Possible mechanism for glass-like thermal conductivities in crystals with off-center atoms

Abstract

In the filled Ga/ Ge clathrate, Eu and Sr are off center in site 2 but Ba is on center. All three filler atoms ͑Ba, Eu, Sr͒ have low-temperature Einstein modes; yet only for the Eu and Sr systems is there a large dip in the thermal conductivity, attributed to the Einstein modes. No dip is observed for Ba. Here we argue that it is the off-center displacement that is crucial for understanding this unexplained difference in behavior. It enhances the coupling between the "rattler" motion and the lattice phonons for the Eu and Sr systems, and turns on/off another scattering mechanism (for 1 K Ͻ T Ͻ 20 K) produced by the presence/absence of off-center sites. The random occupation of different off-center sites produces a high density of symmetry-breaking defects which scatter phonons. It may also be important for improving our understanding of other glassy systems.

Universal Relation between Guest Free Space and Lattice Thermal Conductivity Reduction by Anharmonic Rattling in Type-I Clathrates

“…This is consistent with the increase of anharmonicity of the guest vibration. 22,28 Despite having similar Einstein temperatures, the peak height for M 0 is higher than for SGG because of the smaller Debye temperature for M 0 (Table II) which leads to a higher "background level" (blue dashed lines in Fig. 4).…”

“…The majority of previous works on type-I clathrates has focused on the low value and glass-like temperature dependence of κ L , whose origin is thought to be acoustic phonon scattering by "rattling" effects, i.e., low-energy guest vibrations. [22][23][24][25][27][28][29][30] Among type-I clathrates, Ba 8 In 16 Ge 30 shows one of the lower values of κ L = 0.9 W/m K at 300 K. 31 In this work, we report a partial control of the cage size of this compound by substituting Si and Sn for Ge, to clarify the relation between the free space available for the guest atom in the tetrakaidecahedron, its vibration energy, and the lattice thermal conductivity. …”

Synthesis, Crystal Structure, and Transport Properties of Na22Si136

“…The off-centering of the guest atoms in type I clathrates results in unique phenomena in these compounds. 7,35,36 Further detailed analysis for a series of Na x Si 136 (0< x < 24) will be reported elsewhere.…”

Glasslike versus crystalline thermal conductivity in carrier-tunedBa8Ga16X30clathrates(X<

“…Still, good arguments were made by the authors in terms of describing how off-center vibration does indeed help enhance the coupling between guest vibration modes and the framework phonons. 13,14 A second challenge to the TRR model is: can the shift from crystalline to glasslike behavior be explained solely by phonon scattering mechanisms within the framework, i.e., by interactions between phonons and charge carriers? This question was raised in a series of papers by Bentien et al 15,16,18 which we now discuss.…”

“…One proposes that the tunnelling states are not required, only an off-center vibration of the guest ions, 13,14 and another proposes that the guest ions don't play a major role at all at low temperatures, but rather it is the phonon scattering on charge carriers that leads to the glasslike behavior. 15,16,17,18 In this work we address the issue by performing single crystal x-ray diffraction (SCXRD), thermal conductivity κ(T ) and heat capacity C p (T ) experiments on Ba 8 Ga 16 Sn 30 (BGS) and Ba 8 Ga 16 Ge 30 (BGG) crystals, which have been tuned through the crystal growth process to display n-type or p-type majority charge carriers as a result of small imbalances in their Ga:Ge or Ga:Sn ratios.…”