Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains

Abstract

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behavior. In the higher temperature, higher symmetry phases of these materials, several complex structural features are observed. The local structure can differ greatly from the average structure and there is evidence that dynamic 2D structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI3 is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large‐scale simulations, it is revealed that the low frequency motion of octahedral tilts implies a double‐well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and the length and timescales of the motion are reported. Finally, the spatial arrangement of these features and their interactions are investigated and visualized, providing a comprehensive picture of local structure in the higher symmetry phases.

Phase Transitions, Dielectric Response, and Nonlinear Optical Properties of Aziridinium Lead Halide Perovskites

“…Raman data indicate, therefore, the presence of two unique AZR + cations in the LT phase of AZRPbCl 3 . It is worth noting that this type of behavior was not reported for MAPbX 3 or FAPbX 3 perovskites, but two distinct A-site cations in the LT phase were reported for MHyPbX 3 polar analogues and the monoclinic γ-phase or P 2 1 / m phase of CsPbCl 3 . ,− ,,− Thus, Raman spectra support the conclusion derived from the powder X-ray diffraction data that the LT structure of AZRPbCl 3 may be similar to the monoclinic structure of CsPbCl 3 .…”

“…This implies that even when the material is in the (average) cubic phase, when the temperature is approaching the PT temperature, the octahedra will have a preferred in-phase alignment along one axis (here in the z -direction). This local symmetry breaking effect of cubic phase under cooling is also found in MAPbBr 3 but is not present in inorganic halide perovskites such as CsPbI 3 . , …”

Understanding Correlations in BaZrO3: Structure and Dynamics on the Nanoscale

“…This allows us to extract tilt angles around the α-axis (α = x , y , z ) for an octahedron located at ( n x , n y , n z ) at time t , θ α ( n x , n y , n z , t ). Here, we follow a similar notation as in refs and .…”

“…For both G ⊥ (d) and G ∥ (d) the correlation alternates between positive and negative values when increasing the neighbor distance because the R-tilt mode dominates that motion. 17,29,30 We thus show only |G ⊥ (d)| and |G ∥ (d)| in Figure 6b. The alternation of the correlation is demonstrated in Figure S16, where the joint probability distribution over two angles is shown.…”

Transferable Machine Learning Interatomic Potential for Bond Dissociation Energy Prediction of Drug-like Molecules

“…One of the primary tasks of fitting MLIPs is to collect appropriate structures for the reference database, which determines the domain of applicability of the model. For example, some successful MLIPs in materials modeling are fitted to relatively small unit cells containing tens of atoms, enabling MD simulations of up to a hundred thousand atoms for a few nanoseconds. , The situation is equivalent for MLIPs for small organic molecules, where the chemical space of potential compounds is too vast for the training set to encompass every possible compound. , Therefore, MLIPs based on local environments are expected to extrapolate to novel chemical entities if the training database contains a sufficient number of atomic environments that are similar to those in the new compound. Efforts to achieve transferability in small molecule MLIPs are evident in existing benchmark data sets − and general purpose transferable MLIPs, such as the ANI series. ,, However, applications of these MLIPs beyond near-equilibrium closed-shell structures remain limited.…”