Size- and Temperature-Dependent Lattice Anisotropy and Structural Distortion in CsPbBr3 Quantum Dots by Reciprocal Space X-ray Total Scattering Analysis

Lead halide perovskite nanocrystals (NCs) have emerged as next-generation semiconductors capable of unifying superior photoemission properties, facile and inexpensive preparation, compositional and structural versatility. Among them, CsPbBr3 is a model system in theoretical and experimental studies owing to its intrinsic chemical stability. Nonetheless, knowledge of the precise magnitude and the size- and temperature-dependent lattice and structural distortions is lacking, and the static/dynamic nature of disorder in NCs remains an open question. Herein, robust reciprocal space X-ray total scattering analysis is applied and accurate lattice distortions, Pb-Br bond distances, and Pb-Br-Pb angles versus NCs size are extracted. The lattice anisotropy increases upon expansion on downsizing while, upon contraction on cooling, the lattice distortion behaves differently at intermediate (9 nm) and ultrasmall (5 nm) sizes and from the bulk. Bond distances (stretched by ≈1%) do not show any size dependence, whereas equatorial and axial angles denote more symmetric octahedral arrangements in the smallest sizes, where they differ by ≈2° compared to ≈8° in the bulk. Anomalously high atomic displacement parameters of axial bromine ions persisting down to cryogenic temperatures suggest statically disordered octahedral tilts. These results provide insights having important implications on size-dependent emission properties and the exciton fine structure.