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Solid solutions of Yb2-xAxCdSb2 (A = Ca, Sr, Eu; x ≤ 1) are of interest for their promising thermoelectric (TE) properties. Of these solid solutions, Yb2-xCaxCdSb2 has end members with different crystal structures. Yb2CdSb2 crystallizes in the polar space group Cmc21, whereas Ca2CdSb2 crystallizes in the centrosymmetric space group Pnma. Other solid solutions, Yb2-xAxCdSb2 (A = Sr, Eu), crystallize in the polar space group for x ≤ 1, and compositions with x ≥ 1 have not been reported. Both structure types are composed of corner-sharing CdSb4 tetrahedra condensed into sheets that differ by the stacking of the layers. Single crystals of the solid solution Yb2-xCaxCdSb2 (x = 0-1) were studied to elucidate the structural transition between the Yb2CdSb2 and Ca2CdSb2 structure types. For x ≤ 1, the structures remain in the polar space group Cmc21. As the Ca content is increased, a positional disorder arises in the intralayer cation sites (Yb2/Ca2) and the Cd site, resulting in inversion of the CdSb4 tetrahedral chain. This phenomenon could be indicative of an intergrowth of the opposing space group. The TE properties of polycrystalline samples of Yb2-xCaxCdSb2 (x ≤ 1) were measured from 300 to 525 K. The lattice thermal conductivity is extremely low (0.3-0.4 W/m·K) and the Seebeck coefficients are high (100-180 μV/K) across the temperature range. First-principles calculations show a minimum in the thermal conductivity for the x = 0.3 composition, in good agreement with experimental data. The low thermal conductivity stems from the acoustic branches being confined to low frequencies and a large number of phonon scattering channels provided by the localized optical branches. The TE quality factor of the Yb1.7A0.3CdSb2 (A = Ca, Sr, Eu) series has been calculated and predicts that the A = Ca and Sr solid solutions may not improve with carrier concentration optimization but that the Eu series is worthy of additional modifications. Overall, the x = 0.3 compositions provide the highest zT because they provide the best electronic properties with the lowest thermal conductivity.

Thermoelectric materials can convert heat into electricity. They are used to generate electricity when other power sources are not available or to increase energy efficiency by recycling waste heat. The Yb21Mn4Sb18 phase was previously shown to have good thermoelectric performance due to its large Seebeck coefficient (∼290 μV K−1) and low thermal conductivity (0.4 W m−1 K−1). These characteristics stem respectively from the unique [Mn4Sb10]22− subunit and the large unit cell/site disorder inherent in this phase. The solid solutions, Yb21Mn4−xCdxSb18 (x = 0, 0.5, 1.0, 1.5) and Yb21−yCayMn4Sb18 (y = 3, 6, 9, 10.5) have been prepared, their structures characterized and thermoelectric properties from room temperature to 800 K measured. A detailed look into the structural disorder for the Cd and Ca solid solutions was performed using synchrotron powder X-ray diffraction and pair distribution function methods and shows that these are highly disordered structures. The substitution of Cd gives rise to more metallic behavior whereas Ca substitution results in high resistivity. As both Cd and Ca are isoelectronic substitutions, the changes in properties are attributed to changes in the electronic structure. Both solid solutions show that the thermal conductivities remain extremely low (∼0.4 W m−1 K−1) and that the Seebeck coefficients remain high (>200 μV K−1). The temperature dependence of the carrier mobility with increased Ca substitution, changing from approximately T−1 to T−0.5, suggests that another scattering mechanism is being introduced. As the bonding changes from polar covalent with Yb to ionic for Ca, polar optical phonon scattering becomes the dominant mechanism. Experimental studies of the Cd solid solutions result in a max zT of ∼1 at 800 K and, more importantly for application purposes, a ZTavg ∼ 0.6 from 300 K to 800 K.

FeAs2−xSex (x = 0.30–1.0) samples were synthesized as phase pure powders by conventional solid-state techniques and as single crystals (x = 0.50) from chemical vapor transport. The composition of the crystals was determined to be Fe1.025(3)As1.55(3)Se0.42(3), crystallizing in the marcasite structure type, Pnnm space group. FeAs2−xSex (0 < x < 1) was found to undergo a marcasite-to-arsenopyrite (P21/c space group) structural phase transition at x ∼ 0.65. The structures are similar, with the marcasite structure best described as a solid solution of As/Se, whereas the arsenopyrite has ordered anion sites. Magnetic susceptibility and thermoelectric property measurements from 300–2 K were performed on single crystals, FeAs1.50Se0.50. Paramagnetic behavior is observed from 300 to 17 K and a Seebeck coefficient of −33 μV K−1, an electrical resistivity of 4.07 mΩ cm, and a very low κl of 0.22 W m−1 K−1 at 300 K are observed. In order to determine the impact of the structural transition on the high-temperature thermoelectric properties, polycrystalline FeAs2−xSex (x = 0.30, 0.75, 0.85, 1.0) samples were consolidated into dense pellets for measurements of thermoelectric properties. The x = 0.85 sample shows the best thermoelectric performance. The electronic structure of FeAsSe was calculated with DFT and transport properties were approximately modeled above 500 K.

BaFe2As2 superconductors doped with Co or Ni were synthesized by heat treatment of the reactive BaH2 precursor and elemental Fe, Co, Ni, and As in a sealed niobium container. The hydride route prov...

Polycrystalline samples of CoAsSb were prepared by annealing a stoichiometric mixture of the elements at 1073 K for 2 weeks. Synchrotron powder X-ray diffraction refinement indicated that CoAsSb adopts arsenopyrite-type structure with space group P21/c. Sb vacancies were observed by both elemental and structural analysis, which indicate CoAsSb0.883 composition. CoAsSb was thermally stable up to 1073 K without structure change but decomposed at 1168 K. Thermoelectric properties were measured from 300 to 1000 K on a dense pellet. Electrical resistivity measurements revealed that CoAsSb is a narrow-band-gap semiconductor. The negative Seebeck coefficient indicated that CoAsSb is an n-type semiconductor, with the maximum value of −132 μV/K at 450 K. The overall thermal conductivity is between 2.9 and 6.0 W/(m K) in the temperature range 300–1000 K, and the maximum value of figure of merit, zT, reaches 0.13 at 750 K. First-principles calculations of the electrical resistivity and Seebeck coefficient confirmed ...

