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Thermal expansion (positive or negative) is the result of phonon anharmonicity. These phonons can be probed using inelastic neutron scattering (INS). By measuring the generalized density of states (GDOS), a dynamical fingerprint of polycrystalline samples is created, which is often compared to DFT calculations. However, finite difference phonon calculations are based on the assumption of independent harmonic phonon modes. For calculating thermal properties, the quasi-harmonic approximation (QHA) is often used to introduce temperature affects into calculations. How valid an approximation this is, depends on each compound. One test of the QHA is to calculate thermal expansion and compare to measurements. However it is possible that these can match fortuitously.
With GDOS measurements at only two different temperatures, one can extract a somewhat crude but very useful plot of anharmonicity vs phonon energy. This can be compared with the implicit anharmonicity calculated with the QHA (see attachment). Comparing the calculated with the measured anharmonicity directly provides a great additional way to support or reject the use of the QHA in your calculations.
References:
1. S. d’Ambrumenil, M. Zbiri, S. J. Hibble, A. M. Chippindale, D. S. Keeble, C. Wright, and N. H. Rees. Phys. Rev. B, 100(17):174302, 2019.
2. S. d’Ambrumenil. Negative Thermal Expansion in Transition-Metal Cyanides. PhD thesis, University of Reading, Institut Laue-Langevin, 2020.
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