Tuning Einstein Oscillator Frequencies of Cation Rattlers: A Molecular Dynamics Study of the Lattice Thermal Conductivity of CsPbBr_3
arxiv(2024)
摘要
The pure CsPbBr_3 perovskite is an archetypal example of a strongly
anharmonic crystal that poses a major challenge for computational methods to
describe its thermodynamic properties. Its lattice dynamics exhibits
characteristics of a phonon liquid: mode coupling, low lifetimes and
`rattlers'. To study the thermal conduction in this crystal, including the
effect of dynamic disorder introduced by the Cs rattlers, we apply large-scale
molecular dynamics (MD) simulations combined with machine-learning interatomic
potentials. We simulate its ultra-low lattice thermal conductivity in the cubic
phase and obtain phonon spectra by measuring velocity autocorrelation
functions. The thermal conductivity at 500 K is computed to be
0.53±0.04 W/ m K, which is similar to that of demineralized
water under normal indoor conditions. MD based insight into the heat transport
mechanism of halide perovskites is presented. In the analysis the Cs cations
are interpreted as damped Einstein oscillators. The phonon bandstructure of a
system with artificially raised Cs masses demonstrates an increased
interference of the Cs rattling with the acoustic phonon modes. We show that
the thermal conductivity of the CsPbBr_3 perovskite can still be slightly
decreased by tuning the cation rattling frequency into the range of the low
lying acoustic mode
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