Since the report of high zT in Si95Ge5 there has been significant interest in low Ge alloy compositions for thermoelectric applications. The application of YbP was explored as a means to lower thermal conductivity. A series of 3% phosphorus (P) doped n-type Si95Ge5 (SiGe) alloy was reacted with YbH2 (0, 1, 2%). YbP was formed in the SiGe alloy matrix from the reaction between YbH2 and P during the Spark Plasma Sintering (SPS) process. Thermoelectric property measurements were performed on sintered pellets from room temperature to 1273 K. X-ray diffraction patterns were collected from the ground powder samples and confirmed the main phase possessed diamond structured Si95Ge5 (space group: Fd 3 ¯ m) as well as the presence of YbP (space group: Fm 3 ¯ m). The carrier concentration of the sample was controlled by the amount of YbH2 added, removing some of the phosphorus to form YbP. n-type Si95Ge5 alloy samples with higher YbP amounts showed higher electrical resistivity and lower thermal conductivity attributed to loss of the P dopant. Another composite series of n-type Si95Ge5 with YbP were synthesized with additional P compositions (1, 2%). The thermoelectric properties were characterized from room temperature to 1273 K, and the samples possess electrical resistivity, carrier concentrations, and thermal conductivity as expected from the additional P dopant. The presence of YbP lowered lattice thermal conductivity when the sample was appropriately doped. The Seebeck coefficients were measured with both off-axis and uniaxial axis experimental configurations. These results show that the off-axis measurements overestimate the Seebeck coefficients of the Si95Ge5 alloy samples. This is attributed to a cold finger effect and therefore only the uniaxial data are combined for zT calculations. The n-type Si95Ge5 samples with YbP and less than 3% P dopant show similar zT compared with the Si95Ge5 sample with no YbP inclusions and 3% P dopant with a peak zT of 0.6 at 1200 K.

Yb14ZnSb11 has been of interest for its intermediate valency and possible Kondo designation. It is one of the few transition metal compounds of the Ca14AlSb11 structure type that show metallic behavior. While the solid solution of Yb14Mn1-xZnxSb11 shows an improvement in the high temperature figure of merit of about 10% over Yb14MnSb11, there has been no investigation of optimization of the Zn containing phase. In an effort to expand the possible high temperature p-type thermoelectric materials with this structure type, the rare earth (RE) containing solid solution Yb14-xRExZnSb11 (RE = Y, La) was investigated. The substitution of a small amount of 3+ rare earth (RE) for Yb2+ was employed as a means of optimizing Yb14MnSb11 for use as a thermoelectric material. Yb14ZnSb11 is considered an intermediate valence Kondo system where some percentage of the Yb is formally 3+ and undergoes a reduction to 2+ at ~85 K. The substitution of a 3+ RE element could either replace the Yb3+ or add to the total amount of 3+ RE and provides changes to the electronic states. RE = Y, La were chosen as they represent the two extremes in size as substitutions for Yb: a similar and much larger size RE, respectively, compared with Yb3+. The composition x = 0.5 was chosen as that is the typical amount of RE element that can be substituted into Yb14MnSb11. These two new RE containing compositions show a significant improvement in Seebeck while decreasing thermal conductivity. The addition of RE increases the melting point of Yb14ZnSb11 so that the transport data from 300 K to 1275 K can be collected. The figure of merit is increased five times over that of Yb14ZnSb11 and provides a zT ~0.7 at 1275 K.

Water-soluble poly(allylamine) Mn2+-doped Si (SiMn) nanoparticles (NPs) were prepared and show promise for biologically related applications. The nanoparticles show both strong photoluminescence and good magnetic resonance contrast imaging. The morphology and average diameter were obtained through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM); spherical crystalline Si NPs with an average diameter of 4.2 ± 0.7 nm were observed. The doping maximum obtained through this process was an average concentration of 0.4 ± 0.3% Mn per mole of Si. The water-soluble SiMn NPs showed a strong photoluminescence with a quantum yield up to 13%. The SiMn NPs had significant T1 contrast with an r1 relaxivity of 11.1 ± 1.5 mM-1 s-1 and r2 relaxivity of 32.7 ± 4.7 mM-1 s-1 where the concentration is in mM of Mn2+. Dextran-coated poly(allylamine) SiMn NPs produced NPs with T1 and T2 contrast with a r1 relaxivity of 27.1 ± 2.8 mM-1 s-1 and r2 relaxivity of 1078.5 ± 1.9 mM-1 s-1. X-band electron paramagnetic resonance spectra are fit with a two-site model demonstrating that there are two types of Mn2+ in these NP's. The fits yield hyperfine splittings (A) of 265 and 238 MHz with significant zero field splitting (D and E terms). This is consistent with Mn in sites of symmetry lower than tetrahedral due to the small size of the NP's.

Zintl phases Yb14MnSb11 and Yb14MgSb11, which share the same complex structure type, have been demonstrated as the best p-type thermoelectric materials for the high temperature region (800-1200 K). A new iso-structural compound, Yb14MgBi11, was synthesized in order to investigate the structure and thermoelectric properties of the Bi analogs. Yb14MgBi11 crystallizes in the Ca14AlSb11 structure-type with the space group I41/acd [a = 16.974(2) A, c = 22.399(4) A, V = 6454(2) A3, R1/wR2 = 0.0238/0.0475]. The structure follows the previous description of this structure type and the trend observed in previous analogs. Thermoelectric properties of Yb14MgBi11 are measured together with Yb14MnBi11 and both compounds are metallic. Compared to Yb14MgSb11, Yb14MgBi11 has a higher carrier concentration with a similar mobility and effective mass. The lattice thermal conductivity of Yb14MgBi11 is extremely low, which is as low as 0.16-0.36 W(mK)-1. The zT values of Yb14MgBi11 and Yb14MnBi11 reach 0.2 at 875 K.

