Cesium lead iodide, CsPbI3, is an optoelectronic material of large interest for various technological applications; however, fundamental questions surrounding the vibrational dynamics of this material, especially regarding its role in structural phase transitions, remain to be elucidated. Here, in a combined variable temperature inelastic neutron scattering (INS) and machine-learning based molecular dynamics (MD) simulation study, we show that the stable phase at room temperature, i.e., the nonperovskite δ-phase, exhibits phonon modes with weak anharmonicity with only a weak temperature dependence from 10 K all the way up to the transition to the cubic perovskite α-phase at approximately 600 K. In contrast, the α-phase features anharmonic and damped vibrational dynamics, mainly associated with overdamped tilting motions of the PbI6 octahedra. Crucially, these overdamped tilting modes, which relate to the tetragonal and orthorhombic distorted perovskite phases (β- and γ-phase, respectively) formed at lower temperatures, stay overdamped by more than 100 K above the respective phase transition. This suggests a flat energy landscape of octahedral tilting motions in α-CsPbI3 and with structural fluctuations on the picosecond time scale with tilting patterns that locally resemble the structure of the β- and γ-phases. The vibrational dynamics of α-CsPbI3 are also characterized by pronounced anharmonic motions with large thermal displacements of the Cs+ ions, but these modes remain underdamped at 600 K.
Lavén, Rasmus, et al. "Unraveling the Nature of Vibrational Dynamics in CsPbI<sub>3</sub> by Inelastic Neutron Scattering and Molecular Dynamics Simulations." Journal of Physical Chemistry Letters, vol. 16, no. 19, May. 2025. https://doi.org/10.1021/acs.jpclett.5c00778
Lavén, Rasmus, Fransson, Erik, Erhart, Paul, et al., "Unraveling the Nature of Vibrational Dynamics in CsPbI<sub>3</sub> by Inelastic Neutron Scattering and Molecular Dynamics Simulations," Journal of Physical Chemistry Letters 16, no. 19 (2025), https://doi.org/10.1021/acs.jpclett.5c00778
@article{osti_2573134,
author = {Lavén, Rasmus and Fransson, Erik and Erhart, Paul and Juranyi, Fanni and Granroth, Garrett E. and Karlsson, Maths},
title = {Unraveling the Nature of Vibrational Dynamics in CsPbI<sub>3</sub> by Inelastic Neutron Scattering and Molecular Dynamics Simulations},
annote = {Cesium lead iodide, CsPbI3, is an optoelectronic material of large interest for various technological applications; however, fundamental questions surrounding the vibrational dynamics of this material, especially regarding its role in structural phase transitions, remain to be elucidated. Here, in a combined variable temperature inelastic neutron scattering (INS) and machine-learning based molecular dynamics (MD) simulation study, we show that the stable phase at room temperature, i.e., the nonperovskite δ-phase, exhibits phonon modes with weak anharmonicity with only a weak temperature dependence from 10 K all the way up to the transition to the cubic perovskite α-phase at approximately 600 K. In contrast, the α-phase features anharmonic and damped vibrational dynamics, mainly associated with overdamped tilting motions of the PbI6 octahedra. Crucially, these overdamped tilting modes, which relate to the tetragonal and orthorhombic distorted perovskite phases (β- and γ-phase, respectively) formed at lower temperatures, stay overdamped by more than 100 K above the respective phase transition. This suggests a flat energy landscape of octahedral tilting motions in α-CsPbI3 and with structural fluctuations on the picosecond time scale with tilting patterns that locally resemble the structure of the β- and γ-phases. The vibrational dynamics of α-CsPbI3 are also characterized by pronounced anharmonic motions with large thermal displacements of the Cs+ ions, but these modes remain underdamped at 600 K.},
doi = {10.1021/acs.jpclett.5c00778},
url = {https://www.osti.gov/biblio/2573134},
journal = {Journal of Physical Chemistry Letters},
issn = {ISSN 1948-7185},
number = {19},
volume = {16},
place = {United States},
publisher = {American Chemical Society},
year = {2025},
month = {05}}
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 764https://doi.org/10.1016/j.nima.2014.07.029