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At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

The new ternary silicide Gd{sub 5}CoSi{sub 2}: Structural, magnetic and magnetocaloric properties

Gd{sub 5}CoSi{sub 2} was prepared by annealing at 1003 K. Its investigation by the X-ray powder diffraction shows that the ternary silicide crystallizes in a tetragonal structure deriving from the Cr{sub 5}B{sub 3}-type (I4/mcm space group; a=7.5799(4) and c=13.5091(12) A as unit cell parameters). The Rietveld refinement shows a mixed occupancy on the (8h) site between Si and Co atoms. Magnetization and specific heat measurements performed on Gd{sub 5}CoSi{sub 2} reveal a ferromagnetic behaviour below T{sub C}=168 K. This magnetic ordering is associated to an interesting magnetocaloric effect; the adiabatic temperature change {Delta}T{sub ad} is about 3.1 and 5.9 K, respectively, for a magnetic field change of 2 and 4.6 T. -- Graphical abstract: The adiabatic temperature change {Delta}T{sub ad} was determined by combining the heat capacity measurements and the magnetization data. As expected, a peak near the Curie temperature of the Gd{sub 5}CoSi{sub 2} ternary silicide is observed, with a maximum of {Delta}T{sub ad} around 3.1 and 5.9 K for {Delta}H=2 and 4.6 T, respectively. Display Omitted Research Highlights: {yields} We prepare and characterize for the first time the ternary silicide Gd{sub 5}CoSi{sub 2}. {yields} Gd{sub 5}CoSi{sub 2} crystallizes in the tetragonal structure deriving from the Cr{sub 5}B{sub 3}-type. {yields} Gd{sub 5}CoSi{sub 2} shows a ferromagnetic behaviour below 168 K associated with magnetocaloric properties.

36 MATERIALS SCIENCE↗
DOE OSTI2011

A comparison of the transport properties of lithium-stuffed garnets and the conventional phases Li{sub 3}Ln{sub 3}Te{sub 2}O{sub 12}

The structures of new phases Li{sub 6}CaLa{sub 2}Sb{sub 2}O{sub 12} and Li{sub 6.4}Ca{sub 1.4}La{sub 2}Sb{sub 2}O{sub 12} have been characterised using neutron powder diffraction. Rietveld analyses show that both compounds crystallise in the space group la3-bar d and contain the lithium cations in a complex arrangement with occupational disorder across oxide tetrahedra and distorted oxide octahedra, with considerable positional disorder in the latter. Variable temperature neutron diffraction experiments on Li{sub 6.4}Ca{sub 1.4}La{sub 2}Sb{sub 2}O{sub 12} show the structure is largely invariant with only a small variation in the lithium distribution as a function of temperature. Impedance spectroscopy measurements show that the total conductivity of Li{sub 6}CaLa{sub 2}Sb{sub 2}O{sub 12} is several orders of magnitude smaller than related lithium-stuffed garnets with {sigma}=10{sup -7} S cm{sup -1} at 95 {sup o}C and an activation energy of 0.82(3) eV. The transport properties of the conventional garnets Li{sub 3}Gd{sub 3}Te{sub 2}O{sub 12}, Li{sub 3}Tb{sub 3}Te{sub 2}O{sub 12}, Li{sub 3}Er{sub 3}Te{sub 2}O{sub 12} and Li{sub 3}Lu{sub 3}Te{sub 2}O{sub 12} have been evaluated and consistently show much lower values of conductivity, {sigma}{<=}4.4x10{sup -6} S cm{sup -1} at 285 {sup o}C and activation energies in the range 0.77(4){<=}E{sub a}/eV{<=}1.21(3). -- Graphical abstract: The lithium-stuffed garnets Li{sub 6}CaLa{sub 2}Sb{sub 2}O{sub 12} and Li{sub 6.4}Ca{sub 1.4}La{sub 1.6}Sb{sub 2}O{sub 12} accommodate lithium in a complex distribution across oxide tetrahedra and octahedra. The total conductivity of Li{sub 6}CaLa{sub 2}Sb{sub 2}O{sub 12} is considerably lower than reported for related fast-ion conducting garnets due to a much larger intra-grain contribution to the resistivity than is commonly found for this family of compounds. Research highlights: {yields} Lithium stuffed garnets have a complex distribution of Li{sup +} across multiple sites. {yields} They show fast lithium ion conductivity with an activation energy of 0.82 eV. {yields} This conductivity is much lower than related garnets due to grain boundary effects. {yields} The stoichiometric garnets show minimal ion mobility.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Effects of surfactants on morphology in synthesis of {alpha}-Mn{sub 2}O{sub 3} nanostructures