Yb14MnSb11 is the most efficient bulk p-type thermoelectric material for high temperature applications. Materials with Y and Sc substitutions in Yb14MnSb11 were made both in Sn-flux and by ball milling. These small 3+ rare earth (RE) cations were introduced with the goal of providing chemical pressure on the structure. The RE3+ cation is smaller than Yb2+ and also donates one additional electron to this p-type semiconductor. In Yb14−xRExMnSb11 (RE = Sc, Y) the maximum x was about 0.5. X-ray diffraction experiments on the single crystals obtained from Sn-flux showed that Sc preferentially substitutes for Yb(1) and Yb(3), and decreases the size of the unit cell by about 0.3%. Y substitutes on all Yb sites and increases the size of the unit cell by about 0.2%. Samples with Yb14−xRExMnSb11 (x~0.3) were prepared via powder metallurgy and spark plasma sintering for transport and thermal conductivity measurements. Electron microprobe of the Sc-substituted sample showed small regions (≤1 μ m) containing greater amounts of Sc, and X-ray powder diffraction of the ball milled Sc sample could be fitted as phase pure Yb14−xScxMnSb11. Y-substituted samples showed larger regions of excess Y in electron microprobe, and small amounts of Yb4Sb3 in X-ray powder diffraction. The Sc sample has slightly reduced carrier concentration over optimized Yb14MnSb11, while the Y samples have even lower carrier concentrations. These carrier concentrations lead to comparable resistivity to Yb14MnSb11 in the Sc-substituted material, and higher resistivities in the Y-substituted material. All materials had similar Seebeck coefficients that slightly exceed Yb14MnSb11 at high temperatures, with the Sc-substituted sample having the highest despite having a higher carrier concentration. Sc-substituted samples also had a slightly higher thermal conductivity over the Y-substituted samples, which had comparable thermal conductivity to Yb14MnSb11. The zT values of the Sc and Y substituted samples are similar (zT1000 K~0.8), however below that of Yb14MnSb11 due to the compensation of Seebeck and resistivity.

The thermal stability and thermoelectric properties of type I clathrate K8Al8Si38 up to 873 K are reported. K8Al8Si38 possesses a high absolute Seebeck coefficient value and high electrical resistivity in the temperature range of 323 to 873 K, which is consistent with previously reported low temperature thermoelectric properties. Samples with Ba partial substitution at the K guest atom sites were synthesized from metal hydride precursors. The samples with the nominal chemical formula of K8–xBaxAl8+xSi38–x (x = 1, 1.5, 2) possess type I clathrate structure (cubic, Pm3n), confirmed by X-ray diffraction. The guest atom site occupancies and thermal motions were investigated with Rietveld refinement of synchrotron powder X-ray diffraction. Transport properties of Ba-containing samples were characterized from 2 to 300 K. The K–Ba alloy phases showed low thermal conductivity and improved electrical conductivity compared to K8Al8Si38. Electrical resistivity and Seebeck coefficients were measured over the tempera...

Two series of crystals were prepared via Sn flux synthesis with the compositional fluxes of Yb14−xLnxMnSb11, where x = 0.1–0.9 and x = 0.4. By X-ray structural measurements and microprobe analysis, the maximum amount of Ln incorporated in the unit cell matrix were determined to be 0.37 ± 0.04 for the La–Nd and 0.45 ± 0.04 for Sm–Tm elements with solid substitution solution formation. The Ln incorporation did not change the unit cell significantly but the cell volume decreased going from the largest La–Nd to the smallest Tm–Lu cations. The Ln0.30–0.40 samples demonstrated congruent melting and their melting points increased by ∼30–50 °C compared to the pristine matrix. The temperatures were attributed to the ordered structural state due to the Ln distribution in the unit cell only through the one regular system site. Based on geometrical fitting between crystal radii of Yb2+ and Ln3+ in six-coordination, the Yb(2) sites were found to be more preferable for substitution by La–Nd, Yb(1) by Sm–Ho and Yb(3) by Tm and Lu atoms. Thermal losses as a temperature function of the alloyed by La and Lu samples were determined by a step-by-step heating procedure with analysis of the vapor condensate deposited on the viewing window of the chamber. This experiment demonstrated a high mobility of the tetrahedral Mn and Sb along with Yb ions in the Yb14MnSb11 matrix with incongruent sublimation beyond 1300 °C and a decrease of the thermal weight losses by half if the matrix was alloyed by La. Occupation of the Yb sites by Ln atoms varied the geometry of the MnSb4 tetrahedron as well as electron properties and bonding in this structural fragment, and these changes are considered in the context of the coupling between chemical structure and thermal stability of the compounds. The improved thermal stability due to increasing the total ionic state of the alloyed samples was found to possibly be a useful factor for the long-time use of these materials for space applications.

The syntheses of YbTe1−x Sb x (x = 0, 0.05, 0.2, 0.5, 0.8, 1) were investigated by solid state reactions and formed into dense pellets by spark plasma sintering. X-ray powder diffraction and microprobe analysis indicated no solubility of Sb in YbTe, and these phases are better described as composite phases (YbTe)1−x (YbSb) x (x = 0, 0.05, 0.2, 0.5, 0.8, 1). Thermal conductivity, electrical resistivity, and Seebeck coefficients were acquired for the larger values of x (x = 0.2, 0.5, 0.8, 1) from room temperature to 773 K, and the figure of merit was calculated. Thermal conductivities for x = 0, 0.05 are also reported; however, measurements of Seebeck coefficients and electrical resistivity were not possible due to large resistivity. The figure of merit for all samples was low, and the maximum zT measured was zT 791K = 0.018 for YbSb. Low figures of merit were primarily the result of very high resistivity in YbTe rich samples, and high thermal conductivity, and a small Seebeck coefficient in all samples.

Compounds of the Yb14MnSb11 structure type are the highest efficiency bulk p-type materials for high temperature thermoelectric applications, with reported figures of merit (ZTs) as high as ∼1.3 at 1275 K. Further optimization of ZT for this structure type is possible with the development of a simple synthetic route. However, this has been difficult to achieve because of the small amount of Mn required compared with Yb and Sb. A simple synthetic route for Yb14MnSb11 has been developed utilizing a combination of ball milling and annealing to produce phase-pure material followed by spark plasma sintering for consolidation. The materials have been characterized by powder X-ray diffraction before and after spark plasma sintering. The stoichiometric reaction of Yb, Sb, and MnSb provides phase-pure powder by X-ray diffraction. Upon cycling to temperatures greater than 1272 K, Yb14MnSb11 shows the presence of Yb11Sb10. Additional samples with 5% and 10% excess Mn were also investigated. Adding 5–10% excess Mn do...

