Unraveling Phase Stability and Distortion Mechanisms in RbGeX₃ (X = I, Br, Cl) Perovskites via First-Principles...

In this study, first-principles calculations were employed to systematically explore the stability and structural evolution of all-inorganic lead-free perovskites RbGeX₃ (X = I, Br, Cl) in three optically active phases: Pm $\overline{3}$ m, R3m, and Pna2₁. Total energy and phonon spectra indicate that Pna2₁ is the most stable phase, followed by R3m and Pm $\overline{3}$ m. Ab initio molecular dynamics (AIMD) simulations at 300 and 500 K further confirm the thermal robustness of RbGeX₃. The distortion parameters (D, σ², ψ) indicate that the R3m and Pna2₁ phases exhibit varying degrees of symmetry breaking compared to the Pm $\overline{3}$ m phase. Analysis of the soft phonon modes reveals that instability in Pm $\overline{3}$ m originates from off-center displacements of Ge atoms coupled with halide vibrations, whereas in R3m it is driven by X-site-induced tilting of GeX₆ octahedra. After AIMD simulations, the structures of both Pm $\overline{3}$ m and R3m exhibit significant octahedral tilting, while the Pna2₁ 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.