Cubic and chain-like structure of {alpha}-Mn{sub 2}O{sub 3} with a high surface area was prepared by air oxidation of manganese chloride through sol process by adding hexamine and mercaptosuccinic acid as wetting agent, respectively. The as-synthesized products were characterized with X-ray powder diffraction (XRD), Energy Dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and selected area electron diffraction (SAED). The possible formation mechanism of {alpha}-Mn{sub 2}O{sub 3} cubic and chain-like nanostructures has been proposed and discussed. -- Graphical abstract: Cubic and chain-like nanostructure of {alpha}-Mn{sub 2}O{sub 3} has been synthesized by air oxidation of manganese chloride as precursor, hexamine, and mercaptosuccinic acid as wetting agent, respectively. Display Omitted Research highlights: {yields} Cubic nanostructure of {alpha}-Mn{sub 2}O{sub 3} is obtained using hexamine as surfactant. {yields} Chain-like structure of {alpha}-Mn{sub 2}O{sub 3} is obtained using mercaptosuccinic acid as surfactant. {yields} The linking nature of mercaptosuccinic acid is proved. {yields} Mercaptosuccinic acid accelerates the growth of material.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Crystallographic and magnetic characterisation of the brownmillerite Sr{sub 2}Co{sub 2}O{sub 5}

Sr{sub 2}Co{sub 2}O{sub 5} with the perovskite-related brownmillerite structure has been synthesised via quenching, with the orthorhombic unit cell parameters a=5.4639(3) A, b=15.6486(8) A and c=5.5667(3) A based on refinement of neutron powder diffraction data collected at 4 K. Electron microscopy revealed L-R-L-R-intralayer ordering of chain orientations, which require a doubling of the unit cell along the c-parameter, consistent with the assignment of the space group Pcmb. However, on the length scale pertinent to NPD, no long-range order is observed and the disordered space group Imma appears more appropriate. The magnetic structure corresponds to G-type order with a moment of 3.00(4) {mu}{sub B} directed along [1 0 0]. -- Graphical abstract: Possible ordering of the tetrahedral chains in Sr{sub 2}Co{sub 2}O{sub 5}. Display Omitted Research highlights: {yields} Sr{sub 2}Co{sub 2}O{sub 5} has alternating (L and R) chains in a given tetrahedral layer. {yields} Order between layers is limited. {yields} NPD shows no evidence for the doubled unit cell-averaged structure. {yields} The length scale for a given technique is critical in considering space group.

36 MATERIALS SCIENCE↗
DOE OSTI2011

High-temperature crystal structure and transport properties of the layered cuprates Ln{sub 2}CuO{sub 4}, Ln=Pr, Nd and Sm

