RESUMO
Engineering lattice thermal conductivity requires to control the heat carried by atomic vibration waves, the phonons. The key parameter for quantifying it is the phonon lifetime, limiting the travelling distance, whose determination is however at the limits of instrumental capabilities. Here, we show the achievement of a direct quantitative measurement of phonon lifetimes in a single crystal of the clathrate Ba7.81Ge40.67Au5.33, renowned for its puzzling 'glass-like' thermal conductivity. Surprisingly, thermal transport is dominated by acoustic phonons with long lifetimes, travelling over distances of 10 to 100 nm as their wave-vector goes from 0.3 to 0.1 Å-1. Considering only low-energy acoustic phonons, and their observed lifetime, leads to a calculated thermal conductivity very close to the experimental one. Our results challenge the current picture of thermal transport in clathrates, underlining the inability of state-of-the-art simulations to reproduce the experimental data, thus representing a crucial experimental input for theoretical developments.Phonon lifetime is a fundamental parameter of thermal transport however its determination is challenging. Using inelastic neutron scattering and the neutron resonant spin-echo technique, Lory et al. determine the acoustic phonon lifetime in a single crystal of clathrate Ba7.81Ge40.67Au5.33.
RESUMO
The title compound, C(10)H(18), a decalin stereoisomer, crystallizes with Z' = 0.5 in the space group P2(1)/n. The trans-decalin molecule is located on an inversion centre with both rings in a chair conformation, making for a quasi-flat overall shape. Despite the absence of hydrogen bonds, it crystallizes easily. In this work the unknown crystal structure of trans-decalin has been solved and refined using X-ray powder diffraction data.
RESUMO
The material class of skutterudites is believed to have strong potential for thermoelectric application due to the very low thermal conductivity of the filled structures. It is generally assumed that the atoms filling the skutterudite cages act as 'rattlers' and essentially induce a disordered lattice dynamics referred to as 'phonon glass'. Here, we present neutron spectroscopy experiments and ab initio computational work on phonons in LaFe(4)Sb(12) and CeFe(4)Sb(12). Our results give unequivocal evidence of essentially temperature-independent lattice dynamics with well-defined phase relations between guest and host dynamics, indicative of a quasi-harmonic coupling between the guests and the host lattice. These conclusions are in disagreement with the 'phonon glass' paradigm based on individual 'rattling' of the guest atoms. These findings should have an essential impact on the design and improvement of thermoelectric materials and on the development of microscopic models needed for these efforts.
