University of California, Santa Cruz

Skutterudites are a promising class of thermoelectric materials containing voids in which rare earth "rattler" atoms can be inserted to increase the figure of merit (ZT).  Recent extended X-ray absorption fine structure (EXAFS) studies suggest that in skutterudites, the nearly square rings (such as As₄ in CeFe₄As₁₂) are quite rigid and may vibrate with low energy modes in one direction, similar to the rattler atom vibrations. That work suggests that the motions of the square rings and the rattler atoms are coupled. In addition, for LnCu₃Ru₄O₁₂, the second neighbor pairs about Ln have stiffer effective springs than the nearest neighbor pairs. To investigate these systems, a one dimensional, four mass, linear chain spring model is developed to describe the recent experimental results, and provide insight about the low energy vibrations in such systems. Our model solves the resulting coupled network of overlapping weak and strong springs and determines the eigenfrequencies and eigenvectors. The dispersion curves show an acoustic mode, two different low energy optical rattling modes involving both the rattler and square, and a non interacting optical mode. Each rattler mode can couple to the acoustic mode, which generates avoided crossings characterized by flattening of the modes; this has important consequences for thermal transport.