The role of the quartic anharmonic term in the thermal conductivity of complex crystals
Low thermal conductivities (~ 1-2 W m-1 K-1) are observed in structurally complex crystals with a large number of atoms in the unit cell (more than 50 atoms), such as guest-host structures (clathrates and skutterudites), where however the measurement of phonon lifetimes remains beyond the standard resolution limit. Moreover, in some of them the typical Umklapp peak in the K(T) dependence (1/T) is smoothed out and replaced by a weakly temperature dependent glass-like 'plateau', whose origin in crystals is intensively debated. Recently, theoreticians have proposed that the strong quartic anharmonicity within perturbation theory, to the first order, of rattling guest atoms causes an anomalous hardening of the vibrational frequencies of low-lying optical modes with temperature, resulting in an improved agreement with experimental results, including the deviation of phonon lifetimes and thermal conductivity from the expected 1/T dependence. We propose here to track the low-lying optical modes energies as a function of temperature on the clathrate Ba7.81Ge40.67Au5.33 to validate this theoretical finding. This proposal is part of the PhD thesis work of S. Turner.
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Shelby R. Turner; BOURDAROT Frederic; DE BOISSIEU Marc; GIORDANO Valentina; PAILHES Stephane; RAYMOND Stephane and SCHOBER Helmut. (2019). The role of the quartic anharmonic term in the thermal conductivity of complex crystals. Institut Laue-Langevin (ILL) doi:10.5291/ILL-DATA.7-01-494
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