High-temperature crystal structure of the layered cuprates Ln{sub 2}CuO{sub 4}, Ln=Pr, Nd and Sm with tetragonal T'-structure was refined using X-ray powder diffraction data. Substantial anisotropy of the thermal expansion behavior was observed in their crystal structures with thermal expansion coefficients (TEC) along a- and c-axis changing from TEC(a)/TEC(c){approx}1.37 (Pr) to 0.89 (Nd) and 0.72 (Sm). Temperature dependence of the interatomic distances in Ln{sub 2}CuO{sub 4} shows significantly lower expansion rate of the chemical bond between Pr and oxygen atoms (O1) belonging to CuO{sub 2}-planes (TEC(Pr-O1)=11.7 ppm K{sup -1}) in comparison with other cuprates: TEC (Nd-O1)=15.2 ppm K{sup -1} and TEC (Sm-O1)=15.1 ppm K{sup -1}. High-temperature electrical conductivity of Pr{sub 2}CuO{sub 4} is the highest one in the whole studied temperature range (298-1173 K): 0.1-108 S/cm for Pr{sub 2}CuO{sub 4}, 0.07-23 S/cm for Nd{sub 2}CuO{sub 4} and 2x10{sup -4}-9 S/cm for Sm{sub 2}CuO{sub 4}. The trace diffusion coefficient (D{sub T}) of oxygen for Pr{sub 2}CuO{sub 4} determined by isotopic exchange depth profile (IEDP) technique using secondary ion mass spectrometry (SIMS) varies in the range 7.2x10{sup -13} cm{sup 2}/s (973 K) and 3.8x10{sup -10} cm{sup 2}/s (1173 K) which are in between those observed for the manganese and cobalt-based perovskites. -- Graphical abstract: Anomaly anisotropic thermal expansion behavior was observed for Pr{sub 2}CuO{sub 4} in comparison with Ln{sub 2}CuO{sub 4}, Ln=Pr and Nd having tetragonal T'-structure with thermal expansion coefficients (TEC) along a- and c-axis changing from TEC(a)/TEC(c){approx}1.37 (Pr) to 0.89 (Nd) and 0.72 (Sm). It was found that the trace diffusion coefficient (D{sub T}) of oxygen in Pr{sub 2}CuO{sub 4} determined by secondary ion mass spectrometry (SIMS) varies in the range 7.2x10{sup -13} cm{sup 2}/s (973 K) and 3.8x10{sup -10} cm{sup 2}/s (1173 K) which are in between those observed for the manganese and cobalt-based perovskites. Display Omitted Research highlights: {yields} Anisotropic high-temperature thermal expansion behavior of T'-Ln{sub 2}CuO{sub 4}, Ln=Pr, Nd and Sm. {yields} Anomalous expansion behavior of Pr{sub 2}CuO{sub 4} in comparison with Ln{sub 2}CuO{sub 4}, Ln=Nd and Sm. {yields} High-temperature electrical conductivity of Pr{sub 2}CuO{sub 4} is higher in comparison with other T'-Ln{sub 2}CuO{sub 4}. {yields} Values of the oxygen trace diffusion coefficient for Pr{sub 2}CuO{sub 4} are between those reported for the Mn- and Co-based perovskites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Determining the structure of tetragonal Y{sub 2}WO{sub 6} and the site occupation of Eu{sup 3+} dopant

The compound Y{sub 2}WO{sub 6} is prepared by solid state reaction at 750 {sup o}C using sodium chloride as mineralizer. Its structure is solved by ab-initio methods from X-ray powder diffraction data. This low temperature phase of yttrium tungstate crystallizes in tetragonal space group P4/nmm (No. 129), Z=2, a=5.2596(2) A, c=8.4158(4) A. The tungsten atoms in the structure adopt an unusual [WO{sub 6}] distorted cubes coordination, connecting [YO{sub 6}] distorted cubes with oxygen vacancies at the O{sub 2} layers while other yttrium ions Y{sub 2} form [YO{sub 8}] cube coordination. Y{sup 3+} ions occupy two crystallographic sites of 2c (C{sub 4v} symmetry) and 2a (D{sub 2d} symmetry) in the Y{sub 2}WO{sub 6} host lattice. With Eu{sup 3+} ions doped, the high resolution emission spectrum of Y{sub 2}WO{sub 6}:Eu{sup 3+} suggests that Eu{sup 3+} partly substituted for Y{sup 3+} in these two sites. The result of the Rietveld structure refinement shows that the Eu{sup 3+} dopants preferentially enter the 2a site. The uniform cube coordination environment of Eu{sup 3+} ions with the identical eight Eu-O bond lengths is proposed to be responsible for the intense excitation of long wavelength ultraviolet at 466-535 nm. -- Graphical abstract: The excitation and emission spectra of Y{sub 2}WO{sub 6}:Eu{sup 3+} indicates the effect of long wavelength-excited property, which is ascribed to the cube coordination environment of Eu{sup 3+} ions with the uniform Eu-O bond length. Display Omitted Research highlights: Y{sub 2}WO{sub 6} is prepared using sodium chloride as mineralizer. The structure of the compound Y{sub 2}WO{sub 6} is solved by ab-initio methods. The site occupation of Eu{sup 3+} is determined. The cube coordination of Eu{sup 3+} ions is responsible for the long wavelength excitation.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Perovskite-type SrTi{sub 1-x}Nb{sub x}(O,N){sub 3} compounds: Synthesis, crystal structure and optical properties