A series of alkali metal containing compounds with type I clathrate structure, A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38, were synthesized and characterized. Room temperature lattice parameters of A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38 were determined to be 10.424916(10), 10.470174(13), 10.535069(15), and 10.48071(2) A, respectively. The type I clathrate structure (cubic, Pm3n) was confirmed for all phases, and in the case of K8Al8Si38 and K8Ga8Si38, the structures were also refined using synchrotron powder diffraction data. The samples were consolidated by Spark Plasma Sintering (SPS) for thermoelectric property characterization. Electrical resistivity was measured by four probe AC transport method in the temperature range of 30 to 300 K. Seebeck measurements from 2 to 300 K were consistent with K8Al8Si38 and K8Ga8Si38 being n-type semiconductors, while Rb8Ga8Si38 and Cs8Ga8Si38 were p-type semiconductors. K8Al8Si38 shows the lowest electrical resistivity and the highest Seebeck coefficient. This phase a...

Inorganic semiconductor nanoparticles are of significant interest for applications that benefit from their size-dependent properties. The work presented here focuses on the characterization of solution-based microwave synthesized Ge nanocrystals (NCs). Three differently capped Ge NCs were investigated: oleylamine (OAM), dodecanethiol (DDT), and a functionalized N4,N4,N4′,N4′-tetraphenylbiphenyl-4,4′-diamine (TPD) ligand, which is commonly used as hole-transporting units. The optical gaps followed the expected trend for quantum confinement; however, the absolute value depended upon the ligand. We found that the DDT-capped Ge NCs feature consistently larger bandgaps than OAM-capped Ge NCs of a similar size. Cyclic voltammetry (CV) was used to determine the valence band energy for OAM-capped Ge NCs, and the conduction band energy was estimated from the optical gap. By contrast, DDT-capped Ge NCs and the OAM/DDT-capped Ge NCs did not exhibit an oxidative signal in the cyclic voltammetry. This was attributed t...

Bulk nanocrystalline Mg2Si thermoelectric materials were synthesized and consolidated in a one-step process through a solid-state reaction between magnesium hydride and silicon, using the spark plasma sintering (SPS) method. The hydrogen produced in the process alleviates the problem of the oxidation of Mg. The samples were reactively sintered at temperatures in the range 723–823 K and under a uniaxial pressure in the range of 71–164 MPa in 5 min. Powder X-ray diffraction (XRD) analysis showed the products to be pure Mg2Si. The grain size of the consolidated samples was less than 500 nm, as determined by transmission electron spectroscopy (TEM). Residual nano-pores were observed by scanning electron microscopy at grain boundaries; their presence is believed to be the consequence of hydrogen evolution during the reactive sintering. The effect of synthesis temperature and pressure on crystallite size, density, and transport properties was determined. The results showed that use of MgH2 instead of Mg in the onestep method prevents the formation of MgO. The addition of 1 at.% Bi as a dopant improved the power factor significantly. Samples with 1 at.% Bi had a ZT of 0.6 at 775 K.

After the discovery of Yb14MnSb11 as an outstanding p-type thermoelectric material for high temperatures (≥900 K), site substitution of other elements has been proven to be an effective method to further optimize the thermoelectric properties. Yb14−xRExMnSb11 (RE = Pr and Sm, 0 < x < 0.55) compounds were prepared by powder metallurgy to study their thermoelectric properties. According to powder X-ray diffraction, these samples are iso-structural with Yb14MnSb11 and when more than 5% RE is used in the synthesis the presence of (Yb,RE)4Sb3 is apparent after synthesis. After consolidation and measurement, (Yb,RE)Sb and (Yb,RE)11Sb10 appear in the powder X-ray diffraction patterns. Electron microprobe analysis results show that consolidated pellets have small (Yb,RE)Sb domains and that the maximum amount of RE in Yb14−xRExMnSb11 is x = 0.55, however, (Yb,RE)11Sb10 cannot be distinguished by electron microprobe analysis. By replacing Yb2+ with RE3+, one extra electron is introduced into Yb14MnSb11 and the carrier concentration is adjusted. Thermoelectric performance from room temperature to 1275 K was evaluated through transport and thermal conductivity measurements. The measurement shows that Seebeck coefficients initially increase and then remain stable and that electrical resistivity increases with substitutions. Thermal conductivity is slightly reduced. Substitution of Pr and Sm leads to enhanced zT. Yb13.82Pr0.18Mn1.01Sb10.99 has the best maximum zT value of ∼1.2 at 1275 K, while Yb13.80Sm0.19Mn1.00Sb11.02 has its maximum zT of ∼1.0 at 1275 K, respectively, ∼45% and ∼30% higher than Yb14MnSb11 prepared in the same manner.

We have developed a fast, easy, and scalable synthesis method for Ba1xKxFe2As2 (0 ≤ x ≤ 1) superconductors using hydrides BaH2 and KH as a source of barium and potassium metals. Synthesis from hydrides provides better mixing and easier handling of the starting materials, consequently leading to faster reactions and/or lower synthesis temperatures. The reducing atmosphere provided by the evolved hydrogen facilitates preparation of oxygen-free powders. By a combination of methods we have shown that Ba1xKxFe2As2 obtained via hydride route has the same characteristics as when it is prepared by traditional solid-state synthesis. Refinement from synchrotron powder X-ray diffraction data confirms a linear dependence of unit cell parameters upon K content as well as the tetragonal to orthorhombic transition at low temperatures for compositions with x < 0.2. Magnetic measurements revealed dome-like dependence of superconducting transition temperature Tc upon K content with a maximum of 38 K for x close to 0.4. Electron diffraction and high-resolution high-angle annular dark-field scanning transmission electron microscopy indicates an absence of Ba/K ordering, while local inhomogeneity in the Ba/K distribution takes place at a scale of several angstroms along [110] crystallographic direction.

A thermochemical study of three germanium allotropes by differential scanning calorimetry (DSC) and oxidative high-temperature drop solution calorimetry with sodium molybdate as the solvent is described. Two allotropes, microcrystalline allo-Ge (m-allo-Ge) and 4H-Ge, have been prepared by topotactic deintercalation of Li_7Ge_(12) with methanol (m-allo-Ge) and subsequent annealing at 250 °C (4H-Ge). Transition enthalpies determined by differential scanning calorimetry amount to 4.96(5) ± 0.59 kJ/mol (m-allo-Ge) and 1.46 ± 0.55 kJ/mol (4H-Ge). From high-temperature drop solution calorimetry, they are energetically less stable by 2.71 ± 2.79 kJ/mol (m-allo-Ge) and 5.76 ± 5.12 kJ/mol (4H-Ge) than α-Ge, which is the stable form of germanium under ambient conditions. These data are in agreement with DSC, as well as with the previous quantum chemical calculations. The morphology of the m-allo-Ge and 4H-Ge crystallites was investigated by a combination of scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Even though the crystal structures of m-allo-Ge and 4H-Ge cannot be considered as truly layered, these phases retain the crystalline morphology of the layered precursor Li_7Ge_(12). Investigation by diffuse reflectance infrared Fourier transform spectroscopy and UV–vis diffuse reflectance measurements reveal band gaps in agreement with quantum chemical calculations.

