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Anharmonic motion and aspherical nuclear probability density functions in cesium halides

Publikation: Bidrag til tidsskrift/Konferencebidrag i tidsskrift /Bidrag til avis › Tidsskriftartikel › Forskning › peer review

DOI

The cesium halides (CsX) are ionic high-symmetry compounds, which at first would seem like well-understood systems. However, recent studies have shown that using the simple Perdew-Burke-Ernzerhof (PBE) functional in density-functional theory (DFT) calculations, CsX materials do not adopt their namesake structure. Furthermore, peculiar low thermal conductivities have been observed experimentally in both CsCl and CsI at room temperature, and the origin has been linked to low-temperature anharmonicity derived from different types of experiments. In the case of CsCl the anharmonicity was observed from x-ray diffraction as an octahedral nuclear probability density function (nPDF), which, in contrast to expectations, becomes spherical at elevated temperature. Here, we study the nPDF of CsBr and CsI from single-crystal x-ray diffraction to compare with the findings of CsCl. It is shown that the aspherical features become less pronounced when substituting for a heavier halide. From periodic DFT calculations on CsCl, CsBr, and CsI probing the potential-energy surfaces this can be explained by progressively more similar masses upon substitution linked with Pauli repulsion. The apparent disappearance of the anharmonic features in CsCl with increasing temperatures can be understood as relatively larger population of acoustic phonons compared to the optical phonons following the Bose-Einstein distribution function. Finally, it is shown that theory can reproduce the correct equilibrium structures as well as phonon dispersions comparable to experimental values when adding a functional form of van der Waals interactions to a PBE DFT calculation.

OriginalsprogEngelsk
Artikelnummer104113
TidsskriftPhysical Review B
Vol/bind105
Nummer10
ISSN2469-9950
DOI
StatusUdgivet - mar. 2022

Bibliografisk note

Funding Information:
We would like to acknowledge Jiawei Zhang for computing the phonon dispersion of CsI with vasp for comparative purposes between our result and the one given by Wei et al. Likewise, we would like to acknowledge the beam-time crew Venkatesha Hathwar, Mattia Sist, and Hidataka Kasai for measuring the single-crystal x-ray diffraction data, and the beamline scientist at BL02B1 Kunihisa Sugimoto. We thank Carlo Gatti for fruitful discussions. The computational results were obtained at the Centre for Scientific Computing at Aarhus University. This work was supported by the Villum Foundation.

Publisher Copyright:
© 2022 American Physical Society.

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