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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Stabilization of over-stoichiometric Mn{sup 4+} in layered Na{sub 2/3}MnO{sub 2}

Sodium manganates with nominal composition Na{sub 2/3}MnO{sub 2} were prepared by solid state reaction between Na{sub 2}CO{sub 3} and MnCO{sub 3} at 1000 {sup o}C. The composition and structure of Na{sub x}MnO{sub 2} were controlled by the rate of cooling from the temperature of preparation. This is a consequence of the capability of Na{sub 2/3}MnO{sub 2} to accommodate over-stoichiometric Mn{sup 4+} ions up to 12.5%. Structural characterization was carried out by XRD powder diffractions, TEM analysis and Raman spectroscopy. The composition and oxidation state of manganese were determined by chemical analysis and magnetic susceptibility measurements. The manganese distribution in the layers was analysed using electron paramagnetic resonance (EPR) spectroscopy. By quenching from 1000 {sup o}C, the orthorhombic distorted modification is stabilized. A phase separation into orthorhombic and hexagonal modifications takes place when Na{sub 2/3}MnO{sub 2} is slow cooled. The structure changes are concomitant with an increase in the oxidation state of Mn. The over-stoichiometric Mn{sup 4+} ions are accommodated in the hexagonal modification by creation of vacancies in the MnO{sub 2}-layers. - Graphical abstract: The composition and structure of Na{sub 2/3}MnO{sub 2} were controlled by the rate of cooling from the temperature of preparation. By quenching from 1000 {sup o}C, the orthorhombic distorted modification is stabilized. A phase separation into orthorhombic and hexagonal modifications takes place when Na{sub 2/3}MnO{sub 2} is slow cooled. The over-stoichiometric Mn{sup 4+} ions are accommodated in the hexagonal modification by creation of vacancies in the MnO{sub 2}-layers.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Synthesis and structural chemistry of La{sub 18}Li{sub 8}Rh{sub 4}MO{sub 39} (M=Ti, Mn, Ru)

Polycrystalline samples of La{sub 18}Li{sub 8}Rh{sub 4}MO{sub 39} (M=Ti, Mn, Ru) have been prepared by a solid-state method and studied by neutron powder diffraction. They are isostructural with La{sub 18}Li{sub 8}Rh{sub 5}O{sub 39} and adopt the cubic space group Pm3{sup -bar }n with a {approx}12.22 A. Their structure consists of a La-O framework containing intersecting channels that run along <111>. These channels are occupied by chains made up of alternating, face-sharing trigonal-prismatic and octahedral coordination polyhedra; there are two crystallographically distinct types of octahedral site. The prisms are occupied by Li and the transition metals are disordered over the two octahedral sites. - Graphical abstract: The substitution of Ti, Mn or Ru into La{sub 18}Li{sub 8}Rh{sub 5}O{sub 39} results in the loss of the cation ordering seen in the parent compound.

36 MATERIALS SCIENCE↗
DOE OSTI2010

p-Type thermoelectric properties of the oxygen-deficient perovskite Ca{sub 2}Fe{sub 2}O{sub 5} in the brownmillerite structure

Brownmillerite calcium ferrite was synthesized in air at 1573 K and thermoelectric properties (direct current electrical conductivity {sigma}, Seebeck coefficient {alpha}, thermal conductivity {kappa}, thermal expansion {alpha}{sub L}) were measured from 373 to 1050 K in air. Seebeck coefficient was positive over all temperatures indicating conduction by holes, and electrical properties were continuous through the Pnma-Imma phase transition. Based on the thermopower and conductivity activation energies as well as estimated mobility, polaron hopping conduction was found to dominate charge transport. The low electrical conductivity, <1 S/cm, limits the power factor ({alpha}{sup 2{sigma}}), and thus the figure of merit for thermoelectric applications. The thermal conductivity values of {approx}2 W/mK and their similarity to Ruddlesden-Popper phase implies the potential of the alternating tetrahedral and octahedral layers to limit phonon propagation through brownmillerite structures. Bulk linear coefficient of thermal expansion ({approx}14x10{sup -6} K{sup -1}) was calculated from volume data based on high-temperature in situ X-ray powder diffraction, and shows the greatest expansion perpendicular to the alternating layers. - Graphical abstract: Representative microstructure of Ca{sub 2}Fe{sub 2}O{sub 5} samples used for thermoelectrical measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Divergent effects of static disorder and hole doping in geometrically frustrated {beta}-CaCr{sub 2}O{sub 4}

The gallium substituted and calcium deficient variants of geometrically frustrated {beta}-CaCr{sub 2}O{sub 4}, {beta}-CaCr{sub 2-2x}Ga{sub 2x}O{sub 4} (0.02{<=}x{<=}0.25) and {beta}-Ca{sub 1-y}Cr{sub 2}O{sub 4} (0.075{<=}y{<=}0.15), have been investigated by X-ray powder diffraction, magnetization and specific heat measurements. This allows for a direct comparison of the effects, in a geometrically frustrated magnet, of the static disorder that arises from non-magnetic substitution and the dynamic disorder that arises from hole doping. In both cases, disturbing the Cr{sup 3+} lattice results in a reduction in the degree of magnetic frustration. On substitution of Ga, which introduces disorder without creating holes, a gradual release of spins from ordered antiferromagnetic states is observed. In contrast, in the calcium-deficient compounds the introduction of holes induces static ferrimagnetic ordering and much stronger perturbations of the {beta}-CaCr{sub 2}O{sub 4} host. - Graphical abstract: Scaled plot of the temperature dependence of the inverse susceptibility for {beta}-CaCr{sub 2-2x}Ga{sub 2x}O{sub 4} (closed symbols) and {beta}-Ca{sub 1-y}Cr{sub 2}O{sub 4} (open symbols). Data sets depicted in the same color contain equivalent amounts of Cr{sup 3+}.

