Anharmonicity and scissoring modes in the negative thermal expansion materials ScF and CaZrF
arXiv:1911.12695 · doi:10.1103/PhysRevB.101.064306
Abstract
We use a symmetry-motivated approach to analysing X-ray pair distribution functions to study the mechanism of negative thermal expansion in two ReO-like compounds; ScF and CaZrF. 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 than for ScF, 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 due to the rock-salt ordering mixing the characters of two of these scissoring modes. Additionally, we show that in ScF 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.
arXiv comment9 pages, 7 figures
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Cited by in corpus (4)
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- Soft mode anisotropy in negative thermal expansion material ReO
- Recovery of harmonic-like behaviour of the polar mode in BaTiO at high pressure