Atomic-Level Description of Thermal Fluctuations in Inorganic Lead Halide Perovskites

Abstract

A comprehensive microscopic description of thermally induced distortions in lead halide perovskites is crucial for their realistic applications, yet still unclear. Here, we quantify the effects of thermal activation in CsPbBr 3 nanocrystals across length scales with atomic-level precision, and we provide a framework for the description of phase transitions therein, beyond the simplistic picture of unit-cell symmetry increase upon heating. The temperature increase significantly enhances the short-range structural distortions of the lead halide framework as a consequence of the phonon anharmonicity, which causes the excess free energy surface to change as a function of temperature. As a result, phase transitions can be rationalized via the soft-mode model, which also describes displacive thermal phase transitions in oxide perovskites. Our findings allow to reconcile temperature-dependent modifications of physical properties, such as changes in the optical band gap, that are incompatible with the perovskite time- and space-average structures.

Decoupled Structural Responses upon Light and Thermal Functional Activation in CsPbBr3 Perovskites

“…In Figure 1b (bottom panel) we report the difference XAS spectra across the Br K-edge between: (i) the 120 °C and the 25 °C in our experiment (orange); (ii) 130 °C and 27 °C from MD simulations (red), (iii) the thermal structural changes from averaged structures at 160 °C and 22 °C obtained from pair-distribution function (PDF) refinements (gray dotted) [4]. The agreement of the experiment and MD spectra shows that the T-induced XAS changes are due to an increase of dynamical disorder in the lattice [3], rather than to an increase of unit-cell symmetry. Moreover, we show that the experiment cannot be reproduced by the average structure determined through conventional PDF refinement methods (gray dotted).…”

“…Ab initio MD simulations were performed using the CP2K package in the isobaric (NpT) ensemble at 27 ⁰C and 130 ⁰C, initialized with orthorhombic and cubic supercells of 320 atoms, respectively. The XAS spectra were computed based on DFT calculations with the Quantum ESPRESSO distribution, explicitly accounting for core hole effects on supercells of 160 atoms (320 atoms for MD-based simulations) [2,3].…”

“…We also investigated the purely thermal response of CsPbBr3 with temperature-dependent XAS and first principles molecular dynamics (MD) simulations across its phase diagram. We show that the thermally-activated lattice cannot be reduced to a cubic average structure, because of the presence of dynamically distorted local configurations and lattice anharmonicity [3].…”

Accurate and efficient band-gap predictions for metal halide perovskites at finite temperature

“…This indicates that a harmonic description of the phonons would be inappropriate. Indeed, cubic structures are generally subject to important anharmonic effects 44,49 , which cause the excess free energy surface to change as a function of temperature 49 . The cubic phases at high temperature correspond to average structures, which differ noticeably from the local atomic-scale description 3,49 .…”

“…Indeed, cubic structures are generally subject to important anharmonic effects 44,49 , which cause the excess free energy surface to change as a function of temperature 49 . The cubic phases at high temperature correspond to average structures, which differ noticeably from the local atomic-scale description 3,49 . For instance, the average B-X-B bond angle corresponds to 180 ∘ for the cubic structure, but in molecular dynamics simulations the distribution is peaked at a lower angle (~168 ∘ , ref.…”