Anharmonicity and scissoring modes in the negative thermal expansion materials ScF$_3$ and CaZrF$_6$
| Autor: | Bird, Tobias, Woodland-Scott, Jessica, Hu, Lei, Wharmby, Michael, Chen, Jun, Goodwin, Andrew, Senn, Mark |
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| Rok vydání: | 2019 |
| Předmět: | |
| Zdroj: | Phys. Rev. B 101, 064306 (2020) |
| Druh dokumentu: | Working Paper |
| DOI: | 10.1103/PhysRevB.101.064306 |
| Popis: | We use a symmetry-motivated approach to analysing X-ray pair distribution functions to study the mechanism of negative thermal expansion in two ReO$_3$-like compounds; ScF$_3$ and CaZrF$_6$. Both average and local structure suggest that it is the flexibility of M-F-M linkages (M = Ca, Zr, Sc) due to dynamic rigid and semi-rigid "scissoring" modes that facilitates the observed NTE behaviour. The amplitudes of these dynamic distortions are greater for CaZrF$_6$ than for ScF$_3$, which corresponds well with the larger magnitude of the thermal expansion reported in the literature for the former. We show that this flexbility is enhanced in CaZrF$_6$ due to the rock-salt ordering mixing the characters of two of these scissoring modes. Additionally, we show that in ScF$_3$ anharmonic coupling between the modes responsible for the structural flexibility and the rigid unit modes contributes to the unusually high NTE behaviour in this material. Comment: 9 pages, 7 figures |
| Databáze: | arXiv |
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