When Does a Crystal Conduct Heat Like a Glass?
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When Does a Crystal Conduct Heat Like a Glass? B. C. Sales, B. C. Chakoumakos, V. Keppens1, R. Jin, D. Mandrus and J. R. Thompson Solid State Division, Oak Ridge National Laboratory Oak Ridge, TN 37831-6056 1 National Center for Physical Acoustics, The University of Mississippi Oxford, Mississippi 38677 ABSTRACT Semiconducting crystalline materials that are poor conductors of heat are important as thermoelectric materials and for technological applications involving thermal management. In the present article neutron scattering, electrical and thermal transport, heat capacity, magnetic and acoustic measurements are reported on single crystals of three semiconductors with the type I clathrate structure: Ba8Ga 16Ge 30, Sr8Ga 16 Ge 30 and Eu8Ga 16 Ge 30 . Taken together these measurements suggest specific structural features that result in a crystal with the lowest possible thermal conductivity, namely that of a glass with the same chemical composition. Weakly bound atoms that rattle within oversized atomic cages in a crystal result in a low thermal conductivity, but the present data show that both rattling atoms and tunneling states are necessary to produce a true glass-like thermal conductivity. INTRODUCTION One of the more interesting ideas in the area of thermoelectric materials research is the concept of designing a solid that conducts heat like a glass but maintains the good electrical properties associated with crystals [1]. The essence of the idea is to synthesize semiconducting compounds in which one of the atom types is weakly bound in an oversized atomic cage. Such an atom will undergo large localized vibrations and will be referred to as a “rattler”. If the characteristic vibration frequency of the rattler is low enough, the rattler can resonantly scatter the acoustic phonons that carry most of the heat in a crystal, which can result in a crystal with a very low lattice thermal conductivity. If most of the electronic conduction takes place via the framework atoms (i.e., electronic bands associated primarily with the atoms that form the cages), then a good thermoelectric material can result. The validity of this concept was first demonstrated in a class of cubic compounds known as filled skutterudites that were shown to have superior thermoelectric properties in the 700-1000 K temperature range [2-5] The thermal conductivity of glasses, κ, is not only very small [6,7], but displays a universal temperature dependence: κ varies as T2 below 1 K, has a plateau in the region from 10-20 K, and is roughly independent of temperature above 100 K. This behavior is qualitatively different from ordinary crystalline behavior, in which κ reaches a maximum between 20 to 50 K and shows no plateau. Although filled skutterudites exhibit a low lattice thermal conductivity, the temperature dependence of κ is still characteristic of a crystalline solid [4] Recently Nolas and co-workers [8,9] reported a cubic crystalline semiconductor, Sr8Ga16Ge30, with good electronic properties, but with a lattice thermal conductivity that exhib
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