First-principles insights into double perovskite fluorides for energy harvesting applications

The DFT calculations utilizing the FP-LAPW approach were conducted for a detailed study of structural, electronic, optical, elastic, and thermoelectric properties of cubic K₂TiHgF₆ and Na₂TiHgF₆ Heusler fluorides. It was found that both materials have a cubic crystal structure of type Fm3̄m and display negative values of formation and cohesion energy, which indicates good thermodynamic stability, with the elastic parameters being consistent with the Born stability criteria. Calculations within the framework of the TB-mBJ approach reveal semiconductor nature, the band gap value for which amounts to 2.14 eV for K₂TiHgF₆ and 1.88 eV for Na₂TiHgF₆, with valence and conduction bands being composed of F 2p/Hg 5d and Ti 3d levels, respectively. Based on the optical computations, there are very low values of the static dielectric constant, good transparency below the absorption edge, high ultraviolet absorbance, high optical conductivity, and strong plasmon resonances at 6.5 to 8.0 eV, which suggest good ultraviolet photonic activity. The elastic moduli show moderate stiffness, low compressibility, low elastic anisotropy, and mechanically stable structures, with Na₂TiHgF₆ being more rigid compared to K₂TiHgF₆. From the thermoelectric analysis, it is evident that these compounds have with negative Seebeck coefficients, high electrical conductivity, and low thermal conductivity at high temperatures.