Lattice Dynamics–Driven Stability in Lead-Free Double Perovskites Ba₂B′NbO₆ (B′ = Gd, Tb): Implications for Sustainable Photovoltaic Applications

Abstract Perovskite materials are at the forefront of next-generation photovoltaic technologies, yet their large-scale implementation remains limited by concerns over long-term stability and environmental impact. In this work, we present a detailed lattice dynamical study of the lead-free double perovskites Ba₂B′NbO₆ (B′ = Gd, Tb) in the tetragonal (I4/m) structure, using normal coordinate analysis within a short-range force constant framework based on the Wilson GF matrix method. Infrared- and Raman-active phonon modes are systematically computed and assigned. Optimized stretching and bending force constants are used to quantify interatomic interactions, and potential energy distribution (PED) analysis is employed to characterize the nature of the vibrational modes. The results show that rare-earth substitution governs the lattice dynamics through a competition between mass effects and electronic contributions—particularly the higher polarizability of Tb³⁺ relative to Gd³⁺—leading to stronger B′–O bonding despite only weak structural distortion. High-frequency modes dominated by NbO₆ and B′O₆ octahedral stretching confirm a rigid, well-connected lattice framework. Taken together, the phonon spectra, force constants, and PED contributions point to a dynamically stable structure with strong metal–oxygen interactions and no lattice instabilities. Although the wide band gap of these oxide double perovskites limits their use as primary light absorbers, their lead-free composition, chemical stability, and tunable lattice properties make them well suited to auxiliary roles in photovoltaic architectures—such as protective layers, transport layers, and tandem device components. This study establishes a clear link between lattice dynamics, rare-earth electronic effects, and structural stability, offering design guidelines for robust, eco-friendly perovskite-based materials for real-world energy applications.