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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 particles. The largest weight loss is observed for the amorphous 4-nm-diameter particles, which show a weight loss of ∼4.5%, which is attributed primarily to hydrogen. The products have been investigated by powder XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), Fourier transform infrared (FTIR) analysis, thermogravimetry/differential scanning calorimetry (TG/DSC), and thermogravimetry/mass spectroscopy (TG/MS). [ABSTRACT FROM AUTHOR]

By converting waste heat into electricity and improving the efficiency of refrigeration systems, thermoelectric devices could play a significant role in solving today's energy problems. Increasing the thermoelectric efficiency (as measured by the thermoelectric material's figure-of-merit, zT) is critical to the development of this technology. Complex Zintl phases, in particular, make ideal candidates for thermoelectric materials because the necessary “electron–crystal, phonon–glass” properties can be engineered with an understanding of the Zintl chemistry. A recent example is the discovery that Yb14MnSb11, a transition metal Zintl compound, has twice the zT as the material currently in use at NASA. This perspective outlines a strategy to discover new high zT materials in Zintl phases, and presents results pointing towards the success of this approach. [ABSTRACT FROM AUTHOR]

High quality synchrotron powder X-ray diffraction (PXRD) data measured from 90 to 900 K were used to carry out crystal structure refinements of the thermoelectric Zintl phase, Yb14MnSb11, made up of Yb cations and polyanions along with Sb anions. The crystal structure exhibits near linear thermal expansion with high-temperature expansion coefficients of 2.20(3) × 10–4K–1and 3.67(3) × 10–4K–1for aand cof Yb14MnSb11. A subtle structural transition is observed between 500 and 600 K, which primarily involves the [MnSb49–] tetrahedron where the Mn–Sb bond length is increased by about 0.015 Å. The atomic displacement parameters (ADPs) of the Mn atom were found to be larger than the ADPs of the Sb and Yb atoms reflecting the lower mass of Mn. On the other hand the heavy Yb atoms were found to vibrate significantly more than the lighter Sb atoms, and this presumably reflects the stronger chemical bonding of the covalently bonded anions relative to the isolated Yb cations. Modeling of the ADPs gives a Debye temperature of 168(2) K. The chemical bonding in Yb14MnSb11was analyzed using Atomic Hirshfeld Surfaces. Overall, the analysis supports the presence of the structural elements of Yb cations, [MnSb49–] tetrahedra, linear [Sb37–] trimers and isolated Sb3–. There are, however, indications of a complex network of directional interactions between the Sb anions and the Yb cations. [ABSTRACT FROM AUTHOR]

The synthesis and transport propertiesof the family of coinagemetal-stuffed Zintl compounds, Eu9Cd4–xCM2+x–y□ySb9(CM = coinagemetal, □ = vacancies), is presented as a function of coinagemetal substitution. Eu9Cd4–xCM2+x–y□ySb9compounds areshown to be rare examples of metallic Zintl phases with low thermalconductivities. While the lattice thermal conductivity is low, whichis attributed to the complex structure and presence of interstitials,the electronic contribution to thermal conductivity is also low. Inthese p-type compounds, the carriers transmit lessheat than expected, based on the Wiedemann–Franz law and metallicconduction, κe= L0T/ρ. Density functional theory(DFT) calculations indicate that the Fermi level resides in a pseudo-gap,which is consistent with the metallic description of the properties.While the contribution from the interstitial CM states to the Fermilevel is small, the interstitial CMs are required to tune the positionof the Fermi level. Analysis of the topology of electron localizationfunction (ELF) basins reveals the multicenter Eu−Cd(CM)−Sbinteractions, as the Eu and Sb states have the largest contributionat the top of the valence band. Regardless of the success of the Zintlconcept in the rationalization of the properties, the representationof the CM-stuffed Eu9Cd4Sb9structureas Eu cations encapsulated into a polyanionic (Cd/Cu)Sb network isoversimplified and underestimates the importance of the Eu–Sbbonding interactions. These results provide motivation to search formore efficient thermoelectric materials among complex metallic structuresthat can offer less electronic thermal conductivity without deterioratingthe electrical conductivity. [ABSTRACT FROM AUTHOR]

