Uniaxial Strain Engineering of Electronic, Elastic and Optical Properties of Halide Double Perovskites K₂NaTIX₆ (X = I, Br, and Cl): A DFT Insight

The electronic and optical properties of lead-free halide double perovskites K₂NaTIX₆ (X = Cl, Br, and I), with low toxicity and suitable stability, were studied using density functional theory (DFT). The investigation focused on the effects of uniaxial strain ranging from − 4 to 4%. The study revealed that K₂NaTIX₆ perovskites retained their direct band gap when subjected to uniaxial strains. Furthermore, the bandgap energies exhibited an increase (decrease) as the tensile (compressive) strain was intensified. The lattice parameters and bond lengths exhibited a consistent increase (reduction) as the tensile (compressive) strain effect. The strain from − 4 to 4% caused a drop (increase) in the effective mass of holes (electrons). Therefore, tensile strain was more appropriate for the transportation of electrons and holes. The exciton binding energies of investigated HDPs vary from 0.45 to 0.057 eV, which will increase in the tensile lattices, indicating increase the production of free carriers. The work highlights the capacity of lattice strain engineering to enhance the photophysical characteristics of HDPs, thereby leading to an evolution in their optoelectronic performance. The valence band of K₂NaTIX₆ was located at the Fermi energy level and mostly influenced by Cl-3p and TI-3d orbitals. On the other hand, the conduction band was mainly influenced by TI-s and Cl/Br/I-3p/4p/5p orbitals. The intensity of the peak in the imaginary component ε₂(ω) increases and shifted towards lower energy as the strain varied from − 4 to 4%. The optical absorption spectra exhibited a blue shift at the edges when subjected to the effect tensile (compressive) strain. The static reflectivity drops from − 4 to 4% and the reflectivity peak is shifted towards lower (higher) energy with a compressive (tensile) strain. Hence, this study offered a theoretical understanding of the photoelectronic characteristics of K₂NaTIX₆ (X = Sb, Bi) when subjected to uniaxial strain.