₆ (X = Cl, Br, I) lead-free double perovskites for energy harvesting applications

In the current investigation, we theoretically explored the halide-based double perovskites Li₂CuAlX₆ (X = Cl, Br, I), focusing on their structural, optoelectronic, and thermoelectric attributes. The computation of tolerance factor and enthalpy of formation confirms the thermodynamically stable cubic configuration of all the studied compositions. The study of the band structure reveals the direct bandgap nature with the reduction of bandgap value from visible to infrared region upon replacement of halogens in the compositions. It also enhances the absorption coefficient, dielectric function, refractive index, and reflectivity of Li₂CuAlX₆ (X = Cl, Br, I) highlighting their feasibility for solar cell industry. Employing semiclassical transport theory based BoltzTrap code, the temperature-dependent various transport parameters including electronic and thermal conductivities, Seebeck coefficient, and power factor are computed which enlightened the path of these materials for thermoelectric devices suitable to work at room temperature.

Left sided figure illustrates the atomic configuration within the unit cell of double perovskites Li₂CuAlX₆ (X = Cl, Br, I). The unit cell adopts a face-centered cubic structure with space group \(Fm\overline{3 }m\) $Fm\overline{3}m$ . According to the crystallographic perspective, the Li atom is situated at interstitial positions, having (0.25, 0.25, 0.25) fractional coordinates, whereas Cu, Al, and X atoms occupy (0, 0, 0), (0.5, 0.5, 0.5), and (x, 0, 0) positions, respectively. Notably, Li atoms are surrounded by 12 halogen ions, and Cu/Al atoms are surrounded by six halogen ions, giving them coordination numbers of 12 for Cu and 6 for Al. The Li and Cu atoms are represented as green and blue spheres, while Al and X atoms are depicted as grey and red spheres. The figure of merit (ZT) versus temperature is presented in right sided figure.