Disparate atomic displacements in skutterudite-type LaFe3CoSb12, a model for thermoelectric behavior

Mean-square atomic displacements in lanthanum triiron cobalt dodecaantimoni de, determined as a function of temperature using single-crystal neutron di ffraction, show that the La atom exhibits an anomalously large displacement at room temperature, U-eq = 0.0196 (9) Angstrom(2), because it is too smal l to fill the atomic cage formed by the corner-linked octahedral framework of M4Sb12, M = Fe, Co. Site-occupancy refinements show 25% vacancies on the La site and an actual Fe:Co ratio of 2.17:1. Analysis of the temperature d ependence of the atomic displacements identifies a significant temperature- independent component for the La atom ascribed to static disorder, which am ounts to 19% of the room-temperature value. The large-amplitude rattling of the La atom can be effectively linked to the dramatic decrease of the latt ice contribution to the thermal conductivity, which is a key factor for imp roving the thermoelectric behavior of these materials. This structure-prope rty relationship offers a new paradigm for the exploration of thermoelectri c materials.