FIRST-PRINCIPLES INVESTIGATION OF THE STRUCTURAL, ELECTRONIC, OPTICAL, AND MECHANICAL PROPERTIES OF RB2YCUX6 (X = CL, I) DOUBLE PEROVSKITES

Abstract

Lead-free halide double perovskites have garnered considerable attention as promising materials for optoelectronic and energy-related applications due to their compositional flexibility and tunable electronic and optical properties. In this work, a systematic first-principles investigation of the structural, electronic, optical, and mechanical properties of Rb₂YCuCl₆ and Rb₂YCuI₆ double perovskites is performed using density functional theory as implemented in the CASTEP code. The generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) is employed together with on-the-fly generated ultrasoft pseudopotentials. A plane-wave cutoff energy of 350 eV and a 4×4×4 Monkhorst-Pack k-point mesh are used for the calculations. The calculated electronic band structures reveal band gaps of 3.08 eV for Rb₂YCuCl₆ and 2.60 eV for Rb₂YCuI₆, indicating that halogen substitution provides an effective means of tuning the electronic structure. The total and partial density of states are analyzed to determine the orbital contributions to the valence- and conduction-band regions. Furthermore, the optical response is investigated through the dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity, and optical conductivity. The mechanical behavior is evaluated from the elastic constants and the derived bulk modulus, shear modulus, Young's modulus, Poisson's ratio, Pugh ratio, and elastic anisotropy. A comparative analysis reveals that replacing Cl with I significantly alters the electronic and optical properties of the studied double perovskites. These results provide useful theoretical insight into the composition-property relationship of Rb-based lead-free double perovskites and their potential for future optoelectronic applications.