Resolving the true ground-state structure and origin of high-symmetry instability in lead-free perovskite RbSrI₃

Inorganic halide perovskite RbSrI3 has recently attracted attention as a lead-free material for radiation detection and optoelectronic applications. However, most theoretical studies have assumed an ideal cubic structure as the ground-state phase, despite the strong tendency of halide perovskites toward polymorphism and experimental evidence for an orthorhombic structure. Here, we resolve this discrepancy through a comprehensive first-principles density functional theory (DFT) investigation using the GGA-PBE functional and the projector augmented-wave method as implemented in VASP. The HSE06 hybrid functional was additionally employed to validate the calculated electronic band gaps. Energetic, dynamical, and finite-temperature thermodynamic analyses consistently identify the distorted orthorhombic Cmcm phase as the true ground state, while the cubic phase is dynamically unstable. Importantly, we show that energetic ordering alone is insufficient to establish the proper ground state, since lattice-dynamical stability is essential for distinguishing a true minimum from an unstable or saddle-point structure. The instability of the high-symmetry phase is traced to an A-site-driven lattice mismatch, in which the undersized Rb+ cation gives rise to soft Rb-I vibrational modes that drive cooperative octahedral rotations. Crystal orbital Hamilton population analysis further shows that octahedral tilting strengthens Rb-I interactions while reducing antibonding Sr-I contributions, thereby stabilizing the distorted structure. Octahedral tilting also modifies the optoelectronic response, increasing the GGA-PBE band gap from 3.32 eV in cubic Pm-3m to 3.76 eV in orthorhombic Cmcm phase. Higher-level HSE06 calculations confirm this trend, giving corresponding band gaps of 4.34 and 4.77 eV, respectively. The structural distortion also modifies the optical spectra significantly. These results establish the correct structural reference for RbSrI3 and provide a microscopic framework linking geometric mismatch, lattice dynamics, chemical bonding, and optoelectronic behavior in lead-free halide perovskites.