Ba1.9Ca2.4Mg9.7Si7wasgrown as large crystals from the reaction of silicon with barium andcalcium in Mg/Al flux. This compound is a charge-balanced Zintl phasewith the Zr2Fe12P7structure typein hexagonal space group P6̅ (a= 11.196(2) Å, c= 4.4595(9) Å); bariumoccupies the Zr sites, the lighter alkaline earths (Mg, Ca) mix onthe Fe sites, and silicon occupies the phosphorus sites. An isostructuralgermanide (Ba1.2Sr0.9Mg11.9Ge7, a= 11.064(1) Å, c= 4.3709(6) Å)) was similarly synthesized from reactions ofgermanium with barium and strontium in Mg/Al flux. Density of statescalculations indicate these phases are semimetals, in agreement withtheir charge-balanced nature. Measurements of electrical resistivity,thermal conductivity, and Seebeck coefficient were carried out from300–1000 K to investigate the thermoelectric properties ofBa1.9Ca2.4Mg9.7Si7. Thiscompound is an n-type semimetal with low thermal conductivity (2 W/(m·K)),moderate Seebeck coefficient (−200 μV/K at 900 K), anda thermoelectric figure of merit ZT of 0.35 at 900 K. [ABSTRACT FROM AUTHOR]

A facile size-controlled synthesis (microwave/conventional)of quasi-spherical germanium nanoparticles is reported.Oleylamineserves as a solvent, a binding ligand, and a reducing agent in thesynthesis. Reactions were carried out with microwave-assisted heating,and the results have been compared with those produced by conventionalheating. Germanium iodides (GeI4, GeI2) wereused as the Ge precursor, and size control in the range of 4–11nm was achieved by controlling the ratio of Ge4+/Ge2+in the precursor mix. Longer reaction times and higher temperatureswere also observed to have an effect on the nanoparticle size distribution.Microwave heating resulted in crystalline nanoparticles at lower temperaturesthan conventional resistive heating because of the ability of germaniumiodides to convert electromagnetic radiation directly to heat. Thereported approach for germanium nanoparticle preparation avoids theuse of strong reducing agents (LiAlH4, n-BuLi, NaBH4) and HF for etching and, thus, can be consideredsimple, safe, and amenable to industrial-level scaleup. The as-preparednanoparticles are a stable dispersion (hexane or toluene) for weekswhen stored under an inert atmosphere (N2/Ar). The stabilityof the colloidal dispersion was observed to be dependent on the nanoparticlesize, with smaller nanoparticles exhibiting longer stability. On exposureto ambient conditions, oxidation occurs over a period of time andresults in slow precipitation of the nanoparticles. The nanoparticleshave been characterized by powder X-ray diffraction (PXRD), transmissionelectron microscopy (TEM), and spectroscopic techniques (UV-Vis-NIR,FTIR, Raman). [ABSTRACT FROM AUTHOR]

Two rare-earth Zintl phases, Yb11GaSb9and Yb11InSb9, 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 Yb11GaSb9and Yb11InSb9are 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 Yb11MSb9(M= Ga, In) has the potential for high thermoelectric efficiency at high temperature if charge-carrier doping can be controlled. [ABSTRACT FROM AUTHOR]

