Theoretical investigation of alkali-metal doping in Si clathrates

Low temperature transport properties and heat capacity of single-crystal Na8Si46

The low temperature thermal conductivity, resistivity, and Seebeck coefficient of single-crystal Na(8)Si(46) are investigated revealing the intrinsic low temperature transport properties of this material. Metallic conduction is observed, with a higher residual resistance ratio than any other known type I clathrate. Heat capacity together with thermal conductivity provide insight into the Na disorder inside the polyhedra formed by the Si framework. Single-crystal structural refinement and thermal property analyses reveal anisotropic disorder for Na inside the tetrakaidecahedra due to a reduction in the symmetry inside these polyhedra, unlike that for Na inside the dodecahedra.

Intrinsic electrical and thermal properties from single crystals of Na24Si136

The observation of intrinsic structural, electrical, and thermal properties from measurements on single-crystal specimens of clathrate-II Na24Si136 is reported, revealing metallic conduction in agreement with electronic structure calculations. Low-temperature heat capacity measurements corroborate a substantial electronic density of states at the Fermi level, and reveal an Einstein-like mode that can be attributed to Na guest "rattling". The large thermal conductivity of Na24Si136, compared to literature data for other intermetallic clathrates, can be understood in terms of the predominant electronic contribution for the fully filled Na24Si136 composition.

Binding between endohedral Na atoms in Si clathrate I; a first principles study

We investigate the binding nature of the endohedral sodium atoms with the density functional theory methods, presuming that the clathrate I consists of a sheaf of one-dimensional connections of Na@Si(24) cages interleaved in three perpendicular directions. Each sodium atom loses 30% of the 3s(1) charge to the frame, forming an ionic bond with the cage atoms; the rest of the electron contributes to the covalent bond between the nearest Na atoms. The presumption is proved to be valid; the configuration of the two Na atoms in the nearest Si(24) cages is more stable by 0.189 eV than that in the Si(20) and Si(24) cages. The energy of the beads of the two distorted Na atoms is more stable by 0.104 eV than that of the two infinitely separated Na atoms. The covalent bond explains both the preferential occupancies in the Si(24) cages and the low anisotropic displacement parameters of the endohedral atoms in the Si(24) cages in the [100] directions of the clathrate I.

Tailoring band gap and hardness by intercalation: an ab initio Study of I(8)@Si-46 and related doped clathrates

We present an ab initio study of the structural and electronic properties of the recently synthesized I(8)@Si-46 clathrate which is shown to be a degenerate p-type doped system. The intercalation significantly opens the band gap to a 1.75 eV value within the density functional theory. We study further the intercalation by other neighboring elements. A quasiparticle study reveals that such systems can display a band gap in the "green-light" energy range. Finally, we show that the bulk modulus can be increased to values equivalent to the one of the diamond phase.

Silicon clathrate with an f-electron system

A novel crystal of Ba6Ce2Au4Si42 with Ba and Ce encapsulated into silicon-polyhedral clusters is self-assembled from the state of elemental mixture. Each atom in the crystal is arranged in its well-defined position with a nanoscale period, causing unique interactions between the conduction and the magnetic electrons originating from the independent sources of Ba and Ce, respectively. In this system, the long-distant magnetic f electrons can interact with each other through nanoscale spacing with isotropic three dimensionality, leading to the occurrence of a unique spontaneous spin ordering at 6.5 K.

Structural principles and amorphouslike thermal conductivity of Na-doped Si clathrates

The postulated low thermal conductivity and the possibility of altering the electronic conductivity of metal-doped clathrates with semiconducting host elements have stimulated great interest in exploring these compounds as promising thermoelectric materials. The electronic and thermal properties of the prototypical Na xSi (46) system are studied in detail here. It is shown that, despite the fact that the Na/Si clathrate is metallic, its thermal conductivity resembles that of an amorphous solid. A theoretical model is developed to rationalize the structural stability of the peculiar structural topology, and a general scheme for rational design of high efficiency thermoelectric materials is presented.