Intentional impurity doping lies at the heart of the silicon technology. The dopants provide electrons or holes as necessary carriers of the electron current and can significantly modify the electric, optical and magnetic properties of the semiconductors. P-doped amorphous Si ( a -Si) was prepared by a solid state and solution metathesis reaction of a P-doped Zintl phase precursor, NaSi 0.99 P 0.01 , with an excess of NH 4 X (X = Br, I). After the salt byproduct was removed from the solid state reaction, the a -Si material was annealed at 600 °C under vacuum for 2 h, resulting in P-doped nanocrystalline Si ( nc -Si) material embedded in a -Si matrix. The product from the solution reaction also shows a combination of nc -Si embedded in a -Si; however, it was fully converted to nc -Si after annealing under argon at 650 °C for 30 min. Powder X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) show the amorphous nature of the P-doped Si material before the annealing and the nanocrystallinity after the annealing. Fourier Transform Infrared (FTIR) spectroscopy shows that the P-doped Si material surface is partially capped by H and O or with solvent. Electron microprobe wavelength dispersive spectroscopy (WDS) as well as energy dispersive spectroscopy (EDS) confirm the presence of P in the Si material. 29 Si and 31 P solid state magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy data provide the evidence of P doping into the Si structure with the P concentration of approximately 0.07 at.%.

Eu7Cd4Sb8-xAsx (x = 2, 3, 4, and 5) solid solutions are prepared from the elements in a molten Sn flux (alumina crucible in evacuated silica tubes, 1.

The high temperature p-type thermoelectric material Yb(14)MnSb(11) has been of increasing research interest since its high temperature thermoelectric properties were first measured in 2006. Subsequent substitutions of Zn, Al, and La into the structure have shown that this material can be further optimized by altering the carrier concentration or by reduction of spin-disorder scattering. Here the properties of the Yb(14-x)Ca(x)MnSb(11) solid solution series where isovalent Ca(2+) is substituted for Yb(2+) will be presented. Crystals of the Yb(14-x)Ca(x)MnSb(11) solid solution series were made by Sn-flux (x = 2, 4, 6, 8) with the following ratio of elements: (14-x)Yb: xCa: 6 Mn: 11Sb: 86Sn, and their structures determined by single crystal X-ray diffraction. The density of the material significantly decreases by over 2 g/cm(3) as more Ca is added (from x = 1 to 8), because of the lighter mass of Ca. The resulting lower density is beneficial from a device manufacturing perspective where there is often a trade-off with the specific power per kilogram. The compounds crystallize in the Ca(14)AlSb(11) structure type. The Ca substitution contributes to systematic lengthening the Mn-Sb bond while shortening the Sb-Sb bond in the 3 atom linear unit with increasing amounts of Ca. Temperature dependent thermoelectric properties, Seebeck, electrical resistivity, and thermal conductivity were measured from room temperature to 1273 K. Substitution of Yb with Ca improves the Seebeck coefficient while decreasing the thermal conductivity, along with decreasing the carrier concentration in this p-type material resulting in an enhanced thermoelectric figure of merit, zT, compared to Yb(14)MnSb(11).

We demonstrate the synthesis of water-soluble allylamine-terminated Fe-doped Si (Si(xFe)) nanoparticles as bimodal agents for optical and magnetic imaging. The preparation involves the synthesis of a single-source iron-containing precursor, Na(4)Si(4) with x% Fe (x = 1, 5, 10), and its subsequent reaction with NH(4)Br to produce hydrogen-terminated Si(xFe) nanoparticles. The hydrogen-capped nanoparticles are further terminated with allylamine via thermal hydrosilylation. Transmission electron microscopy indicates that the average particle diameter is ∼3.0 ± 1.0 nm. The Si(5Fe) nanoparticles show strong photoluminescence quantum yield in water (∼10%) with significant T(2) contrast (r(2)/r(1) value of 4.31). Electron paramagnetic resonance and Mössbauer spectroscopies indicate that iron in the nanoparticles is in the +3 oxidation state. Analysis of cytotoxicity using the resazurin assay on HepG2 liver cells indicates that the particles have minimal toxicity.

A microwave-assisted reaction has been developed to produce hydrogen-terminated silicon quantum dots (QDs). The Si QDs were passivated for water solubility via two different methods: hydrosilylation produced 3-aminopropenyl-terminated Si QDs, and a modified Stöber process produced silica-encapsulated Si QDs. Both methods produce water-soluble QDs with maximum emission at 414 nm, and after purification, the QDs exhibit intrinsic fluorescence quantum yield efficiencies of 15 and 23%, respectively. Even though the QDs have different surfaces, they exhibit nearly identical absorption and fluorescence spectra. Femtosecond transient absorption spectroscopy was used for temporal resolution of the photoexcited carrier dynamics between the QDs and ligand. The transient dynamics of the 3-aminopropenyl-terminated Si QDs is interpreted as a formation and decay of a charge-transfer (CT) excited state between the delocalized π electrons of the carbon linker and the Si core excitons. This CT state is stable for ~4 ns before reverting back to a more stable, long-living species. The silica-encapsulated Si QDs show a simpler spectrum without CT dynamics.

Phase stability of the type-I clathrate compound Ba_(8)Al_(x)Si_(46−x) and the thermoelectric property dependence on chemical composition are presented. Polycrystalline samples were prepared by argon arc melting and annealing. Results of powder X-ray diffraction and electron microprobe analysis show that the type-I structure is formed without framework deficiency for 8≤x≤15. Lattice constant a increases linearly with the increase of x. Thermoelectric properties were measured for x=12, 14 and 15. The Seebeck coefficients are negative, with the absolute values increasing with x. The highest figure of merit zT=0.24 was observed for x=15 at T=1000 K, with carrier electron density n=3×10^(21) cm^(−3). A theoretical calculation based on the single parabolic band model reveals the optimum carrier concentration to be n~4×10^(20) cm^(−3), where zT~0.7 at T=1000K is predicted.

