RUS Studies of Crypto-Clathrates: Perfect Crystals with the Elastic Properties of Glasses
MISSISSIPPI UNIV UNIVERSITY
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The low-temperature thermal and elastic properties of glasses are known to be quite different from those of perfect crystals. For example the specific heat in amorphous solids is much larger than the Debye specific heat of crystals, and the thermal conductivity is considerably smaller. Some disordered crystals and quasicrystals were found to have properties very similar to those found in glasses, but remarkably, no amount of disorder introduced into a crystalline solid can produce a thermal conductivity which is lower than that of its amorphous counterpart. Recently, motivated by a search for improved thermoelectric materials, several compounds have been identified that combine the high electron mobilities found in crystals with the low thermal conductivities characteristic of glasses. The common structural feature of these materials is that they contain loosely bound atoms that reside in a large crystalline cage these materials are thus inclusion compounds or crypto-clathrates. A particular class of crypto-clathrate is formed by the filled skutterudite antimonides RM4Sb12, with M a transition metal and R a rare-earth. These filled skutterudites are derived from a regular unfilled skutterudite such as CoSb3, by filling the void in this skutterudite structure by a rare-earth. The presence of the rare-earth has a marked effect on the lattice dynamics of these materials. Resonant Ultrasound Spectroscopy measurements were performed as a function of temperature for both the filled and unfilled skutterudites to determine the elastic constants for both structures. These data reveal that an unusual elastic behavior complements the glasslike thermal properties of the La-filled skutterudite the elastic moduli of the filled compound display a strong temperature-dependence at low temperatures, which indicates the presence of low-energy vibrational modes in addition to the normal acoustic phonons.