Theoretical Investigation of Structural, Electronic, and Optical Properties of NaGeBr3-xYx (Y= I, Cl) Halide-Based Perovskites for Optoelectronic Applications

This thesis presents a first-principles investigation of the structural, thermodynamic, electronic, mechanical, anisotropic, and optical properties of lead-free mixed-halide perovskites NaGeBr3−xYx (Y =I, Cl; x = 0, 1, 2, and 3) for potential optoelectronic applications. The study is motivated by the growing demand for environmentally friendly, lead-free, all-inorganic perovskite materials with tunable electronic and optical responses. Density functional theory calculations were performed using the Vienna Ab initio Simulation Package (VASP) within the GGA-PBE exchange–correlation functional. The parent NaGeBr3 compound was selected as the reference system, while Br→I and Br→Cl substitutions were considered as two chemically opposite halide-alloying pathways. The optimized structural results show that the end-member compounds NaGeBr3, NaGeI3, and NaGeCl3 preserve cubic symmetry with space group Pm¯ 3m, whereas the intermediate mixed-halide compositions adopt tetragonal symmetry with space group P4/mmm. Iodide substitution expands the lattice due to the larger ionic size of I−, increasing the unit-cell volume from 166.31 ˚ A3 for NaGeBr3 to 204.88 ˚ A3 for NaGeI3. In contrast, chloride substitution contracts the lattice, reducing the volume to 142.44 ˚ A3 for NaGeCl3. The absence of octahedral tilting indicates that the structural distortions in the intermediate compositions mainly arise from anisotropic GeX bond-length variation and ordered halide arrangement. The calculated formation energies are negative for all compositions, confirming their energetic favorability with respect to elemental reference phases. The chloride-substituted series showsenhanced thermodynamic stability, while the iodide-rich compositions become relatively less stable within the adopted reference framework. The electronic-structure results reveal direct narrowband-gap semiconducting behavior for the investigated compositions. NaGeBr3 shows a direct band gap of 0.5005 eV, while partial Br→I substitution significantly narrows the band gap, reaching 0.2206 eV for NaGeBr2I1. The density-of-states analysis shows that the band-edge states are mainly governed by the Ge–halide framework, with the valence-band maximum dominated by halide-p/Ge4s hybridized states and the conduction-band minimum mainly contributed by Ge-4p states. Mechanical-property calculations confirm that all investigated systems satisfy the Born stability criteria and exhibit ductile behavior according to Pugh’s ratio and Poisson’s ratio. Br→Cl substitution stiffens the lattice, increasing the bulk modulus from 22.595 GPa for NaGeBr3 to 26.582 GPa for NaGeCl3, whereas Br→I substitution softens the lattice, reducing the bulk modulus to 18.013 GPa for NaGeI3. The anisotropy analysis shows that elastic anisotropy is mainly controlled by shear deformation. Optical calculations demonstrate strong halide-dependent tunability. Iodide incorporation enhances dielectric screening, refractive index, extinction coefficient, reflectivity, and low-energy optical response, while chloride incorporation generally lowers the dielectric and refractive response but strengthens high-energy optical activity. The results establish NaGeBr3 as a promising lead-free parent compound for halide-site engineering and show that NaGeBr3−xYx perovskites offer tunable structural, electronic, mechanical, and optical properties for future optoelectronic applications.

• Supervision