The synthesis, crystal structure, thermal stability and absorbance spectra of perovskite-type oxynitrides with the general formula SrTi{sub 1-x}Nb{sub x}(O,N){sub 3} (x=0.05, 0.10, 0.20, 0.50, 0.80, 0.90, 0.95) have been investigated. Oxide samples were prepared by a polymerized complex synthesis route and post-treated under ammonia at 850 {sup o}C for 24 h to substitute nitrogen for oxygen. Synchrotron X-ray powder diffraction (XRD) evidenced that the mixed oxide phases were all transformed into oxynitrides with perovskite-type structure during a thermal ammonolysis. SrTi{sub 1-x}Nb{sub x}(O,N){sub 3} with compositions x{<=}0.80 crystallized in a cubic and samples with x{>=}0.90 in a tetragonal structure. The Rietveld refinement indicated a continuous enlargement of the lattice parameters towards higher niobium content of the samples. Thermogravimetric analysis (TGA) and hotgas extraction revealed the dependence of the nitrogen incorporation upon the degree of niobium substitution. It showed that more nitrogen was detected in the samples with higher niobium content. Furthermore, TGA disclosed stability for all oxynitrides at T{<=}400 {sup o}C. Diffuse reflectance spectroscopy indicated a continuous decrease of the band gap's width from 3.24 eV (SrTi{sub 0.95}Nb{sub 0.05} (O,N){sub 3}) to 1.82 eV (SrTi{sub 0.05}Nb{sub 0.95}(O,N){sub 3}) caused by the increasing amount of nitrogen towards the latter composition. -- Graphical Abstract: The system SrTi{sub 1-x}Nb{sub x}(O,N){sub 3} is described and the changes in crystal structure, nitrogen content and width of the band gap are discussed. Display Omitted Research highlights: {yields} Synthesis of SrTi{sub 1-x}Nb{sub x}(O,N){sub 3} perovskite-type oxynitrides via thermal ammonolysis. {yields} Co-substitution of Nb{sup 5+} enabled to adjust the amount nitrogen insertion. {yields} Tuning of the optical band gap through nitrogen.{yields} Changes of crystal structure from cubic to tetragonal with increasing niobium content.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Dye-coated europium monosulfide

Nanoparticles of EuS were synthesized using europium dithiocarbamate complexes. The resulting nanoparticles were coated with the dye, 1-pyrene carboxylic acid and the resulting material was characterized using X-ray powder diffraction, TEM, and UV-visible spectroscopy. Fluorescence spectroscopy was used to determine the relative energy of the conduction band edge to the excited state energy of the dye. -- Graphical abstract: Dye sensitized magnetic semiconductor materials were prepared by synthesizing EuS nanoparticles using single source precursors and coating with the dye, 1-pyrene carboxylic acid. Display Omitted highlights: > Synthesized EuS nanoparticles, 11{+-}2.4 nm characterized using XRD, TEM, and UV-vis. spect. > Grafted a dye to the surface and characterized the product using XRD, FTIR, UV-vis., and TEM. > Studied the photophysical properties using fluorescence spectroscopy. > Determined the relative dye excited state to the conduction band of the semiconductor.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Synthesis and structural analysis of Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7}, a new pyrochlore solid solution

