Precessional Dynamics of Octahedra in CsPbBr₃

The crystal structure of ABX₃ halide perovskites consists of corner-sharing BX₆ octahedra, whose collective distortions define the different crystallographic phases. Because these materials are mechanically soft, with a shallow energy landscape, the octahedra display pronounced dynamical fluctuations that play a key role in determining their structural transitions and physical properties. Directly resolving this motion is challenging: experimentally, the dynamics occur on time scales that elude conventional spectroscopic techniques, while ab initio molecular dynamic is computationally prohibitive for the system sizes required. Here, we develop a machine-learning potential that enables large-scale molecular dynamics simulations with near-DFT accuracy. We follow the temporal evolution of individual octahedra in CsPbBr₃ and identify the characteristic events that define their rotational behavior. Our simulations reveal that the commonly assumed tilting sequence─where an octahedron switches from a positive tilt to an untilted configuration, and then to a negative tilt (the a⁺ ⇒ a⁰ ⇒ a^- pathway in Glazer notation)─rarely occurs under the specific conditions investigated in this study. Instead, the dominant mode of motion arises from a combined precession-nutation process in which the octahedron continuously reorients around its axis while undergoing smaller-amplitude oscillations. These findings suggest that similar complex rotational dynamics should be expected across halide perovskites, offering new microscopic insight into their structural behavior and guiding future experimental efforts aimed at detecting these motions.