The ability of some materials with a perfectly ordered crystal structure to mimic the heat conduction of amorphous solids is a remarkable physical property that finds applications in numerous areas of materials science, for example, in the search for more efficient thermoelectric materials that enable to directly convert heat into electricity. Here, we unveil the mechanism in which glass-like thermal conductivity emerges in tetrahedrites, a family of natural minerals extensively studied in geology and, more recently, in thermoelectricity. By investigating the lattice dynamics of two tetrahedrites of very close compositions (Cu12Sb2Te2S13 and Cu10Te4S13) but with opposite glasslike and crystal thermal transport by means of powder and single-crystal inelastic neutron scattering, we demonstrate that the former originates from the peculiar chemical environment of the copper atoms giving rise to a strongly anharmonic excess of vibrational states.

Optical properties of copper clusters of 3 to 5 nm in diameter, produced by laser vaporization and embedded in alumina, are investigated and compared to a semi-quantal model [based on time-dependent local-density approximation (TDLDA) and density functional theory (DFT)], including the absorption and screening properties of the ionic core background and the surrounding matrix. To begin with, the experiments show that the alumina trapped clusters are oxidized if codeposited at room temperature whereas they do remain metallic by holding the substrate temperature at 400°C. On the other hand, reducing under H 2 -N 2 atmosphere of samples made at room temperature is an alternative to elaborate embedded metallic copper clusters. Consequently, the alumina matrix evaporated on a substrate at 400°C is optically characterized and the oxidization level of the copper clusters is carefully investigated through x-ray photoelectron spectroscopy (XPS). Concerning the optical properties of the copper clusters, a strong damping and a broadening of the surface plasmon resonance (SPR) with decreasing size is observed, in good agreement with theoretical calculations. This damping, much more important than in gold clusters, is correlated to a strong coupling of the collective excitation with interband transitions. These results allow us to validate the semiquantal model for the three systems Au, Ag, and Cu, in which the size effects are ruled by the electronic spillout phenomenon, the surface of ineffective ionic core polarization and the local porosity at the metal/matrix interface. Lastly, the inhomogeneous effects such as size dispersion are shown to be of less importance compared to intrinsic size effects in the optical properties.

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