This paper presents a study of the synthesis and structural properties of the new pyrochlore-type Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7} series. Ten compositions with 0.0{<=}y{<=}0.9 were prepared by solid-state reaction with thermal treatments at 873, 1073 and 1323 K under atmospheric pressure conditions. Structural refinements from X-ray powder diffraction data by the Rietveld method show that all compounds of the Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7} solid solution crystallize in a {alpha}-pyrochlore structure. The main structural difference when bismuth is substituted by strontium concerns the x position of the O1 (x, (1/8), (1/8)). This substitution significantly increases the Bi/Sr-O1 distance and diminishes the Ir-O1 distance; this implies that the Ir-O1-Ir bond angle increases. With the Sr substitution, the IrO{sub 6} local configuration goes from a flattened trigonal antiprism, y<0.5, to an elongated one, y>0.5, passing through an octahedral array, y{approx}0.5. The electrical consequences of these structural changes observed in this system are qualitatively explained with electronic structure calculations, this behavior agrees very well with those observed in other pyrochlore systems A{sub 2}M{sub 2}O{sub 7} (A=rare earth cations or Tl{sup +}, Pb{sup 2+}, or Bi{sup 3+}, and M=Ru or Ir). -- Graphical abstract: Electrical behavior of the Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7} solid solution. Display Omitted Highlights: {yields} Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7} crystallizes in the {alpha}-pyrochlore structure. {yields} Local geometry of Ir goes from a flattened trigonal antiprism to an elongated one. {yields} Electrical resistivity increases with the Sr content in the solid solution. {yields} Electronic structure analysis relates resistivity with Sr disorder and bond strength.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Nanosized copper ferrite materials: Mechanochemical synthesis and characterization

Nanodimensional powders of cubic copper ferrite are synthesized by two-steps procedure of co-precipitation of copper and iron hydroxide carbonates, followed by mechanochemical treatment. X-ray powder diffraction, Moessbauer spectroscopy and temperature-programmed reduction are used for the characterization of the obtained materials. Their catalytic behavior is tested in methanol decomposition to hydrogen and CO and total oxidation of toluene. Formation of nanosized ferrite material is registered even after one hour of milling time. It is established that the prolonging of treatment procedure decreases the dispersion of the obtained product with the appearance of Fe{sub 2}O{sub 3}. It is demonstrated that the catalytic behavior of the samples depends not only on their initial phase composition, but on the concomitant ferrite phase transformations by the influence of the reaction medium. -- Graphical abstract: It is demonstrated that the catalytic behavior of the obtained copper ferrites depends not only on their initial phase composition, but on the concomitant phase transformations by the influence of the reaction medium. Display Omitted Highlights: {yields} Two-step co-precipitation-ball-milling procedure for copper ferrites preparation. {yields} The phase composition of ferrites depends on the milling duration. {yields} Ferrites transforms under the reaction medium, which affects their catalytic behavior. {yields} Ferrites decompose to magnetite and carbides during methanol decomposition. {yields} Agglomeration and further crystallization of ferrite occur during toluene oxidation.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

SiO{sub 2} nanospheres with tailorable interiors by directly controlling Zn{sup 2+} and NH{sub 3}.H{sub 2}O species in an emulsion process

SiO{sub 2} nanospheres with tailorable interiors were synthesized by a facile one-spot microemulsion process using TEOS as silica source, wherein cyclohexane including triton X-100 and n-octanol as oil phase and Zn{sup 2+} or NH{sub 3}.H{sub 2}O aqueous solution as dispersive phase, respectively. The products were characterized by Scanning Electron Microscopy, Transmission Electron Microscopy and X-ray Powder Diffraction. It was suggested that the as-synthesized silica nanospheres possessed grape-stone-like porous or single hollow interior, and also found that the ammonia dosage and aging time played key roles in controlling the size and structure of silica nanospheres. Furthermore, the comparative results confirmed that in-situ zinc species [ZnO/Zn(OH){sub 2}] acted as the temporary templates to construct grape-stone-like interior, and a simultaneously competing etching process occurred owing to the soluble Zn(NH{sub 3}){sub 4}{sup 2+} complex formation while the additional excessive ammonia was introduced. With the aging time being extended, the in-situ nanocrystals tended to grow into bigger ones by Ostwald Ripening, producing single hollow interior. - Graphical Abstract: Formation process of SiO{sub 2} nanospheres with porous and single hollow interior. Highlights: > ZnO/Zn(OH){sub 2} nanocrystals as the temporary templates shape the interior structures of SiO{sub 2} nanospheres. > Fabrication of porous and single hollow interiors needs no additional processes such as roasting or dissolving. > Tailorable interiors can be easily obtained through adjusting the aging time of temporary templates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Facile synthesis of Cu{sub 2}O nanocube/polycarbazole composites and their high visible-light photocatalytic properties

