Glass-like thermal transport in symmetry-broken clathrates
We present the quantitative interpretation for the glass-like behavior of thermal conductivities κ(T) for type-I clathrate compounds involving off-centered guest ions. It is shown that the dipole-dipole interaction generated in cage/guest-ion systems is crucial to reproduce the characteristics of thermal conductivities for these symmetry-broken clathrates. The above scenario also explains well the difference of κ(T) between the p-type and the n-type β-BGS found recently by K. Suekuni et al. (Phys. Rev. B, 77 (2008) 235119.)
Inelastic Neutron Scattering Study of the Structural Glass Transition in a K (Br,CN) Mixed Crystal
Etude par diffusion inelastique de neutrons des excitations de rotations-translations couplees dans (KBr) 0,992 (KCN) 0,008 . Observation de la transition tunnel A 1g -T 2g et de l'excitation librationnelle A 1g -E g a la transition entre l'etat paraelastique et l'etat de structure vitreuse. La diminution tres importante du couplage rotation-translation T 2g suggere que le nombre d'etats tunnel des ions libres diminue pour donner des modes de reorientations en amas qui sont effectivement decouples du reseau
Universal Relation between Guest Free Space and Lattice Thermal Conductivity Reduction by Anharmonic Rattling in Type-I Clathrates
Since the early days of studies on thermoelectric clathrates it has been evidenced that the lowering of thermal conductivity in these compounds is related to the guest ion's rattling characteristics. Here we report on the relation between the guest vibration and lattice thermal conductivity κ L in single crystalline samples of type-I clathrates Ba 8 In 16 Ge 30- x M x (M = Si, Sn) with n-type carriers. On going from M x = Si 1.6 to M x = Sn 2.2 , the lattice parameter increases by 1% from 11.20 to 11.30 A. This is accompanied by a decrease from 39 to 35 K in the local vibration energy of the Ba guest ions in the tetrakaidecahedron cages, and a strong suppression of κ L , which shows glasslike temperature dependence for M x = Sn 2.2 . We further demonstrate that the lowering of κ L at 150 K shows universality behavior and scales very well with the guest free space for vibration in all type-I clathrates, largely independent of the guest or framework ions.