The local structure of skutterudites: A view from inside the unit cell

摘要

The skutterudites form a large class of compounds with many unusual properties, attributed in part to the novel crystal structure. The unit cell is cubic and is composed of eight sub-cubes formed by transition metal atoms. Six of the sub-cubes contain rings of atoms; the other two sub-cubes can be empty but are usually filled with rare earth or alkali earth atoms. These "filler" atoms can vibrate at low energies and hence are called "rattler" atoms. Here, the dynamics of various atom pairs are reviewed with a focus on the rattler atoms. Most of the work is based on extended X-ray absorption fine structure (EXAFS) studies but results obtained using other techniques, such as inelastic scattering experiments or atomic displacement parameters in diffraction, are also included. Although the main framework of the unit cell is often considered quite stiff, the stiffest springs in the system are only factors of 3-5 larger than the springs connecting the rattler to its neighbors. In addition, the environment about the atoms in the ring structures (e.g. Sb-4 in CeFe4Sb12) has a low symmetry and our recent EXAFS experiments suggest that the rings can be considered to be quasi-rigid units, and treated as a large atom. The restoring forces on the rings are asymmetric, with large forces perpendicular to the ring and weak forces in the direction toward a rattler. This suggests that some low energy modes that have been observed in these systems may be a correlated motion of the rattler atoms and the rings. In addition, the unusual result that the second neighbor effective spring constants are stiffer than the nearest neighbor bonds has been observed for several oxy-skutterudites. A simple one-dimensional (1D) model, of a chain of rattlers and rings, weakly coupled to the rest of the lattice has been developed which can explain these unusual results. These calculations also indicate that the thermal conductivity might be further suppressed using a composite formed of several types of nanoparticles rather than just multiple filling on the rattler sites.