Crystal structures of thermoelectric n- and p-type Ba8Ga16Ge30 studied by single crystal, multitemperature, neutron diffraction, conventional X-ray diffraction and resonant synchrotron X-ray diffraction

Comprehensive single-crystal structural investigations of n- and p-type Ba8Ga16Ge30 have been carried out using multitemperature neutron and conventional X-ray diffraction as well as resonant synchrotron X-ray diffraction. The data show that the guest atom positions and dynamics are very similar in the two structures, although the barium atoms are slightly more displaced from the cage centers in the p-type structure than in the n-type structure (Deltad = 0.025 A). For both structures Fourier difference maps calculated from very high-resolution neutron diffraction data (sin theta/lambda > 2 A-1) show that the Ba nuclear density at lowest temperatures (15 K) is distributed in a torus around the crystallographic 6d site with maxima in the 24j positions. At room temperature the maxima have shifted to the 24k position. Analysis of atomic displacement parameters give Einstein temperatures of approximately 60(1) K for both structures. Thus, the fundamental difference in the low temperature thermal conductivity observed for p- and n-type Ba8Ga16Ge30 appear not to be directly related to the guest atom behavior as is commonly assumed in thermoelectric research. The neutron data and the resonant synchrotron X-ray data facilitate refinement of Ga/Ge framework occupancies. The Ga atoms have a clear preference for the 6c site with the preference being somewhat stronger for the n-type structure.     

Synthesis, structure, and high-temperature thermoelectric properties of boron-doped Ba8Al14Si31 clathrate I phases

Single crystals of boron-doped Ba8Al14Si31 clathrate I phase were prepared using Al flux growth. The structure and elemental composition of the samples were characterized by single-crystal and powder X-ray diffraction; elemental analysis; and multinuclear (27)Al, (11)B, and (29)Si solid-state NMR. The samples' compositions of Ba8B0.17Al14Si31, Ba8B0.19Al15Si31, and Ba8B0.32Al14Si31 were consistent with the framework-deficient clathrate I structure Ba8Al(x)Si(42-3/4x)cube(4-1/4x) (x = 14, cube = lattice defect). Solid-state NMR provides further evidence for boron doped into the framework structure. Temperature-dependent resistivity indicates metallic behavior, and the negative Seebeck coefficient indicates that transport processes are dominated by electrons. Thermal conductivity is low, but not significantly lower than that observed in the undoped Ba8Al14Si31 prepared in the same manner.     

Structural Principles and Thermoelectric Properties of Polytypic Group 14 Clathrate‐II Frameworks

We have investigated the structural principles and thermoelectric properties of polytypic group 14 clathrate‐II frameworks using quantum chemical methods. The experimentally known cubic 3C polytype was found to be the energetically most favorable framework, but the studied hexagonal polytypes (2 H, 4 H, 6 H, 8 H, 10 H) lie energetically close to it. In the case of germanium, the energy difference between the 3C and 6H clathrate‐II polytypes is ten times smaller than the difference between the experimentally known 3C‐Ge (α‐Ge) and 4H‐Ge polytypes. The thermoelectric properties of guest‐occupied clathrate‐II structures were investigated for compositions Na–Rb–Ga–Ge and Ge–As–I. The clathrate‐II structures show promising thermoelectric properties and the highest Seebeck coefficients and thermoelectric power factors were predicted for the 3C polytype. The structural anisotropy of the largest studied hexagonal polytypes affects their thermoelectric power factors by over a factor of two.     

Structure and thermoelectric characterization of Ba8Al14Si31

A molten Al flux method was used to grow single crystals of the type I clathrate compound Ba8Al14Si31. Single-crystal neutron diffraction data for Ba8Al14Si31 were collected at room temperature using the SCD instrument at the Intense Pulsed Neutron Source, Argonne National Laboratory. Single-crystal neutron diffraction of Ba8Al14Si31 confirms that the Al partially occupies all of the framework sites (R1 = 0.0435, wR2 = 0.0687). Stoichiometry was determined by electron microprobe analysis, density measurements, and neutron diffraction analysis. Solid-state (27)Al NMR provides additional evidence for site preferences within the framework. This phase is best described as a framework-deficient solid solution Ba8Al14Si31, with the general formula, Ba(8)Al(x)Si(42-3/4x)[](4-1/4x) ([] indicates lattice defects). DSC measurements and powder X-ray diffraction data indicate that this is a congruently melting phase at 1416 K. Temperature-dependent resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers.     

