A DFT analysis on the electronic, mechanical, optical, elastic, structural and thermodynamic properties of XBiNb2O7 (X = Cs, Rb, K and Na) for photovoltaic applications

The electronic, mechanical, optical, elastic, structural and thermodynamic properties of the orthorhombic crystal structure from Dion Jacobson (DJ) family member XBiNb₂O₇ with Pmc2_1 space group have been determined by using density functional theory (DFT) calculations. These calculations have been performed by using the CASTEP tool, with implementations of the norm-conserving pseudo-potential plane wave and PBE-GGA approaches. And also, density function perturbation theory (DFPT) is implemented to determine thermodynamic properties. The thermodynamic properties of XBiNb₂O₇ (X = Cs, Rb, K, and Na) compounds have demonstrated different zero-point energy in the range of 1.1732–1.3737 eV and also the specific heat capacity (C_v) is calculated, which rises with temperature variation and approaches to Dulong–Petit limit at 500. The obtained results indicate that these compounds are semiconductor with indirect bandgaps for CsBiNb₂O₇, RbBiNb₂O₇, KBiNb₂O₇, and NaBiNb₂O₇ in the energy range of 2.266–2.277 eV. These compounds are ideal for optoelectric devices. For photovoltaic applications, the optical properties of the compounds including optical conductivity, loss function, reflectivity, extinction coefficients, dielectric function (real and imaginary), refractive index and absorption are also examined. Dielectric function (5-10), optical conductivity (6 fs⁻¹), absorption coefficients (10⁵ cm⁻¹) and refractive index (5–6) are all high in visible and near UV region. The values of Pugh's ratio for the compounds are B/G > 1.75, indicating that elastic or mechanical properties exhibit ductile behavior and mechanical stability. The Born stability criteria is used to examine the compound's mechanical stability, which confirm their suitability for flexible electronic applications. This manuscript offers valuable information about the strategy and progress of materials suitable for future photovoltaic applications.