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 Pm3n. Electron microprobe characterization indicates the composition to be Ba_(8−y)Sr_yAl_(14.2(2))Si_(31.8(2)) (0.77

In thermoelectric materials, chemical substitutions are widely used to optimize thermoelectric properties. The Zintl phase compound, Yb14MgSb11, has been demonstrated as a promising thermoelectric material at high temperatures. It is iso-structural with Ca14AlSb11 with space group I41/acd. Its iso-structural analog, Ca14MgSb11, was discovered to be a semiconductor and have vacancies on the Sb(3) sites, although in its nominal composition it can be described as consisting of fourteen Ca2+ cations with one [MgSb4]9− tetrahedron, one Sb37− linear anion and four isolated Sb3− anions (Sb(3) site) in one formula unit. When Sn substitutes Sb in Ca14MgSb11, optimized Seebeck coefficient and resistivity were achieved simultaneously although the Sn amount is small (

Chemical hydrides are compounds that can potentially uptake and release hydrogen without the use of hydrogen gas. Nanostructure silicon may have great potential as a chemical hydride. The surface can be capped by hydride and dihydride, and hydrogen can be thermally desorbed from the surface. We have prepared large-scale (1−2 g) samples of hydrogen-capped silicon nanoparticles with average diameters of 60, 10, 5, and 4 nm via a low-temperature chemical method to explore the release of hydrogen from the surface as a function of size. The 60- and 10-nm-diameter particles have only hydrogen on the surface. The 60-nm-diameter particles are crystalline, and the 10-nm-diameter particles are amorphous according to powder X-ray diffraction (XRD). The 5- and 4-nm-diameter particles have both hydrogen and solvent capped on the surface. The 4-nm-diameter particles are amorphous and the 5-nm-diameter particles are crystalline by powder XRD. Weight percentages of ∼3.5% at 350 °C are observed for the 10-nm-diameter part...

Two rare-earth Zintl phases, Yb_(11)GaSb_9 and Yb_(11)InSb_9, were synthesized in high-temperature self-fluxes of molten Ga and In, respectively. Structures were characterized by both single-crystal X-ray diffraction and powder X-ray diffraction and are consistent with the published orthorhombic structure, with the space group Iba2. High-temperature differential scanning calorimetry (DSC) and thermal gravimetry (TG) measurements reveal thermal stability to 1300 K. Seebeck coefficient and resistivity measurements to 1000 K are consistent with the hypothesis that Yb_(11)GaSb_9 and Yb_(11)InSb_9 are small band gap semiconductors or semimetals. Low doping levels lead to bipolar conduction at high temperature, preventing a detailed analysis of the transport properties. Thermal diffusivity measurements yield particularly low lattice thermal conductivity values, less than 0.6 W/m K for both compounds. The low lattice thermal conductivity suggests that Yb_(11)MSb_9 (M = Ga, In) has the potential for high thermoelectric efficiency at high temperature if charge-carrier doping can be controlled.

A series of compounds, Yb_(14)Mn_(1−x)Al_xSb_11 with (0 < x < 1), was prepared via Sn flux and the structures investigated. Single-crystal X-ray diffraction reveals that the volume of the unit cell increases with increasing Al content. Bond-distance changes are less than 2% and the increase in volume of the unit cell is associated with the decreasing distortion of the tetrahedron with increasing Al content. The specific heat, C_p, was measured from 300 to 1100 K. The measured C_p for these compounds is 19% higher than the Dulong-Petit value at the peak zT temperature (1223 K). These measured values permit a recalculation of thermal conductivity, revealing an extremely low lattice thermal conductivity of approximately 0.3−0.4 W/(m K) at 1223 K. The maximum zT of Yb_(14)Mn(1−x)Al_xSb_11 is approximately 0.8 and 1.1 for the x = 0 and x = 0.6 compositions, respectively.

Single crystals of SrAl_2Si_2 were synthesized by reaction of the elements in an aluminum flux at 1000 °C. SrAl_2Si_2 is isostructural to CaAl2Si2 and crystallizes in the hexagonal space group P–3m1 (90 K, a=4.1834 (2), c=7.4104 (2) A, Z=1, R1=0.0156, wR2=0.0308). Thermal analysis shows that the compound melts at 1020 °C. Low-temperature resistivity on single crystals along the c-axis shows metallic behavior with room temperature resistivity value of 7.5 mΩ cm. High-temperature Seebeck, resistivity, and thermal conductivity measurements were made on hot-pressed pellets. The Seebeck coefficient shows negative values in entire temperature range decreasing from −78 μV K^(−1) at room temperature to −34 μV K^(−1) at 1173 K. Seebeck coefficients are negative indicating n-type behavior; however, the temperature dependence is consistent with contribution from minority p-type carriers as well. The lattice contribution to the thermal conductivity is higher than for clathrate structures containing Al and Si, approximately 50 mW cm^(−1) K, and contributes to the overall low zT for this compound.

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.

The electronic conductivity of Langmuir−Blodgett monolayers of silane-passivated silicon nanoparticles (core diameter 3.86 ± 0.85 nm) was examined by electrochemical measurements within the context of photoirradiation and at controlled temperatures. Temperature dependence of the dark conductivity indicated that the interparticle charge transfer followed a thermal activation mechanism within the temperature range of 200−320 K; whereas at lower temperature, the ensemble conductance was determined by tunneling between (clusters of) nanoparticles that were of equivalent energy states. When exposed to photoexcitation with photon energy greater than the effective particle bandgap, the particle ensemble conductivity exhibited a drastic enhancement as compared to that in the dark; and, at a specific excitation wavelength, the conductivity became virtually independent of temperature. This suggested efficient ionization of the photoexcited quantum-confined electron−hole pairs by the applied electric field, most pro...