Structural and spectroscopic properties of pure and doped Ba6Ti2Nb8O30 tungsten bronze

Pure and doped Ba(6)Ti(2)Nb(8)O(30) (BTN), obtained by substituting M = Cr, Mn, or Fe on the Ti site (Ba(6)Ti(2-x) M(x)Nb(8)O(30), x = 0.06 and 0.18) and Y and Fe on the Ba and Ti sites, respectively (Ba(6-x)Y(x)Ti(2-x)Fe(x)Nb(8)O(30), x= 0.18), are synthesized. The influence of cation doping on the local structure, the cation oxidation state, and the possible defect formation able to maintain the charge neutrality are investigated by spectroscopic (electron paramagnetic resonance (EPR) and micro-Raman), structural (X-ray powder diffraction) and transport (impedance spectroscopy, thermoelectric power) measurements, in the temperature range of 300-1200 K in air and N(2) flow. Starting from the valence state of the doping ions (Fe(3+), Cr(3+), and Mn(2+)), determined by EPR, and from thermoelectric power measurements, evidencing a negative charge transport, different charge-compensating defect equilibria, based on the creation of positive electron holes or oxygen vacancies and electrons, are discussed to interpret the conductivity results.     

Type-I clathrate Ba8Ni(x)Si(46-x): phase relations, crystal chemistry and thermoelectric properties

The phase relations, crystal structure and thermoelectric properties of the type-I solid solution Ba(8)Ni(x)Si(46-x) were investigated. Based on X-ray diffraction, differential thermal analysis and electron probe microanalysis data, a partial phase diagram was constructed for the Si-rich part of ternary system Ba-Ni-Si at 800 °C. The solubility range of Ni in the clathrate-I phase at 800 °C was determined (2.9 ≤x≤ 3.8) and thermoelectric properties, namely electrical resistivity, Seebeck-coefficient and thermal conductivity, were measured in the temperature range from 300 to 850 K. A shift of the thermoelectric properties from a predominantly metallic to a more semiconducting behavior was observed for an increasing Ni-content. Density functional calculations revealed a significant decrease of the gap width in the density of states induced by the incorporation of Ni. Electrical resistivity and Seebeck coefficients for Ba(8)Ni(x)Si(46-x) with 3.3 ≤x≤ 3.8 have been modeled within the rigid band approximation.     

Thermoelectric properties of single- and polycrystalline RuGa3

The thermoelectric properties of the intermetallic semiconductor RuGa₃ are investigated. Large single crystals were grown to study intrinsic properties. To investigate the influence of grain boundaries in this system, the single crystals were ground to powder and densified using spark plasma sintering treatment. The initial chemical composition is maintained with the introduction of grain boundaries. Electrical resistivity data show semiconducting behavior for single- and polycrystalline samples. The high thermal conductivity (>500 W K⁻¹ m⁻¹) obtained for single crystals at low temperatures is reduced by a factor of 10 in the polycrystalline specimen. The thermopower shows a change between n-type and p-type behavior with a sharp minimum of about −700 μV K⁻¹ at 38 K for single crystals, which is completely suppressed by the introduction of grain boundaries. A comparison with RuIn₃ shows the potential of RuGa₃ as a thermoelectric material in its single- and polycrystalline form.     

Structure and thermoelectric characterization of AxBa8-xAl14Si31 (A = Sr, Eu) single crystals

Single crystals of AxBa8-xAl14Si31 (A = Sr, Eu) were grown using a molten Al flux technique. Single-crystal X-ray diffraction confirms that AxBa8-xAl14Si31 (A = Sr, Eu) crystallize with the type I clathrate structure, and phase purity was determined with powder X-ray diffraction. Stoichiometry was determined to be Sr0.7Ba7.3Al14Si31 and Eu0.3Ba7.7Al14Si31 by electron microprobe analysis. These AxBa8-xAl14Si31 phases can be described as framework-deficient clathrate type I structures with the general formula, AxBa8-xAlySi42-3y/4[]4-1/4y. DSC measurements indicate that these phases melt congruently at 1413 and 1415 K for Sr0.7Ba7.3Al14Si31 and Eu0.3Ba7.7Al14Si31, respectively. Temperature-dependent resistivity indicates metallic behavior, and the negative Seebeck coefficient indicates transport processes dominated by electrons as carriers. Thermal conductivity of these phases remains low with Sr0.7Ba7.3Al14Si31 having the lowest values.     

