Application of density functional theory in analyzing the structural, optoelectronic, and thermodynamics properties of RbCrCl3 and mixed NaGeCl2F perovskites: a first-principles approach

Abstract

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 (C_p) and at constant volume (C_v) 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 C_p, C_v 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.