Rare-earth transition metal compounds Yb_(14)Mn_(1−x)Zn_xSb_(11), isostructural with Ca_(14)AlSb_(11), have been prepared using a metal flux growth technique for thermoelectric property measurements (with x = 0.0, 0.2, 0.3, 0.4, 0.7, 0.9, and 1.0). Single-crystal X-ray diffraction and electron microprobe analysis data indicate the successful synthesis of a solid-solution for the Yb_(14)Mn_(1−x)Zn_xSb_(11) structure type for 0 0.4. High-temperature (298 K–1275 K) measurements of the Seebeck coefficient, resistivity, and thermal conductivity were performed on hot-pressed, polycrystalline samples. As the concentration of Zn increases in Yb_(14)Mn_(1−x)Zn_xSb_(11), the Seebeck coefficient remains unchanged for 0 ≤ x ≤ 0.7 indicating that the free carrier concentration has remained unchanged. However, as the nonmagnetic Zn^(2+) ions replace the magnetic Mn^(2+) ions, the spin disorder scattering is reduced, lowering the resistivity. Replacing the magnetic Mn^(2+) with non magnetic Zn^(2+) provides an independent means to lower resistivity without deleterious effects to the Seebeck values or thermal conduction. Alloying the Mn site with Zn reduces the lattice thermal conductivity at low temperatures but has negligible impact at high temperatures. The reduction of spin disorder scattering leads to an ∼10% improvement over Yb_(14)MnSb_(11), revealing a maximum thermoelectric figure of merit (zT) of ∼1.1 at 1275 K for Yb_(14)Mn_(0.6)Zn_(0.4)Sb_(11).

The antimony-121 Mössbauer spectra of the Yb14MnSb11 and Eu14MnSb11 Zintl compounds have been measured between 2 or 5 and 300 K. The resulting three-dimensional arrays of the spectral counts, velocity, and temperature have been simultaneously fit with a minimum number of free parameters. These fits yield a 0 Kelvin transferred hyperfine field of 2.9(2) T, a Curie temperature of 57(3) K, and a Mössbauer temperature of 182(2) K for Yb14MnSb11; in this case the transferred field arises solely from the ordering of Mn2+. Because Eu14MnSb11 has both Eu2+ and Mn2+ ions that are magnetically ordered, its antimony-121 Mössbauer spectra are more complex and reveal two magnetic transitions, the first at 92(1) K resulting from the ordering of the Mn2+ ions and the second at 9.5(1.0) K resulting from the ordering of the Eu2+ ions; the corresponding 0 Kelvin transferred hyperfine fields are 1.3(1) and 3.7(1) T. The antimony-121 isomer shifts yield electronic configurations of 5s1.745p4.28 and 5s1.745p4.19 for the average antimony anion in Yb14MnSb11 and Eu14MnSb11, respectively.

Zintl phase compounds with large unit cells and complex anionic structures such as Yb11Sb10 hold potential for being good thermoelectric materials. Single crystals of Ge-doped Yb11Sb10 were synthesized using a molten Sn-flux technique. Single crystal X-ray diffraction data were obtained and resulted in a composition of Yb11Sb9.3Ge0.5 which was verified by microprobe. Yb11Sb9.3Ge0.5 is isostructural to Ho11Ge10, crystallizing in a body-centered, tetragonal unit cell, space group I4/mmm, with Z = 4. The unit cell parameters of Yb11Sb9.3Ge0.5 are a = 11.8813(4), c = 17.1276(13) A with a volume of 2417.8(2) A3. These parameters correlate well with the structural refinement of previously published Yb11Sb10. The structure consists of 16 isolated Sb3− anions, 8 dumbbells, 2 square planar rings and 44 Yb2+ cations. The Ge, doped in at 28 % occupancy, was found to be site specific, residing on the 2 square planar rings. Single crystal X-ray diffraction is most consistent with the site that makes up the square ring being less than fully occupied. The doped compound is additionally characterized by X-ray powder diffraction, differential scanning calorimetry and thermogravimetry. High temperature (300–1200 K) thermoelectric properties show that the doped compound has extremely low thermal conductivity (10–30 mW/cmK), lower than that of Yb11Sb10. Temperature dependent resistivity is consistent with a heavily doped semiconductor. Yb11Sb9.3Ge0.5 shows p-type behavior increasing from ∼22 μV/K at room temperature to ∼31 μV/K at 1140 K. The low value and the temperature dependence of the Seebeck coefficient suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.

Ba1.9Ca2.4Mg9.7Si7 was grown as large crystals from the reaction of silicon with barium and calcium in Mg/Al flux. This compound is a charge-balanced Zintl phase with the Zr2Fe12P7 structure type in hexagonal space group P6 (a = 11.196(2) A, c = 4.4595(9) A); barium occupies the Zr sites, the lighter alkaline earths (Mg, Ca) mix on the Fe sites, and silicon occupies the phosphorus sites. An isostructural germanide (Ba1.2Sr0.9Mg11.9Ge7, a = 11.064(1) A, c = 4.3709(6) A)) was similarly synthesized from reactions of germanium with barium and strontium in Mg/Al flux. Density of states calculations indicate these phases are semimetals, in agreement with their charge-balanced nature. Measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient were carried out from 300–1000 K to investigate the thermoelectric properties of Ba1.9Ca2.4Mg9.7Si7. This compound is an n-type semimetal with low thermal conductivity (2 W/(m·K)), moderate Seebeck coefficient (−200 μV/K at 900 K), and a thermoel...

Single crystals of Eu11Cd6-xZnxSb12 (0 ≤ x ≤ 6) are prepared by reaction of the elements in a Sn flux (950 °C; cooling to 650 °C with a rate of 5 °C/h).

The Zintl phases Yb14MgSb11 (I) and Ca14MgSb11 (II) are prepared by annealing mixtures of the elements in the molar ratio Yb(Ca):Mg:Sb of 14:2:11 (Nb tube in evacuated silica tubes, 1275 K for (I) and 1225 K for (II)).

Synthesis of silicon nanoparticles of various sizes from 3 to 9 nm in diameter was accomplished via a low temperature solution route. These nanoparticles are prepared via reduction of SiCl4 with Na naphthalide in dimethoxyethane and capped with octasiloxane. The resulting nanoparticles were characterized by transmission electron microscopy (TEM), high resolution (HR) TEM, selected area electron diffraction (SAED), energy dispersive X-ray (EDX) spectroscopy, powder X-ray diffraction, UV–vis, photoluminescence, and their quantum yields were determined. TEM micrographs show that the nanoparticles are well dispersed and SAED and lattice fringes are consistent with diamond structured silicon. X-ray powder diffraction provides no diffraction peaks. UV–vis and photoluminescence show characteristic shifts corresponding to size, consistent with quantum confinement. The smallest sized nanoparticles show the largest quantum yield, consistent with an indirect bandgap nanoparticles.

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.