Nanostructured Bi(2-x)Cu(x)S3 bulk materials with enhanced thermoelectric performance

Nanostructured Bi(2-x)Cu(x)S(3) (x = 0, 0.002, 0.005, 0.007, 0.01, 0.03) thermoelectric polycrystals were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) methods. The effect of Cu content on the microstructure and thermoelectric property of Bi(2-x)Cu(x)S(3) bulk samples was investigated. It was found that the subtle tailoring of Cu content could reduce both the electrical resistivity and the thermal conductivity at the same time, and consequently enhancing the thermoelectric property. A low electrical resistivity of 1.34 × 10(-4)Ω m(-1) and a low thermal conductivity of 0.52 W m(-1) K(-1) were obtained for the Bi(1.995)Cu(0.005)S(3) sample at 573 K. The low thermal conductivity is supposed to be due to the nanoscopic Cu-rich regions embedded in the host matrix. A peak ZT value of 0.34 at 573 K was achieved for the Bi(1.995)Cu(0.005)S(3) composition, which is the highest value in the Bi(2)S(3) system reported so far.     

Crystal structure, characterization and thermoelectric properties of the type-I clathrate Ba8−ySryAl14Si32 (0.6≤y≤1.3) prepared by aluminum flux

The title compound was prepared as single crystals using an aluminum flux technique. Single crystal and powder X-ray diffraction indicate that this composition crystallizes in the clathrate type-I structure, space group Pm3?n. Electron microprobe characterization indicates the composition to be Ba_8−ySr_yAl_14.2(2)Si_31.8(2) (0.77<y<1.3). Single-crystal X-ray diffraction data (90 and 12 K) were refined with the Al content fixed at the microprobe value (12 K data: R₁=0.0233, wR₂=0.0441) on a crystal of compositions Ba. The Sr atom preferentially occupies the 2a position; mixed Al/Si occupancy was found on all framework sites. These refinements are consistent with a fully occupied framework and nearly fully occupied cation guest sites as found by microprobe analysis. Temperature dependent electrical resistivity and thermal conductivity have been measured from room temperature to 1200 K on a hot-pressed pellet. Electrical resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers. Thermal conductivity is between 22 and 25 mW/cm K. The sample shows n-type conductivity with a maximum figure of merit, zT of 0.3 at 1200 K. A single parabolic band model predicts a five-fold increase in zT at 800 K if carrier concentration is lowered.     

Siting of antimony dopants and gallium in Ba(8)Ga(16)Ge(30) clathrates grown from gallium flux

A series of antimony-doped Ba(8)Ga(16)Ge(30) clathrates was grown as large crystals from gallium flux. These compounds form in the cubic space group Pm(-)3n, with the unit cell parameter varying from 10.784(5) to 10.9008(6) A as the amount of GaSb substituting for germanium atoms in the framework is increased. It was found that more antimony than extra gallium was incorporated into the material and that a specific site (the 24k Wyckoff site) was favored by this element. (71)Ga NMR was carried out to determine the siting of gallium; it fills the 6c site preferentially.     

Synthesis, structure, and properties of BaAl2Si2

Single crystals of BaAl2Si2 were grown from an Al molten flux and characterized using single-crystal X-ray diffraction at 10 and 90 K and neutron diffraction at room temperature. BaAl2Si2 crystallizes with the alpha-BaCu2S2 structure type (Pnma), is isostructural with alpha-BaAl2Ge2, and is an open 3D framework compound, where Al and Si form a covalent cagelike network with Ba2+ cations residing in the cages. BaAl2Si2 has a unit cell of a=10.070(3) A, b=4.234(1) A, and c=10.866(3) A, as determined by room-temperature single-crystal neutron diffraction (R1=0.0533, wR2=0.1034). The structure as determined by single-crystal neutron and X-ray diffraction (10 and 90 K) indicates that BaAl2Si2 (Pnma) is strictly isostructural to other (alpha)-BaCu2S2-type structures, requiring site specificity for Al and Si. Unlike BaAl2Ge2, no evidence for an alpha to beta (BaZn2P2-type, I4/mmm) phase transition was observed. This compound shows metallic electronic resistivity and Pauli paramagnetic behavior.     

