Abstract
Crystalline solids are generally known as excellent heat conductors, amorphous materials or glasses as thermal insulators. It has thus come as a surprise that certain crystal structures defy this paradigm. A prominent example are type-I clathrates and other materials with guest-host structures. They sustain low-energy Einstein-like modes in their phonon spectra, but are also prone to various types of disorder and phonon-electron scattering and thus the mechanism responsible for their ultralow thermal conductivities has remained elusive. Our thermodynamic and transport measurements on various clathrate single crystal series and their comparison with ab initio simulations reveal an all phononic Kondo effect as origin. This insight devises design strategies to further suppress the thermal conductivity of clathrates and other related materials classes, with relevance for thermoelectric waste heat recovery and, more generally, phononic applications. It may also trigger theoretical work on strong correlation effects in phonon systems.
Highlights
Crystalline solids are generally known as excellent heat conductors, amorphous materials or glasses as thermal insulators
A second key ingredient for our work are sample series of single crystals, which allow us to pin down the dominating instrinsic phonon scattering mechanisms by employing an empirical model[20], modified to be in line with the most recent findings[21,23] on the phonon band structure and phonon dynamics in type-I clathrates
The phonon density of states (DOS) and the specific heat are calculated in the harmonic approximation, the thermal expansion in the quasi-harmonic approximation, and the thermal conductivity from anharmonic interatomic force constants[23]
Summary
Crystalline solids are generally known as excellent heat conductors, amorphous materials or glasses as thermal insulators. Our thermodynamic and transport measurements on various clathrate single crystal series and their comparison with ab initio simulations reveal an all phononic Kondo effect as origin This insight devises design strategies to further suppress the thermal conductivity of clathrates and other related materials classes, with relevance for thermoelectric waste heat recovery and, more generally, phononic applications. 16,17), inconsistencies with other results have been pointed out more recently Such modeling can neither explain the exceptionally long phonon lifetimes[12,21] nor the large thermal conductivity differences between structurally very similar n- and p-type versions of these materials[22]. The recent achievement of ab initio calculations of the thermal conductivity of a type-I clathrate, based on intrinsic phonon-phonon Umklapp scattering processes[23], represents a major step forward. To the best of our knowledge, it has never been studied in an all phononic context
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