أنا  Bewar Mohammad Ahmad

In this study, the structural, electronic, optical and thermodynamic properties of the cubic perovskite RbCrCl3 and mixed perovskite tetragonal phase NaGeCl2F are investigated. The perovskites have become interesting due to their good optical and electronic properties, such as, the large band gaps and the high carrier mobility. For the exchange–correlation (XC) energy density functional theory (DFT) was employed and routine ab-initio calculations with the general gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) as the chief means of solving XC energy. From the density of state (DOS), the band structure, and structural characteristics, can be studied using DFT approach. Thus, an analysis of the results obtained by calculating the band structure which shows that NaGeCl2F has a direct band gap, whereas the band gap of RbCrCl3 is indirect. Further investigations, the optical properties of these perovskites were studied through the obtained dielectric function, refractive index and absorption coefficient. Moreover, Gibbs2 employs Quasiharmonic Approximation (QHA) was used in the calculations of the thermodynamic properties of the studied structures. The results suggest that these halide perovskites can be prospective materials for optoelectronics devices because they can allow, reflect or absorb electricity from the electromagnetic spectrum. In the current investigation the derived Volume (V), bulk modulus (B), entropy (S), free Gibbs energy (G), specific heat at constant pressure (Cp) and at constant volume (Cv) with respect to pressure successively at higher temperatures in kelvin. The results show that if the pressure rises then both B and G show a moderate increase in visibility to allow their use in geophysical simulation and pressure equipment’s. It is noted form the results of thermodynamics that the value of Cp, Cv and S for NaGeCl2F slightly decrease with pressure. Nevertheless, for RbCrCl3 these values are not significantly variable with pressure, so that these materials can be used for high-pressure sensors. The real applications of the selected perovskites, RbCrCl3, with its indirect bandgap, is an ideal candidate for photodetectors and photovoltaic devices due to its long carrier diffusion lengths, enabling efficient photon interaction and charge collection. In contrast, NaGeCl2F, with its direct bandgap, is well-suited for LEDs and lasers, where high radiative recombination rates enable efficient photon emission. The combination of these structures provides flexibility in the design of devices, with direct bandgap excelling in light-emitting applications and indirect bandgap supporting long-distance carrier transport for energy-harvesting devices.

This study investigates the structural, electronic, and optical properties of tetragonal-phase RbGeA2X (A = Br, Cl; X = Br, I) lead-free mixed halide perovskites using density functional theory (DFT) with PBE-GGA for exchange-correlation energy. These perovskites show enhanced properties, including high charge carrier mobility, tunable direct band gaps, and strong ultra-violet (UV) absorption. Band structure and density of states (DOS) analyses highlight their suitability for optoelectronic applications. Optical studies of the dielectric function and absorption coefficient of the studied structures confirm their ability to absorb electromagnetic radiation beyond the visible spectrum, making them promising candidates for advanced (UV)-range optoelectronic devices.

In this study, the structural, electronic and optical properties of cubic lead-free halide perovskites LiSnX₃ (X = Br and Cl) under hydrostatic pressure are investigated. The first-principle approach based on density functional theory (DFT) is employed. The exchange-correlation functional is treated using the generalized gradient approximation (GGA), specifically a variant of the Perdew–Burke–Ernzerhof (PBE) method. The aim of the study is to understand the effect of pressure on the properties of LiSnX₃ (X = Br and Cl), with a maximum pressure limit of 6 GPa. The results show a decreasing tendency in the energy band gap as pressure increases. In addition, a prominent reduction in the energy band gap is observed when the halogen atom is changed from Cl to Br under constant pressure. The calculations also investigate the density of states (DOS), showing variations in energy levels near the Fermi level under different pressures. For optical properties, density functional perturbation theory (DFPT) is used in conjunction with the Kramers-Kronig relation. Optical parameters such as the real and imaginary parts of the dielectric constant, refractive index, absorption coefficient, and wavelength are computed under different pressures to understand the optical response of the perovskites to the electromagnetic spectrum. The insights from this study highlight the fundamental properties of LiSnX₃ (X = Br and Cl) under different pressures, which could influence advancements in optoelectronic devices, photonic applications, and solar cell technologies. Moreover, this research contributes to the growing body of knowledge on lead-free halide perovskites, encouraging further developments in the field.

Abstract. Pseudopotentials and density functional theory (DFT) implemented in the ABINIT code were used to study the properties of the GaSb cubic alloy zinc-blende structure. Both the local density approximation and the generalized gradient approximation were used for the exchange-correlation (XC) potential calculation. The calculated lattice parameter aligns well with available experimental and theoretical results. Elastic constants, Young’s modulus, shear modulus, and anisotropy factor were determined, and the pressure dependence of elastic constants was investigated. Band gaps were initially calculated but showed discrepancies with experimental values due to the known band gap problem of DFT. To enhance accuracy, the Green function and screened Coulomb interaction approximation were introduced. The impact of thermal effects on compound properties was investigated using the quasi-harmonic Debye model, presenting variations in volume, heat capacities, thermal expansion coefficient, and Debye temperature concerning pressure and temperature.

We study structural, electronic and optical properties of inorganic lead- free halide perovskites RbGeX3 (X = Cl, Br and I) under hydrostatic pressure, which could facilitate development of new optoelectronic and solar-cell technologies. ab initio first-principles calculations are employed based on the generalized gradient approximation within the framework of density functional theory. We demonstrate that the bandgap of our perovskites decreases with increasing pressure. At a given pressure, the bandgap becomes narrower when the halogen atom is changed from Cl to I. We also examine the density of states and demonstrate that the energy levels near the Fermi level change significantly under pressure. The optical properties are calculated using the density functional perturbation theory and the Kramers–Kronig relation. The optical parameters such as the real and imaginary parts of the dielectric function, the refractive index and the absorption coefficient are calculated under different pressures.