Susan E. Latturner

Band structures and thermoelectric properties of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30

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; a...

Structure and stability of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30

We use density functional calculations and single-crystal x-ray diffraction measurements to study structure and bonding in the solid state clathrates Ba8Ga16Ge30, Ba8Ga16Si30, Sr8Ga16Ge30, and Ba8In16Sn30. 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 guest) within the cages is facile, with estimated frequencies for vibration ranging from 40 to 100 cm−1. While these results may suggest a weak guest-frame bond, we find that the binding energy is over 4 eV per guest. We...

Spin Glass Behavior of Isolated, Geometrically Frustrated Tetrahedra of Iron Atoms in the Intermetallic La21Fe8Sn7C12

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.

Ruthenium intermetallics grown from La-Ni flux: synthesis, structure, and physical properties.

Crystals of three new intermetallic compounds were grown from reactions of ruthenium with other elements in La(0.8)Ni(0.2) eutectic flux. The new boride LaRu(2)Al(2)B crystallizes in a filled CeMg(2)Si(2) structure type (P4/mmm, a = 4.2105(5) A, c = 5.6613(8); Z = 1, R(1) = 0.014), with Ru atoms forming a planar square net; B atoms center alternating Ru squares, which is an unusual coordination of boron by transition metals. Al atoms connect the Ru(2)B layers, forming large voids where La ions reside. The chemical bonding analysis using the electron localization function (ELF) reveals two-center covalent bonding between Al atoms, an absence of direct Ru-Ru interactions, and three-centered bonds between Ru and B or Al atoms. The band structure calculation shows LaRu(2)Al(2)B to be metallic, which is in agreement with the observed temperature independent paramagnetism and heat capacity data. The crystal structure of La(2)Ni(2-x)Ru(x)Al (HT-Pr(2)Co(2)Al-type; x = 0.21(1) and x = 0.76(1); C2/c; a = 9.9001(3) A, b = 5.7353(1) A, c = 7.8452(2) A, beta = 104.275(1); Z = 4, R(1) = 0.016 for x = 0.76(1)) features infinite [Ni(2-x)Ru(x)Al] spiral-twisted chains composed of Al(2)M(2)-rhombic units (M = Ni/Ru) seen in many La-Ni-Al intermetallics. The structure of La(6)SnNi(3.67)Ru(0.76)Al(3.6) (Nd(6)Co(5)Ge(2.2)-type; P6m2, a = 9.620(1) A, c = 4.2767(9) A; Z = 1, R(1) = 0.015) is composed of a three-dimensional [Ni(3.67)Ru(0.76)Al(3.6)](3)(infinity) network with large hexagonal channels accommodating interconnected tin-centered lanthanum clusters Sn@La(9).

Ca2LiC3H: a new complex carbide hydride phase grown in metal flux.

The reaction of carbon and CaH2 in a calcium/lithium flux mixture produces crystals of the new compound Ca2LiC3H. This phase forms with a new structure type in tetragonal space group P4/mbm (a = 6.8236(1) Å, c = 3.7518(1) Å, Z = 2, R1 = 0.0151). This is a stuffed variant of the Cs2(NH2)N3 structure, containing hydride anions in octahedral sites; the structure determination by single-crystal X-ray diffraction surprisingly allowed the hydrogen to be detected. The Ca2LiC3H structure also features the rarely seen C3(4-) carbide anion; the protolysis reaction of this compound with ammonium chloride produces C3H4. The electronic properties of Ca2LiC3H were studied by quantum-chemical calculations including band structure and electron localization function (ELF) analysis; the phase is a charge-balanced semiconductor with a calculated band gap of 0.48 eV. This is in agreement with (7)Li, (13)C, and (1)H MAS NMR data, which show resonances in the ionic region instead of the Knight shifted region. ELF analysis of the theoretical nonhydrided Ca2LiC3 structure confirms the ability of these calculations to properly locate hydrides and supports the structural model based on X-ray diffraction data.

Molten Salt Synthesis and Structural Characterization of Novel Salt-Inclusion Vanadium Bronze Cs5FeV5O13Cl6

Single crystals of a new reduced vanadate phase, Cs(5)FeV(5)O(13)Cl(6), have been grown from the reaction of metal oxides V(2)O(5) and Fe(2)O(3) in the presence of a metal reducing agent in a eutectic CsCl/NaCl flux. This compound adopts a tetragonal structure (P4/nmm, a = 10.943(3) A, c = 10.535(4) A, Z = 2) that consists of reduced vanadate layers separated by ionic layers comprised of [FeCl(6)](3-) anions and Cs(+) cations. There are two distinct vanadium sites in the structure of this compound; V(4+) is in square pyramidal configuration, and V(5+) has a tetrahedral coordination environment. The (51)V NMR Knight shift and the magnetic susceptibility data indicate the delocalization of the unpaired electron of vanadium. Ferrimagnetic ordering is observed at 5 K.

new ternary aluminides grown from aluminum flux

Abstract A series of ternary aluminide intermetallics with a new structure type were formed from the reaction of gold and rare-earth metals in aluminum flux. The REAu3Al7 structure was obtained with all rare earths RE with the exception of La and Eu. These materials crystallize in the rhombohedral space group R-3c, with unit cell parameters a=8.0922(6) A and c=21.066(2) A for PrAu3Al7 as an example. The variation in cell edges with rare-earth size is regular with the exception of the Yb analogue. The possible mixed valency indicated by this result was confirmed by magnetic susceptibility measurements. Density functional theory-based band structure calculations on YbAu3Al7 indicate the ytterbium f-orbitals are located just below the Fermi level, further supporting the mixed valence description of this material.