Cu{sub 2}O nanocube/polycarbazole composites have been prepared by an one-pot solvothermal process using carbazole as a reductant. The polycarbazole layer not only protected and stabilized Cu{sub 2}O particles, but also prohibited the recombination of photogenerated electrons-holes pair and facilitated interfacial charge transfer between polycarbazole and Cu{sub 2}O. The composition, structure and morphology of the obtained products was systematically studied by X-ray powder diffraction (XRD), Field-emission scanning electron microscopy (FE-SEM), High-resolution transmission electron microscopy (HRTEM), Fourier transformation infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and UV-visible spectrophotometer. Furthermore, the visible-light photocatalytic behavior of the Cu{sub 2}O nanocube/polycarbazole composites on the methyl orange was investigated. - Graphical abstract: The uniform and monodisperse Cu{sub 2}O nanocube/polycarbazole composites were prepared by an one-pot solvothermal process. As covered by polycarbazole, the photocatalytic activities of Cu{sub 2}O nanocubes were improved. The polycarbazole not only protected and stabilized Cu{sub 2}O cubes, but also prohibited the recombination of photogenerated electrons-holes pair and facilitated interfacial charge transfer between polycarbazole and Cu{sub 2}O. Highlights: > The Cu{sub 2}O/polycarbazole nanocube composite has a better photocatalytic activity. > We obtained the composite by an one-pot solvothermal process. > Carbazole monomers as reductants.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

An approach to preparing porous and hollow metal phosphides with higher hydrodesulfurization activity

This paper describes an effective method for the synthesis of metal phosphides. Bulk and supported Ni{sub 2}P, Cu{sub 3}P, and CoP were prepared by thermal treatment of metal and the amorphous red phosphorus mixtures. Porous and hollow Ni{sub 2}P particles were also synthesized successfully using this method. The structural properties of these products are investigated using X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), inductively coupled plasma (ICP-AES) and X-ray photoemission spectroscopy (XPS). A rational mechanism was proposed for the selective formation of Ni{sub 2}P particles. In experimental conditions, the Ni{sub 2}P/SiO{sub 2} catalyst exhibits excellent hydrodesulfurization (HDS) activity for dibenzothiophene (DBT). - Graphical abstract: Bulk and supported Ni{sub 2}P, Cu{sub 3}P, and CoP were prepared by thermal treatment of their metal and amorphous red phosphorus mixtures. Porous and hollow Ni{sub 2}P particles were successfully synthesized by this method also. In the experimental condition, a Ni{sub 2}P/SiO{sub 2} catalyst exhibits excellent hydrodesulfurization activity for dibenzothiophene. Highlights: > A new synthetic route by heat treating mixtures of metal and red phosphorus in flowing N{sub 2} to prepare corresponding metal phosphides. > Porous and hollow Ni{sub 2}P particles may successfully be obtained using the route. > It is very easy to synthesize other bulk and supported metal phosphides using the mixing of bulk and supported metal and red phosphorus by the method. > The Ni{sub 2}P/SiO{sub 2} catalyst synthesized by the route shows a good HDS of dibenzothiophene. > Its operation is simple (only heat treating pure metal and red phosphorus), and the reaction time is short (only 0.5 h).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

NdHO, a novel oxyhydride

A new metal oxyhydride; neodymium oxyhydride, NdHO, has been synthesized from a reactant mixture of metal hydride (CaH{sub 2} or NdH{sub 3}) and neodymium oxide (Nd{sub 2}O{sub 3}). The unit cell dimensions decrease smoothly in the series from LaHO, CeHO, PrHO to NdHO, in line with the lanthanide contraction. The crystal structure of NdHO is described on the basis of Rietveld refinement on neutron powder diffraction data: Space group: P4/nmm (no. 129, D{sub 4h}{sup 7}). Axis lengths: a=7.8480(5) A, c=5.5601(8) A. Volume: V=342.46(6) A{sup 3}. The tetragonal structure is derived from the fluorite structure, showing complete ordering of hydride and oxide ions over the anion sublatttice. The formation of NdHO was further substantiated by Raman spectroscopy. - Graphical Abstract: View of the NdHO structure. Highlights: > CaH{sub 2} or NdH{sub 3} react with Nd{sub 2}O{sub 3} in reducing atmospheres forming a new oxyhydride, NdHO. > NdHO crystalises in tetragonal space group P4/nmm with axes a=b=7.8480(5) A and c=5.5601(8) A. > The structure is similar to the fluorite structure with complete ordering of hydride and oxide ions. > Unit cell dimensions decrease smoothly in the series from LaHO, CeHO, PrHO to NdHO. > The formation of NdHO is substantiated by Raman spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Order and disorder in Ca{sub 2}ND{sub 0.90}H{sub 0.10}-A structural and thermal study