High-pressure synthesis and superconductivity of a new binary barium germanide BaGe3

A new binary barium germanide BaGe(3) was prepared by high-pressure and high-temperature reactions using a Kawai type multi-anvil press. It crystallizes in a hexagonal unit cell with a = 6.814(1) ?, c = 5.027(8) ?, and V = 202.2(5) ?(3) (the space group P6(3)/mmc, No. 194). The unit cell contains two layers along the c axis composed of Ba atoms and Ge(3) triangular units. The triangular units stack along the c axis to form 1D columns in which the adjacent Ge(3) units turn to opposite directions. The columns, therefore, can be described as the face-sharing stacking of elongated Ge(6) octahedra. Each Ba atom is surrounded by six columns. BaGe(3) is metallic and shows superconductivity at 4.0 K. The band structure calculations revealed that there are four conduction bands mainly composed of Ge 4p and Ba 5d orbitals. From Fermi surface analysis, we confirmed that three of them have a large contribution of Ge 4pz orbitals in the vicinity of the Fermi level and show a simple 1D appearance. The remaining one contains Ge 4px, 4py, and Ba 5d contributions and shows a 2D property.     

Ba3[Ge2B7O16(OH)2](OH)(H2O) and Ba3Ge2B6O16: novel alkaline-earth borogermanates based on two types of polymeric borate units and GeO4 tetrahedra

Two new barium borogermanates with two types of novel structures, namely, Ba(3)[Ge(2)B(7)O(16)(OH)(2)](OH)(H(2)O) and Ba(3)Ge(2)B(6)O(16), have been synthesized by hydrothermal or high-temperature solid-state reactions. They represent the first examples of alkaline-earth borogermanates. Ba(3)[Ge(2)B(7)O(16)(OH)(2)](OH)(H(2)O) crystallized in a polar space group Cc. Its structure features a novel three-dimensional anionic framework composed of [B(7)O(16)(OH)(2)](13-) polyanions that are bridged by Ge atoms with one-dimensional (1D) 10-membered-ring (MR) tunnels along the b axis. The Ba(II) cations, hydroxide ions, and water molecules are located at the above tunnels. Ba(3)Ge(2)B(6)O(16) crystallizes in centrosymmetric space group P1. Its structure exhibits a thick layer composed of circular B(6)O(16) units connected by GeO(4) tetrahedra via corner sharing, forming 1D 4- and 6-MR tunnels along the c axis. Ba1 ions reside in the tunnels of the 6-MRs, whereas Ba2 ions are located at the interlayer space. Both compounds feature new types of topological structures. Second-harmonic-generation (SHG) measurements indicate that Ba(3)[Ge(2)B(7)O(16)(OH)(2)](OH)(H(2)O) displays a weak SHG response of about 0.3 times that of KH(2)PO(4). Optical, thermal stability, and ferroelectric properties as well as theoretical calculations have also been performed.     

Synthesis, crystal structure, and physical properties of the type-I clathrate Ba(8-δ)Ni(x)□(y)Si(46-x-y)

Type-I clathrate phase Ba(8)Ni(x)□(y)Si(46-x-y) (□ = vacancy) was obtained from the elements at 1000 °C with the homogeneity range 2.4 ≤ x ≤ 3.8 and 0 ≤ y ≤ 0.9. In addition, samples with low Ni content (x = 1.4 and 1.6; y = 0) and small Ba deficiency were prepared from the melt by steel-quenching. Compositions were established by microprobe analysis and crystal structure determination. Ba(8-δ)Ni(x)□(y)Si(46-x-y) crystallizes in the space group Pm ?3n (No. 223) with lattice parameter ranging from a = 10.3088(1) ? for Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) to a = 10.2896(1) ? for Ba(8.00(3))Ni(3.82(4))Si(41.33(6)). Single-crystal X-ray diffraction data together with microprobe analysis indicate an increasing number of framework vacancies toward compositions with higher Ni content. For all compositions investigated, Ni K-edge X-ray absorption spectroscopy measurements showed an electronic state close to that of elemental Ni. All samples exhibit metallic-like behavior with moderate thermopower and low thermal conductivity in the temperature range 300-773 K. Samples with compositions Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) and Ba(7.9(1))Ni(1.6(1))Si(44.4(1)) are superconducting with T(c) values of 6.0 and 5.5 K, respectively.     