RE(AuAl2)nAl2(AuxSi1−x)2: A New Homologous Series of Quaternary Intermetallics Grown from Aluminum Flux

The combination of early rare earth metals (La- to Gd and Yb), gold, and silicon in molten aluminum results in the formation of intermetallic compounds with four related structures, forming a new homologous series: RE[AuAl2]nAl2(AuxSi(1-x))2, with x approximately 0.5 for most of the compound and n = 0, 1, 2, and 3. Because of the highly reducing nature of the Al flux, rare earth oxides instead of metals can also be used in these reactions. These compounds grow as large plate-like crystals and have tetragonal structure types that can be viewed as intergrowths of the BaAl4 structure and antifluorite-type AuAl2 layers. REAuAl2Si materials form with the BaAl4 structure type in space group I4/mmm (cell parameters for the La analogue are a = 4.322(2) A, c = 10.750(4) A, and Z = 2). REAu2Al4Si forms in a new ordered superstructure of the KCu4S3 structure type, with space group P4/nmm and cell parameters of the La analogue of a = 6.0973(6) A, c = 8.206(1) A, and Z = 2. REAu3Al6Si forms in a new I4/mmm symmetry structure type with cell parameters of a = 4.2733(7) A, c = 22.582(5) A, and Z = 2 for RE = Eu. The end member of the series, REAu4Al8Si, forms in space group P4/mmm with cell parameters for the Yb analogue of a = 4.2294(4) A, c = 14.422(2) A, and Z = 1. New intergrowth structures containing two different kinds of AuAl2 layers were also observed. The magnetic behavior of all these compounds is derived from the RE ions. Comparison of the susceptibility data for the europium compounds indicates a switch from 3-D magnetic interactions to 2-D interactions as the size of the AuAl2 layer increases. The Yb ions in YbAu(2.91)Al(6)Si(1.09) and YbAu(3.86)Al(8)Si(1.14) are divalent at high temperatures.

A tale of two metals: new cerium iron borocarbide intermetallics grown from rare-earth/transition metal eutectic fluxes.

R(33)Fe(14-x)Al(x+y)B(25-y)C(34) (R = La or Ce; x ≤ 0.9; y ≤ 0.2) and R(33)Fe(13-x)Al(x)B(18)C(34) (R = Ce or Pr; x < 0.1) were synthesized from reactions of iron with boron, carbon, and aluminum in R-T eutectic fluxes (T = Fe, Co, or Ni). These phases crystallize in the cubic space group Im3m (a = 14.617(1) Å, Z = 2, R(1) = 0.0155 for Ce(33)Fe(13.1)Al(1.1)B(24.8)C(34), and a = 14.246(8) Å, Z = 2, R(1) = 0.0142 for Ce(33)Fe(13)B(18)C(34)). Their structures can be described as body-centered cubic arrays of large Fe(13) or Fe(14) clusters which are capped by borocarbide chains and surrounded by rare earth cations. The magnetic behavior of the cerium-containing analogs is complicated by the possibility of two valence states for cerium and possible presence of magnetic moments on the iron sites. Temperature-dependent magnetic susceptibility measurements and Mössbauer data show that the boron-centered Fe(14) clusters in Ce(33)Fe(14-x)Al(x+y)B(25-y)C(34) are not magnetic. X-ray photoelectron spectroscopy data indicate that the cerium is trivalent at room temperature, but the temperature dependence of the resistivity and the magnetic susceptibility data suggest Ce(3+/4+) valence fluctuation beginning at 120 K. Bond length analysis and XPS studies of Ce(33)Fe(13)B(18)C(34) indicate the cerium in this phase is tetravalent, and the observed magnetic ordering at T(C) = 180 K is due to magnetic moments on the Fe(13) clusters.

Flux Growth and Magnetoresistance Behavior of Rare Earth Zintl Phase EuMgSn

Reactions of europium and tin in 1:1 Mg/Al mixed flux produce large crystals of EuMgSn. This phase crystallizes with the TiNiSi structure type in orthorhombic space group Pnma (a = 8.0849(7) Å, b = 4.8517(4) Å, c = 8.7504(8) Å, Z = 4, R1 = 0.0137). The crystal structure features europium cations positioned between puckered hexagonal layers comprised of magnesium and tin atoms. Magnetic susceptibility measurements indicate the europium in this phase is divalent, which suggests that the compound is possibly valence-balanced as Eu(2+)Mg(2+)Sn(4-). However, EuMgSn is a metal as indicated by density of states calculations and electrical resistivity behavior. This phase exhibits antiferromagnetic ordering at T(N) = 10.9 K at low field (100 G) and the ordering temperature decreases when a higher magnetic field is applied. ac magnetization and field dependence of resistivity at 4.2 K reveal that there is a spin reorientation at 2 T, in agreement with the metamagnetic transition shown in the dc magnetization versus field data. Temperature dependence of resistivity at 2.5 T indicates that EuMgSn has a large magnetoresistance up to -30% near its magnetic ordering temperature.