Theoretical Study of the Lattice Thermal Conductivity in Ge Framework Semiconductors

Abstract

The lattice thermal conductivity of Ge clathrates is investigated by evaluating the linear response theory heat current correlation functions using molecular dynamics. Clathrate crystals with and without guest atoms in their fullerane cages are studied. In comparison with that of diamond-phase Ge, the clathrate conductivity is reduced by approximately 1 order of magnitude due to the open framework itself. The addition of an encapsulated (rattling) Sr guest atom produces a further order of magnitude reduction in the conductivity, making it comparable to that of amorphous Ge. Our results are consistent with experiments, and have impact on the search for improved thermoelectric materials.

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

“…At 100 K and above (not shown), κ L (T ) for BGS is roughly half that of BGG (about 1 and 2 W/m K respectively). This can be understood as a consequence of three main factors: (i) if the rattling of the guest ions is the main contributor to the unusually high phonon scattering level in these materials, 24 the larger cage size in BGS leads to larger rattling of the guest ions; (ii) in the BGS unit cell all 8 guest ions vibrate with equal intensity (single crystallographic site for Ba in the Type-VIII clathrate structure), while in BGG only the 6 guest ions inside the X 24 cages show large rattling; and (iii) the heavier Sn atoms produce lower frequency phonons, which are more easily scattered. Below 100 K, each sample behaves quite differently, depending more on the carrier type than on the compound.…”

A study of low-energy guest phonon modes in clathrate-II NaxSi136(x= 3, 23, and 24)

“…These modes exhibit very little dispersion [3] and fulfill the criteria for localized modes, and can efficiently impede the lattice thermal conduction in these materials [4] through strong interaction with the heat carrying phonons.…”

Les excitations dans la matière condensée : vibrations et phonons

“…Nous nous appliquerons ici à donner un exemple simple pour le couplage des particules par interaction directe. 13 Cela nous permettra d'introduire le concept de modes normaux, qui plus tard sera essentiel pour décrire la dynamique du réseau. Soient x 1 et x 2 les positions de deux atomes de masse m assujetis à bouger dans une direction.…”

“…Le paquet d'onde lui-même doit être plus étendu que la maille cristalline. 13 Une présentation très pédagogique est donnée dans l'article de Johnson et Gutierrez [10]. Figure 6.…”