Unraveling Phase Stability and Distortion Mechanisms in RbGeX 3 (X = I, Br, Cl) Perovskites via First‐Principles Calculations

In this study, first‐principles calculations were employed to systematically explore the stability and structural evolution of all‐inorganic lead‐free perovskites RbGeX 3 (X = I, Br, Cl) in three optically active phases: Pmm, R3m, and Pna2 1 . Total energy and phonon spectra indicate that Pna2 1 is the most stable phase, followed by R3m and Pmm. Ab initio molecular dynamics (AIMD) simulations at 300 and 500 K further confirm the thermal robustness of RbGeX 3 . The distortion parameters ( D , σ 2 , ψ ) indicate that the R3m and Pna2 1 phases exhibit varying degrees of symmetry breaking compared to the Pmm phase. Analysis of the soft phonon modes reveals that instability in Pmm originates from off‐center displacements of Ge atoms coupled with halide vibrations, whereas in R3m it is driven by X‐site‐induced tilting of GeX 6 octahedra. After AIMD simulations, the structures of both Pmm and R3m exhibit significant octahedral tilting, while the Pna2 1 phase shows minimal structural change, indicating greater thermal robustness. Finally, band structure calculations using the Heyd–Scuseria–Ernzerhof hybrid functional show a progressive bandgap increase with decreasing symmetry, offering theoretical guidance for the development of efficient lead‐free perovskites.