The material class of skutterudites is believed to have strong potential for thermoelectric application due to the very low thermal conductivity of the filled structures. It is generally assumed that the atoms filling the skutterudite cages act as 'rattlers' and essentially induce a disordered lattice dynamics referred to as 'phonon glass'. Here, we present neutron spectroscopy experiments and ab initio computational work on phonons in LaFe(4)Sb(12) and CeFe(4)Sb(12). Our results give unequivocal evidence of essentially temperature-independent lattice dynamics with well-defined phase relations between guest and host dynamics, indicative of a quasi-harmonic coupling between the guests and the host lattice. These conclusions are in disagreement with the 'phonon glass' paradigm based on individual 'rattling' of the guest atoms. These findings should have an essential impact on the design and improvement of thermoelectric materials and on the development of microscopic models needed for these efforts.
Lyotropic lamellar phases occur naturally and are a key architectural feature for life to develop as they enable the formation of closed-cell topologies. [1] But in addition to closed-cell topologies, enabling life means that the same solvent must be on both sides of the cell membrane, hence at least a double-layered membrane structure is necessary. For this a lamellar phase must be enabled. Herein we show that the formation of lamellar phases is not exclusive to alkylchain-based surfactants with a well-defined amphiphilic structure but that it can also be obtained with metallacarborane clusters, described previously as q-shaped amphiphiles. [2] Similarly to phospholipid cell membranes the lamellae formed can exist both in the liquid and in the solid states depending on temperature. The determination of the 2D molecular arrangement in the lamella demonstrated that the formation of intermolecular dihydrogen bonds, such as -C-H d+ ··· dÀ H-B-, is the driving force in the lamella self-assembly process. Compared to the common bilayer structure that originates from the hydrophobic effect, [3] q-shaped amphiphiles form lamellae with a peculiar monomolecular structure reminiscent of lamellar sheets observed in inorganic layered systems. [4] Nano-scale ordering of planar organic-inorganic hybrid sheets is controlled by temperature and concentration through a self-assembly process.The lyotropic lamellar phase, characterized by an elementary smectic liquid-crystal symmetry, is by far the most common surfactant mesophase. [5] A vast literature can be found on the topic as it shows practical applications in many different fields, such as in detergents, pharmaceutics, [6] food, [7] or materials synthesis as templates. [8] Its mesostructure consists of parallel stacks of bilayers, the structural unit of biological membranes, separated by water layers. To distinguish between molten and frozen states of the surfactant alkyl chains, lamellar phases are referred to as L a and L b (or "gel phase"). [9] Therefore the surfactant has a liquid-like mobility in L a whereas chain motions are highly restricted in L b , mostly limited to rotation about the chain axis as is the case in rotator phases formed in long-chain alkanes. Surfactants in the bilayers have mostly an all-trans alkyl-chain conformation with possible chain inter-digitation or chain tilt in the case of L b . Lyotropic lamellar phases have only been observed with molecules that have a well-defined amphiphilic character.Herein, we show that metallabis(dicarbollide derivatives), [10] large anions with amphiphilic properties, form lyotropic lamellar phases at high concentrations in water. In previous studies, the surfactant-like properties of cobaltabis(dicarbollide) anion ([COSAN] À with H + as the counterion) were highlighted. [2,11] Even though the central region of [COSAN] À around the cobalt atom is more polar (and locally charged) than its two extremities, [12] H[COSAN] (Figure 1) does not show a classical amphiphilic structure and was therefore named q-shaped amphip...
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