Mg3Sb2 has been prepared by direct reaction of the elements. Powder X-ray diffraction, thermal gravimetric, differential scanning calorimetery, and microprobe data were obtained on hot pressed samples. Single phase samples of Mg3Sb2 were prepared and found to contain oxygen at the grain boundaries and to lose Mg and oxidize at temperatures above 900 K. Thermoelectric properties were characterized by Seebeck, electrical resistivity, and thermal conductivity measurements from 300 to 1023 K, and the maximum zT was found to be 0.21 at ∼875 K.

Core/shell structured iron(Fe)/gold(Au) nanoparticles have been prepared by a reverse micelle method, and their potential application as magnetic resonance (MR) contrast agent investigated. The average nanoparticle size is 19 nm, as determined by x-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Magnetic properties and relaxivities of nanoparticles are presented and compared. The saturation magnetization is 81 emu g−1 Fe for freshly prepared nanoparticles and the particles have high r1(6.87 mM−1 s−1) when the Fe core is kept from oxidation. These nanoparticles may be used as T1 agents in the unoxidized form.

The electronic conductivity of tri-n-octylphosphineoxide (TOPO)-protected CdSe quantum dots (QDs) was studied at the air-water interface using the Langmuir technique within the context of photochemical and photophysical excitation. It was found that, upon photoirradiation with photon energies higher than that of the absorption threshold, the voltammetric currents increased rather substantially with a pair of voltammetric peaks at positive potentials. However, the photoconductivity profiles exhibited a dynamic transition, which was ascribed to the strong affinity of oxygen onto the CdSe surface and the consequent trapping of the photogenerated electrons. The resulting excess of holes led to photocorrosion of the particle cores. The oxygen adsorption and photoetching processes were found to be reversible upon cessation of the photoexcitation. In contrast, only featureless voltammetric responses were observed when the particle monolayers were deposited onto the electrode surface and the film conductance was measured in a vacuum (the overall profiles were analogous to that of a Coulomb blockade). A comparative study was also carried out with a CdSe dropcast thick film immersed in acetonitrile, where the photoconductivity profiles were reversible and almost linear. The latter was attributed to the separation of photogenerated electrons and holes which were subsequently collected at the electrodes under voltammetric control. In the dropcast system, the oxygen effects were minimal which was ascribed to the acetontrile medium that limited the access to oxygen and thus the particles were chemically intact. These studies suggest that chemical environment plays an important role in the determination of the chemical stability and electronic conductivity of CdSe QD thin films.

The framework-deficient clathrate phases Ba7.5Al13Si29 and Eu0.27Ba7.22Al13Si29 were prepared using a molten Al flux. Single-crystal X-ray diffraction confirmed the two phases to be clathrate type I (space group Pmn). For Eu0.27Ba7.22Al13Si29, single-crystal X-ray diffraction revealed the Eu to partially occupy the 2a position. Microprobe analysis of single crystals provided the stoichiometry, and Raman spectroscopy was used to investigate the guest framework interactions. The Raman spectra are consistent with both Ba7.5Al13Si29 and Eu0.27Ba7.22Al13Si29 having minimal guest-host interactions. Magnetic susceptibility data for Eu0.27Ba7.22Al13Si29 imply weak magnetic ordering and indicate a 2+ oxidation state for the Eu ion.

The chemical reactivity of current collector pastes, utilized in electrode performance measurements, were investigated with respect to the solid oxide fuel cell composite cathode, (La 85 Sr 15 ) .98 MnO 3 /8 mol% Y 2 O 3 -stabilized ZrO 2 (LSM/YSZ). Anode substrate single cells were prepared with Pt or Ag paste current collectors applied to the electrodes. Phase purity of the cathodes before and after sintering the current collectors were examined with powder X-ray diffraction (XRD) and the performance of the cells determined with current-potential measurements. XRD experiments provided evidence that Pt paste, containing a bismuth flux, reacted with the composite cathode at temperatures above 800 °C leading to formation of the pyrochlore phase, La 2 Zr 2 O 7 . Cathodes coated with Ag paste, which did not contain a bismuth flux, showed no sign of chemical degradation. Synthesized Bi-subnitrate and commercial Bi 2 O 3 were shown to have similar chemical reactions with LSM/YSZ powders leading to pyrochlore phase formation. Current-potential experiments on single test cells demonstrated that samples prepared with paste containing bismuth flux had a decrease in initial performance.

Measurements of core and valence electronic states of single crystals of the rare earth transition metal Zintl phases Yb14MnSb11 and Yb14ZnSb11 were performed using the X-ray photoelectron spectroscopy station of Beamline 7 at the Advanced Light Source. Sample surfaces of Yb14MnSb11 and Yb14ZnSb11 were measured as received, after Ar+ ion bombardment, and after cleaving in situ. The single crystal structure of Yb14ZnSb11 is also reported. Both compounds are air-sensitive and show Yb3+ due to surface oxidation. In the case of Yb14MnSb11, there is no evidence for Yb3+ that would be intrinsic to the sample, consistent with previously reported X-ray magnetic circular dichroism studies. Detailed analyses of the Yb14ZnSb11 surfaces reveal a significant contribution of both Yb3+ and Yb2+ 4f states in the valence band region. This result is predicted for the Zn analog by Zintl counting rules and support the mixed valency of Yb for Yb14ZnSb11. Further detailed analysis of the core and valence band structure of both Yb14MnSb11 and Yb14ZnSb11 is presented.

Silicon nanoparticles (NPs) of ∼4.5(1.10) nm from a room-temperature solution route are terminated by a silanization method for the first time. Energy-selected emission is observed, consistent with the distribution of sizes obtained by this route. The NPs are photochemically stable in nonpolar organic solvents and when exposed to air/water under ambient conditions for up to 1 year. The nanoparticles were characterized by TEM, HRTEM, EDX, SAED, FTIR, 1H/13C NMR, UV−vis, and photoluminescence (PL) spectroscopy.

The Mg 2 Si x Ge 1 - x solid solution was prepared by heating stoichiometric amounts of elemental Mg, Si, and Ge in an argon atmosphere. Both microprobe and X-ray diffraction analysis of the reaction products suggest that homogeneous alloys can be made by heating the elements to 1200 °C, a temperature roughly 100 °C in excess of the melting temperatures for pure Mg 2 Si and Mg 2 Ge. 2 9 Si solid-state nuclear magnetic resonance was used to characterize the Mg 2 Si x Ge 1 - x solid solution. The 2 9 Si isotropic chemical shift changes consistent with the band gap in these materials while the line width increases with local disorder.