The structure of calcium nitride hydride and its deuterided form has been re-examined at room temperature and studied at high temperature using neutron powder diffraction and thermal analysis. When synthesised at 600 deg. C, a mixture of both ordered and disordered Ca{sub 2}ND{sub 0.90}H{sub 0.10} phases results. The disordered phase is the minor component and has a primitive rocksalt structure (spacegroup Fm3m) with no ordering of D/N on the anion sites and the ordered phase is best described using the rhombohedral spacegroup R-3m with D and N arranged in alternate layers in (111) planes. This mixture of ordered and disordered phases exists up to 580 deg. C, at which the loss of deuterium yields Ca{sub 2}ND{sub 0.85} with the disappearance of the disordered phase. In the new ordered phase there exists a similar content of vacancies on both anion sites; to achieve this balance, a little N transfers onto the D site, whereas there is no indication of D transferring onto the N-sites. These observations are thought to indicate that the D/N ordering is difficult to achieve with fully occupied anion sites. It has previously been reported that Ca{sub 2}ND has an ordered cubic cell with alternating D and N sites in the [100] directions ; however, for the samples studied herein, there were clearly two coexisting phases with apparent broadening/splitting of the primitive peaks but not for the ordered peaks. The rhombohedral phase was in fact metrically cubic; however, all the observed peaks were consistent with the rhombohedral unit cell with no peaks requiring the larger ordered cubic unit cell to be utilised. Furthermore this rhombohedral cell displays the same form of N-D ordering as the Sr and Ba analogues, which are metrically rhombohedral. - Graphical abstract: Ca{sub 2}ND{sub 0.90}H{sub 0.10} forms a mixture of ordered and disordered phases when synthesised at 600 deg. C. The ordered phase disappears at high temperature upon release of structural deuterium/hydrogen, leaving a single, partially disordered phase. Research Highlights: > Emergence of mixture of ordered and disordered phases at synthesis conditions used. > Disordered phase disappears at high temperature, leaving partially ordered phase. > Suggestion of ambiguity in anion ordering scheme.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Structural behavior and thermoelectric properties of the brownmillerite system Ca{sub 2}(Zn{sub x}Fe{sub 2-x})O{sub 5}

The Ca{sub 2}(Zn{sub x}Fe{sub 2-x})O{sub 5} series was synthesized and characterized to determine the influence of zinc dopant on the brownmillerite structure for thermoelectric applications. All single-phase compounds exhibited Pnma symmetry at room temperature up to the solubility limit at x=0.10. High-temperature X-ray powder diffraction was used to show that the nature of the Pnma-Imma(0 0 {gamma})s00 transition in Ca{sub 2}Fe{sub 2}O{sub 5} is modified by the presence of zinc. While the Zn-free composition transitions to an incommensurate phase, the Zn-containing phases transition instead to a commensurate phase, Imma(0 0 {gamma})s00 with {gamma}=1/2. Both the Neel temperature and the onset temperature of the Pnma-Imma(0 0 {gamma})s00 phase transition decreased with increasing zinc concentration. Rietveld analysis of the in situ diffraction pattern for the x=0 sample at 1300 deg. C demonstrates that the structure contains statistically disordered chain orientations as described by space group Imma. Thermoelectric properties were analyzed in air from 100 to 800 deg. C. The positive Seebeck coefficient revealed hole-type conduction for all compositions. Doped samples exhibited electrical conductivities up to 3.4 S/cm and thermal conductivity of 1.5 W/mK. Transport analysis revealed thermally activated mobility consistent with polaron conduction behavior for all compositions. - Graphical Abstract: Preferential substitution by zinc for iron at the tetrahedral sites of brownmillerite dicalcium ferrite modifies the high temperature Imma(0 0 {gamma})s00 phase from incommensurate to commensurate. Highlights: > Preferential tetrahedral occupancy by zinc in brownmillerite dicalcium ferrite. > Ca{sub 2}Fe{sub 2}O{sub 5} exhibits two additional phase transitions above 1000 deg. C: a commensurately modulated Imma(0 0 {gamma})s00 phase and a statistically disordered Imma phase. > Thermal hysteresis in phase transition increased with increasing zinc. > Electrical conductivity up to 3.4 S/cm at 800 deg. C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Synthesis and structural characterization of Al{sub 7}C{sub 3}N{sub 3}-homeotypic aluminum silicon oxycarbonitride, (Al{sub 7-x}Si{sub x})(O{sub y}C{sub z}N{sub 6-y-z}) (x{approx}1.2, y{approx}1.0 and z{approx}3.5)

