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  Impact of temperature and mode polarization on the acoustic phonon range in complex crystalline phases: A case study on intermetallic clathrates

Turner, S. R., Pailhès, S., Bourdarot, F., Ollivier, J., Raymond, S., Keller, T., et al. (2021). Impact of temperature and mode polarization on the acoustic phonon range in complex crystalline phases: A case study on intermetallic clathrates. Physical Review Research, 3(1): 013031, pp. 1-21. doi:10.1103/PhysRevResearch.3.013021.

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 Creators:
Turner, S. R.1, Author
Pailhès, S.1, Author
Bourdarot, F.1, Author
Ollivier, J.1, Author
Raymond, S.1, Author
Keller, T.1, Author
Sidis, Y.1, Author
Castellan, J.-P.1, Author
Lory, P.-F.1, Author
Euchner, H.1, Author
Baitinger, M.2, Author              
Grin, Yu.3, Author              
Schober, H.1, Author
De Boissieu, M.1, Author
Giordano, V. M.1, Author
Affiliations:
1External Organizations, ou_persistent22              
2Michael Baitinger, Chemical Metal Science, Max Planck Institute for Chemical Physics of Solids, Max Planck Society, ou_1863416              
3Juri Grin, Chemical Metal Science, Max Planck Institute for Chemical Physics of Solids, Max Planck Society, ou_1863413              

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 Abstract: The low and weakly temperature-varying lattice thermal conductivity, κL(T), in crystals with a complex unit cell such as type-I clathrates is assumed to originate from a reduced momentum and energy space available for propagative lattice vibrations, which is caused by the occurrence of low-energy optical phonon modes. In the context of ab initio self-consistent phonon (SCP) theory, it has been shown that the cubic and quartic anharmonic interactions result in a temperature-induced energy renormalization of these low-lying optical branches which contributes to the anomalous behavior of κL(T) in structurally ordered type-I clathrates [T. Tadano and S. Tsuneyuki, Phys. Rev. Lett. 120, 105901 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.105901]. By means of inelastic neutron scattering, we provide evidence for this energy renormalization in temperature, which has been resolved for transversely and longitudinally polarized phonons in the single crystal type-I clathrate Ba7.81Ge40.67Au5.33. By mapping the neutron intensity in the momentum space, we demonstrate the coherent character of the low-lying optical phonons. The overall phonon spectrum and dynamical structure factors are satisfactorily reproduced by ab initio harmonic calculations using density functional theory with the meta-GGA SCAN functional and a fully ordered structure. However, a polarization-dependent cutoff energy with opposing temperature shifts for longitudinal and transverse acoustic dispersions is experimentally observed which is not reproduced by the simulations. Anharmonicity affects the energies of the low-lying optical phonons in the transverse polarization, which compares quantitatively well with available results from SCP theory, whereas differences are observed for the longitudinal polarization. © 2021 authors. Published by the American Physical Society.

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Language(s): eng - English
 Dates: 2021-01-082021-01-08
 Publication Status: Published in print
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 Rev. Type: -
 Identifiers: DOI: 10.1103/PhysRevResearch.3.013021
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Title: Physical Review Research
Source Genre: Journal
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Publ. Info: American Physical Society
Pages: - Volume / Issue: 3 (1) Sequence Number: 013031 Start / End Page: 1 - 21 Identifier: ISSN: 26431564