Soft x-ray spectroscopy of Ba24Ge100: electronic phase transition and Ba-atom rattling

The electronic states of Ba24Ge100 are studied by soft x-ray photoelectron spectroscopy (XPS) at a high-energy photon factory. A large reduction in the density of states (DOS) at the Fermi level is clearly shown before and after the electronic phase transition at 200 K. The changes in the spectrum widths and the fine structures of the core-level Ba 4d spectra give a very reasonable indication of the Ba-atom rattlings in the clathrate polyhedra. On-resonance experiments using the excitation from Ba 3d to 4f levels display that the wave functions of Ba 5d and 6s orbitals give only a small contribution to make a Fermi surface through the hybridization with the Ge20 cluster orbitals. Importantly, reliable values of the DOS at the Fermi level NEF are successfully deduced, using two data sets of DOS obtained from high-resolution XPS and the total magnetic susceptibilities by a superconducting quantum interference device, to be 0.149 and 0.0427 states eV(-1) (Ge atom)(-1) for a high-temperature and for a low-temperature phase.     

New barium cobaltite series Ba(n+1)Co(n)O(3n+3)(Co8O8): intergrowth structure containing perovskite and CdI2-type layers

Single crystals of two new cobaltites, Ba2Co9O14 and Ba3Co10O17, were obtained from the flux of K2CO3 in the temperature range 800-890 degrees C. They crystallize in an intergrowth structure containing perovskite block and CdI2-type layers and can be attributed to the n = 1 and 2 members in a new intergrowth series of cobaltites, Ba(n+1)Co(n)O(3n+3)(Co8O8). Both Ba2Co9O14 and Ba3Co10O17 are metastable and transform into the known 2H-perovskite-related oxides at high temperature.     

Neutron diffraction study of the type I clathrate Ba8Al(x)Si(46-x): site occupancies, cage volumes, and the interaction between the guest and the host framework

Samples with the type I clathrate structure and composition Ba(8)Al(x)Si(46-x), where x = 8, 10, 12, 14, and 15, were examined by neutron powder diffraction at 35 K. The clathrate type I structure contains Ba cations as guests in a framework derived from tetrahedrally coordinated Al/Si atoms. The framework is made up of five- and six-membered rings that form dodecahedral and tetrakaidecahedral cages. The change in distances between tetrahedral sites across the series is used to develop a model for the mixed Al/Si occupancy observed in the framework. The calculated volumes of the cages that contain the Ba atoms display a linear increase with increasing Al composition. In the smaller dodecahedral cages, the Ba atomic displacement parameter is symmetry constrained to be isotropic for all compositions. In the larger tetrakaidecahedral cages, the anisotropic atomic displacement of the Ba atom depends upon the composition: the displacement is perpendicular (x = 8) and parallel (x = 15) to the six-membered ring. This difference in direction of the displacement parameter is attributed to interaction with the Al in the framework and not to the size of the cage volume as x increases from 8 to 15. The influence of the site occupation of Al in the framework on displacement of the cation at the 6d site is demonstrated.     

Remarkable enhancement in thermoelectric performance of BiCuSeO by Cu deficiencies

A significant enhancement of thermoelectric performance in layered oxyselenides BiCuSeO was achieved. The electrical conductivity and Seebeck coefficient of BiCu(1-x)SeO (x = 0-0.1) indicate that the carriers were introduced in the (Cu(2)Se(2))(2-) layer by Cu deficiencies. The maximum of electrical conductivity is 3 × 10(3) S m(-1) for Bicu(0.975)Seo at 650 °C, much larger than 470 S m(-1) for pristine BiCuSeO. Featured with very low thermal conductivity (～0.5 W m(-1) K(-1)) and a large Seebeck coefficient (+273 μV K(-1)), ZT at 650 °C is significantly increased from 0.50 for pristine BiCuSeO to 0.81 for BiCu(0.975)SeO by introducing Cu deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.