We use density functional calculations and single-crystal x-ray diffraction measurements to study structure and bonding in the solid state clathrates Ba 8 Ga 16 Ge 30 , Ba 8 Ga 16 Si 30 , Sr 8 Ga 16 Ge 30 , and Ba 8 In 16 Sn 30 . The structures calculated by minimizing the energy provided by the density functional theory agree well with those determined by x-ray scattering. The preferred stoichiometry is found to always have 8 group II, 16 group III, and 30 group IV elements. The resultant structures are shown to be substantially more stable than the constituent elements in their standard states at room temperature and pressure. Calculations show that the group III elements prefer to be located in the six rings of the structure and are distributed to avoid bonding to one another. Motion of the group II atom ͑the guest͒ within the cages is facile, with estimated frequencies for vibration ranging from 40 to 100 cm Ϫ1 . While these results may suggest a weak guest-frame bond, we find that the binding energy is over 4 eV per guest. We demonstrate that the formation of A 8 B 16 C 30 from A 8 and B 16 C 30 takes place through the donation of 16 electrons ͑per unit cell͒ from the bands of A 8 into the empty bands of B 16 C 30 . The guest atoms are thus charge donors. However, the spatial charge distribution of the eight donor orbitals of A 8 is found to be very similar to that of the eight acceptor orbitals of B 16 C 30 . Thus while the guest is an electron donor, it is not ionic in these materials.

Metal flux synthesis in a low-melting eutectic mixture of lanthanum and nickel has produced a family of complex intermetallic carbide phases. La(21)Fe(8)M(7)C(12) (M = Sn, Bi, Sb, Te, Ge) has a new cubic structure featuring tetrahedra of iron atoms capped with carbon on each edge. These tetrahedra are surrounded by a La/M framework and are therefore isolated from each other. The antiferromagnetic coupling of the iron atoms is frustrated by their ideal tetrahedral arrangement; this is evidenced by magnetic susceptibility measurements on the La(21)Fe(8)Sn(7)C(12) analogue. Deviations from Curie-Weiss behavior begin at 100 K; variation in field-cooled vs zero-field-cooled behavior is seen at 5 K indicative of magnetic ordering. AC susceptibility data indicate that the temperature of this transition is frequency-dependent, behavior characteristic of spin glass systems.

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Reactions of bismuth in sulfur/iodine flux mixtures were explored at various temperatures and iodine concentrations. The observed products include Bi2S3, BiSI, and Bi13S18I2. The latter compound, formerly reported as “Bi19I3S27” grows as well formed needles from the flux. This enabled extensive crystallographic studies by single crystal XRD and powder synchrotron XRD. These data allow a more accurate assignment of the disordered bismuth sites in the structure, indicating formation of subvalent Bi2 4+ dimers; this results in a stoichiometry of Bi13S18I2. Thermal decomposition studies and Raman spectroscopy support this structural model, and electronic structure calculations and optical reflectance studies indicate this compound is an indirect band gap semiconductor with band gap of 0.3 eV.

That guest atoms donate electrons to framework atoms is a generally accepted concept. Nevertheless, evidence for the presence of neutral guest atoms in thermoelectric clathrate structures is presented in the form of experimental X‐ray charge density analysis and X‐ray absorption near‐edge structure (XANES) data. The picture shows the deformation density (DD) determined by the maximum entropy method for Ba8Ga16Ge30; the DD shows that both the Ba guest atoms are disordered.

Gd(1.33)Pt(3)Al(8) was synthesized by the combination of Gd and Pt in excess liquid aluminum. Addition of silicon resulted in the incorporation of a small amount of this element into the material to form the isostructural Gd(1.33)Pt(3)Al(7)Si. Both compounds grow as rodlike crystals with hexagonal cross section. The structures were refined in the rhombohedral space group R(-)3m, with cell parameters a = 4.3359(6) A and c = 38.702(8) A for the ternary and a = 4.3280(8) A and c = 38.62(1) A for the quaternary compound. The structure is comprised of stuffed arsenic-like PtAl(2) layers and disordered Gd/Al layers. Analysis of the hk0 zone reflections indicate the presence of an a = radical 3a supercell, but the structure is not ordered along c, as revealed by the highly diffuse reflections in the 0kl zone photos. Therefore, the compounds are disordered variants of the Gd(4)Pt(9)Al(24) type. Magnetic susceptibility studies reveal antiferromagnetic transitions at 15 K for the ternary and 7 K for the quaternary compound. Variation of the reactant ratio produces a different structure comprised of the same structural blocks, including the disordered Gd/Al layer. Gd(0.67)Pt(2)Al(5) and its quaternary analogue Gd(0.67)Pt(2)Al(4)Si form in the hexagonal system P6(3)/mmc with cell parameters a = 4.2907(3) A and c = 16.388(2) A for the ternary and a = 4.2485(6) A and c = 16.156(3) A for the quaternary compound.

Density functional calculations in the generalized gradient approximation are used to study the transport properties of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30. The band structures of these clathrates indicate that they are all semiconductors. Seebeck coefficients, conductivities and Hall coefficients are calculated, to assess the effects of carrier concentration on the quantity S2σ/τ (where S is the Seebeck coefficient, σ is the conductivity, and τ the electron relaxation time) which is proportional to the thermoelectric power factor. In each compound we find that both p- and n-doping will significantly enhance the thermoelectric capabilities of these compounds. For p-doping, the power factors of all four clathrates are of comparable magnitude and have similar temperature dependence, while for n-doping we see significant variations from compound to compound. We estimate that room-temperature ZT values of 0.5 may be possible for optimally n-doped Sr8Ga16Ge30 or Ba8In16Sn30; at 800 K ZT values as large as 1.7 may be possible. For single crystals of high quality, with substantially increased scattering times, the power factor of these materials will be significantly higher. Recent experiments are reviewed in the light of these calculations.

Single crystals of CaMgSi were produced using the metal flux synthesis method in a Mg/Al 1:1 mixture. The large rod-shaped crystals measure up to 7 mm in length. This phase crystallizes with the orthorhombic TiNiSi structure type (space group Pnma; a = 7.4752(2) Å, b = 4.42720(10) Å, c = 8.3149(2) Å; R 1 = 0.021). Despite its relationship to semiconducting Zintl phases Mg2Si and Ca2Si, CaMgSi is metallic at room temperature; this produces a positive (∼160 ppm) 29Si MAS NMR chemical shift and is supported by DOS calculations. A metal to semimetal electronic transition at around 50 K is evident in the resistivity, magnetic susceptibility, and electron paramagnetic resonance measurements. Low temperature powder X-ray diffraction data indicates that a structural distortion accompanies this transition. The electronic heat capacity coefficient (0.4695 mJ/mol·K2) determined from low temperature heat capacity data supports the designation of CaMgSi as a semimetal at low temperature. The hydrogen storage capacity of this phase is negligible (≤0.5 wt % hydrogen), although exposure to hydrogen does destabilize the structure, inducing decomposition at 500 °C.