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

S. Turner (1, 2, 3) , S. Pailhès (3) , F. Bourdarot (4) , J. Ollivier (1) , S. Raymond (4) , T. Keller (5, 6) , Y. Sidis (7) , J.-P. Castellan (7, 8) , P.-F. Lory (1, 2) , H. Euchner (9) , M. Baitinger (10) , Yu. Grin (10) , H. Schober (1) , M. de Boissieu (2) , V. Giordano (3)

1 ILL - Institut Laue-Langevin
2 SIMaP - Science et Ingénierie des Matériaux et Procédés
3 ENERGIE - (nano)Matériaux pour l'énergie
4 MDN - Magnétisme et Diffusion Neutronique
5 Max-Planck-Institut für Festkörperforschung
6 FRM II - Forschungsneutronenquelle Heinz Maier-Leibnitz
7 LLB - UMR 12 - Laboratoire Léon Brillouin
8 Institut für Festkörperphysik
9 HIU - Helmholtz Institute Ulm
10 CPfS - Max-Planck-Institut für Chemische Physik fester Stoffe

The low and weakly temperature-varying lattice thermal conductivity, κ_L(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 κ_L(T) in structurally ordered type-I clathrates [T. Tadano and S. Tsuneyuki, Phys. Rev. Lett. 120, 105901 (2018)]. 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 Ba_7.81Ge_40.67Au_5.33. 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.

Domaines: Matière Condensée [cond-mat]

S. Turner, S. Pailhès, F. Bourdarot, J. Ollivier, S. Raymond, et al.. Impact of temperature and mode polarization on the acoustic phonon range in complex crystalline phases: A case study on intermetallic clathrates. Physical Review Research, 2021, 3 (1), pp.013021. ⟨10.1103/PhysRevResearch.3.013021⟩. ⟨hal-03122426⟩