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

Abstract

The present controversy over the origin of glasslike thermal conductivity observed in certain crystalline materials is addressed by studies on single-crystal x-ray diffraction, thermal conductivity κ(T ) and specific heat Cp(T ) of carrier-tuned Ba8Ga16X30 (X = Ge, Sn) clathrates. These crystals show radically different low-temperature κ(T ) behaviors depending on whether their charge carriers are electrons or holes, displaying the usual crystalline peak in the former case and an anomalous glasslike plateau in the latter. In contrast, Cp(T ) above 4 K and the general structural properties are essentially insensitive to carrier tuning. We analyze these combined results within the framework of a Tunneling/Resonant/Rayleigh scatterings model, and conclude that the evolution from crystalline to glasslike κ(T ) is accompanied by an increase both in the effective density of tunnelling states and in the resonant scattering level, while neither one of these contributions can solely account for the observed changes in the full temperature range. This suggests that the most relevant factor which determines crystalline or glasslike behavior is the coupling strength between the guest vibrational modes and the frameworks with different charge carriers.

Intermediate bands in type-II silicon clathrate with Cu and Ag guest atoms

“…2 The distinctive thermal and vibrational properties of filled clathrate, especially the soft phonon modes from guest atoms, 3 have been intensively studied. [4][5][6] The weak interaction between guests and host facilitates tuning electronic, optical and dynamical properties of clathrates; 7 as a result, they exhibit many intriguing physical properties, such as glasslike thermal conductivity, 8 superconductivity in sp 3 covalent bonded solids 9,10 and magnetism. 11,12 Recently, Si clathrate materials have attracted intense research interest for applications including lithium ion batteries, 13 thermoelectrics, 14 photovoltaic (PV) cells and optoelectronics, [15][16][17] etc.…”

Thermal stability and thermoelectric properties of p-type Ba8Ga16Ge30 clathrates

“…Mixing of the elements was performed in a glove box ͑high-purity Ar atmosphere͒. The n-doped Ba 8 Ga 16 Ge 30 sample was prepared in a quartz crucible, whereas an alumina crucible was used during synthesis of the p-doped Ba 8 Ga 16 Ge 30 samples. Following mixing of the elements, the reaction vessel was sealed in a quartz tube, under argon atmosphere.…”

“…After completed synthesis the formed crystals were removed from the crucible. The p-doped Ba 8 Ga 16 Ge 30 crystals were separated from the molten excess gallium with a pincer, and the remaining excess gallium was removed with a brush and hot water. The n-doped Ba 8 Ga 16 Ge 30 crystals were collected by gently crushing the quartz crucible and carefully removing the quartz pieces.…”

“…28 In the case of, e.g., Ba 8 30 , but when high temperature measurements were conducted, loss of gallium from the samples was observed. 36 The stability of p-type Ba 8 Ga 16 Ge 30 and its usefulness in an application can thus be questioned. In this study the thermal stability of p-doped Ba 8 Ga 16 Ge 30 has been investigated in more detail, and the effects of different heat treatments on its TE properties and its stability for device application assessed.…”

“…A large number of clathrate compounds have now been studied, and most of them are n-doped type-I structures, with the most prominent of the type-I clathrates being Ba 8 Ga 16 Ge 30 with a ZT of 1.35 at 900 K. 27 All TE devices need both n-and p-type materials, which should be compatible with each other in terms of, e.g., thermal expansion, and it is therefore beneficial to find p-type clathrates of equal potential and stability. The amount of work published on n-doped clathrates greatly outweighs the work on p-doped clathrates, and the reason for this is that the structure disfavors bonds between trivalent ͑Ga͒ atoms.…”