Na4Si4and Na4Ge4are ideal chemical precursors for inorganic clathrate structures, clusters, and nanocrystals. The monoclinic Zintl phases, Na4Si4and Na4Ge4, contain isolated homo-tetrahedranide [Si4]4−and [Ge4]4−clusters surrounded by alkali metal cations. In this study, a simple scalable route has been applied to prepare Zintl phases of composition Na4Si4and Na4Ge4using the reaction between NaH and Si or Ge at low temperature (420 °C for Na4Si4and 270 °C for Na4Ge4). The method was also applied to K4Ge4, using KH and Ge as raw materials, to show the versatility of this approach. The influence of specific reaction conditions on the purity of these Zintl phases has been studied by controlling five factors: the method of reagent mixing (manual or ball milled), the stoichiometry between raw materials, the reaction temperature, the heating time and the gas flow rate. Moderate ball-milling and excess NaH or KH facilitate the formation of pure Na4Si4, Na4Ge4or K4Ge4at 420 °C (Na4Si4) or 270 °C (both M4Ge4compounds, M = Na, K). TG/DSC analysis of the reaction of NaH and Ge indicates that ball milling reduces the temperature for reaction and confirms the formation temperature. This method provides large quantities of high quality Na4Si4and Na4Ge4without the need for specialized laboratory equipment, such as Schlenk lines, niobium/tantalum containers, or an arc welder, thereby expanding the accessibility and chemical utility of these phases by making them more convenient to prepare. This new synthetic method may also be extended to lithium-containing Zintl phases (LiH is commercially available) as well as to alkali metal-tetrel Zintl compounds of other compositions, e.g.K4Ge9. [ABSTRACT FROM AUTHOR]

Single crystals of EuGa2Pn2(Pn = P, As) were grown from a molten Ga flux and characterized by single-crystal X-ray diffraction at 100(1) K. They are isostructural and crystallize in a new structure type (monoclinic, P2/m, a= 9.2822(9) Å, b= 3.8967(4) Å, c= 12.0777(11) Å, β = 95.5220(10)°, R1 = 0.0148, wR2 = 0.0325 (EuGa2P2) and a= 9.4953(7) Å, b= 4.0294(3) Å, c= 12.4237(9) Å, β = 95.3040(10)°, R1 = 0.0155, wR2 = 0.0315 (EuGa2As2)). The structures consist of alternating layers of two-dimensional Ga2Pn2anions and Eu cations. The anion layers are composed of Ga2Pn6staggered, ethane-like moieties having a rare Ga−Ga bonding motif; these moieties are connected in a complex fashion by means of shared Pn atoms. Both structures show small residual electron densities that can be modeled by adding a Eu atom and removing two bonded Ga atoms, resulting in structures (<2%) where most of the atoms are the same, but there is a difference in bonding that leads to one-dimensional ribbons of parallel Ga2Pn6staggered, ethane-like moieties. The compounds can be understood within the Zintl formalism, but show metallic resistivity. Magnetization measurements performed on single crystals show low-temperature magnetic anisotropy as well as multiple magnetic ordering events that occur at and below 24 and 20 K for the phosphorus and arsenic analogs, respectively. The magnetic coupling between Eu ions is attributed to indirect exchange via an RKKY interaction, which is consistent with the metallic behavior. The compounds display large negative magnetoresistance of up to −80 and −30% (MR = [(ρ(H) − ρ(0))/ρ(H)] × 100%) for Pn = P, As, respectively, which is maximal at the magnetic ordering temperatures in the highest measured field (5T). [ABSTRACT FROM AUTHOR]

A series of compounds, Yb14Mn1−-xAlxSb11with (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, Cp, was measured from 300 to 1100 K. The measured Cpfor these compounds is 19% higher than the Dulong-Petit value at the peak zTtemperature (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 zTof Yb14Mn1−xAlxSb11is approximately 0.8 and 1.1 for the x= 0 and x= 0.6 compositions, respectively. [ABSTRACT FROM AUTHOR]

Rare-earth transition metal compounds Yb 14Mn 1− xZn xSb 11, isostructural with Ca 14AlSb 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 14Mn 1− xZn xSb 11structure type for 0< x< 0.4. Hot-pressed polycrystalline samples showed that the product from the flux reaction was a pure phase from x= 0 through x= 0.4 with the presence of a minor secondary phase for compositions x> 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 14Mn 1− xZn 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 14MnSb 11, revealing a maximum thermoelectric figure of merit ( zT)of ∼1.1 at 1275 K for Yb 14Mn 0.6Zn 0.4Sb 11. [ABSTRACT FROM AUTHOR]

Capped boron nanoparticles have been synthesized at room temperature by a simple route that does not involve the use of flammable boranes. [ABSTRACT FROM AUTHOR]