A new aluminum silicon oxycarbonitride, (Al{sub 5.8}Si{sub 1.2})(O{sub 1.0}C{sub 3.5}N{sub 1.5}), has been synthesized and characterized by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). The title compound is hexagonal with space group P6{sub 3}/mmc and unit-cell dimensions a=0.322508(4) nm, c=3.17193(4) nm and V=0.285717(6) nm{sup 3}. The atom ratios of Al:Si and those of O:C:N were, respectively, determined by EDX and EELS. The initial structural model was successfully derived from the XRPD data by the direct methods and further refined by the Rietveld method. The crystal is most probably composed of four types of domains with nearly the same fraction, each of which is isotypic to Al{sub 7}C{sub 3}N{sub 3} with space group P6{sub 3}mc. The existence of another new oxycarbonitride (Al{sub 6.6}Si{sub 1.4})(O{sub 0.7}C{sub 4.3}N{sub 2.0}), which must be homeotypic to Al{sub 8}C{sub 3}N{sub 4}, has been also demonstrated by XRPD and TEM. - Graphical abstract: A new oxycarbonitride discovered in the Al-Si-O-C-N system, (Al{sub 7-x}Si{sub x})(O{sub y}C{sub z}N{sub 6-y-z}) (x{approx}1.2, y{approx}1.0 and z{approx}3.5). The crystal is composed of four types of domains (I, II, III and IV), and hence the structure is represented by a split-atom model. Individual crystal structures can be regarded as layered structures, which consist of A-type [(Al, Si){sub 4}(O, C, N){sub 4}] unit layers and B-type [(Al, Si)(O, C, N){sub 2}] single layers. Highlights: > (Al{sub 5.8}Si{sub 1.2})(O{sub 1.0}C{sub 3.5}N{sub 1.5}) as a new aluminum silicon oxycarbonitride. > Crystal structure is determined and represented by a split-atom model. > Existence of another new oxycarbonitride (Al{sub 6.6}Si{sub 1.4})(O{sub 0.7}C{sub 4.3}N{sub 2.0}) is demonstrated. > Both new materials are formed by oxidation and nitridation of (Al, Si){sub 6}(O, C){sub 5}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

One-step growth of Si{sub 3}N{sub 4} stem-branch featured nanostructures: Morphology control by VS and VLS mode

We report here one-step synthesis of Si{sub 3}N{sub 4} nanodendrites by selectively applying a vapor-solid (VS) and vapor-liquid-solid (VLS) strategy via direct current arc discharge method. The resultant nanodendrites were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy and X-ray powder diffraction. The spine-shaped nanodendrites were generated by a noncatalytic growth following a VS mode. The uniform secondary nanowire branches were epitaxial grown from two side surfaces of the nanowire stems. The pine-shaped nanodendrites were obtained through a catalytic growth in a VLS process. These branch nanowires were unsystematically grown from the nanocone-like stems. The photoluminescence spectra of the nanodendrites show a strong white light emission around 400-750 nm, suggesting their potential applications in light and electron emission devices. - Graphical abstract: Spine-shaped and pine-shaped Si{sub 3}N{sub 4} hierarchical nanostructures were synthesized by VS and VLS mode with plasma-assisted dc arc discharge method. Highlights: > Si{sub 3}N{sub 4} stem-branch featured nanostructures have been prepared. > Spine-shaped nanodendrites were generated by a noncatalytic growth following a VS mode. > Pine-shaped nanodendrites were obtained through a catalytic growth in a VLS process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