36 MATERIALS SCIENCE↗
DOE OSTI2010

The local structure and composition of Ba{sub 4}Nb{sub 2}O{sub 9}-based oxycarbonates

X-ray powder-diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), infrared spectroscopy (IR), thermogravimetry (TG) and mass spectroscopy (MS) were performed to investigate the composition and the crystal structure of tetra-barium di-niobate (V) Ba{sub 4}Nb{sub 2}O{sub 9}. The TG, MS and IR studies revealed that the compound is a hydrated oxycarbonate. Assuming that the carbonate stoichiometrically replaces oxygen, the composition of the low-temperature {alpha}-modification, obtained by slow cooling from 1100 {sup o}C, corresponds to Ba{sub 4}Nb{sub 2}O{sub 8.8}(CO{sub 3}){sub 0.2}.0.1H{sub 2}O, while the quenched high-temperature {gamma}-modification has the Ba{sub 4}Nb{sub 2}O{sub 8.42}(CO{sub 3}){sub 0.58}.0.38H{sub 2}O composition. The {alpha}-phase has a composite incommensurately modulated structure consisting of two mutually interacting [Ba]{sub {infinity}} and the [(Nb,{open_square})O{sub 3}]{sub {infinity}} subsystems. The composite modulated crystal structure of the {alpha}-phase can be described with the lattice parameters a=10.2688(1) A, c=2.82426(8) A, q=0.66774(2)c* and a superspace group R3{sup -bar }m(00{gamma})0s. The HRTEM analysis demonstrates the nanoscale twinning of the trigonal domains parallel to the {l_brace}1 0 0{r_brace} crystallographic planes. The twinning introduces a one-dimensional disorder into the [(Nb,{open_square})O{sub 3}]{sub {infinity}} subsystem, which results in an average P6{sup -bar }2c crystal structure of the {alpha}-phase. Possible places for the carbonate group in the structure are discussed using a comparison with other hexagonal perovskite-based oxycarbonates. - Graphical abstract: The composition and the local crystal structure of generally known {alpha}-Ba{sub 4}Nb{sub 2}O{sub 9} was studied. The compound is hydrated oxycarbonate with the composition of Ba{sub 4}Nb{sub 2}O{sub 8.8}(CO{sub 3}){sub 0.2}.0.1 H{sub 2}O. It has a composite incommensurately modulated structure consisting of two mutually interacting subsystems, i.e., [Ba]{infinity} and the [(Nb,)O{sub 3}]{infinity} subsystem.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Srystal structure and physical properties of the new ternary antimonides Ln{sub 3}Pd{sub 8}Sb{sub 4} (Ln=Y, Gd, Tb, Dy, Ho, Er, Tm)

The ternary antimonides Ln{sub 3}Pd{sub 8}Sb{sub 4} (Ln=Y, Gd, Tb, Dy, Ho, Er, Tm) have been synthesized for the first time. The crystal structure of Er{sub 3}Pd{sub 8}Sb{sub 4} has been solved from the X-ray single crystal data: own type structure, space group Fm3-bar m, a=1.3050(1) nm, R{sub F}=0.0484, R{sub W}=0.0524 for 17 free parameters and 401 reflections with F(hkl)>4{sigma}(F). The structure of Er{sub 3}Pd{sub 8}Sb{sub 4} can be viewed as a ternary ordered version of the Sc{sub 11}Ir{sub 4}-type. The lattice parameters of the isotypic compounds Ln{sub 3}Pd{sub 8}Sb{sub 4} (Ln=Y, Gd, Tb, Dy, Ho, Tm) have been refined from the X-ray powder diffraction data. The magnetic and electrical properties of the compounds Ln{sub 3}Pd{sub 8}Sb{sub 4} (Ln=Tb, Ho, Er) have been studied down to 1.75 K. The Ho- and Er-based phases have been found to order antiferromagnetically at 2.5 and 2.0 K, respectively. For all three compounds, the magnetic susceptibility follows in the paramagnetic region the Curie-Weiss behavior with the effective magnetic moments close to the respective free trivalent ion values. All three antimonides studied exhibit metallic character of the electrical conductivity. - Graphical abstract: Projection of the crystal structure of Er{sub 3}Pd{sub 8}Sb{sub 4} onto XY plane and the coordination polyhedra of all the atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Simple synthesis of ultra-long Ag{sub 2}Te nanowires through solvothermal co-reduction method

Ultra-long single crystal {beta}-Ag{sub 2}Te nanowires with the diameter of about 300 nm were fabricated through a solvothermal route in ethylene glycol (EG) system without any template. The long single crystal wires were curves, with high purity, well-crystallized, and dislocation-free and characterized by using X-ray powder diffraction (XRD), Differential scanning calorimetry (DSC) analysis, X-ray photoelectron spectroscope (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high-resolution transmission microscopy (HRTEM). The detailed topotactic transformation process from particles into single crystal wires was studied. Furthermore, the electrical conductivity and Seebeck coefficient have been systematically studied between 300 and 600 K. - Graphical abstract: Ultra-long single crystal {beta}-Ag{sub 2}Te nanowires with the diameter of about 300 nm were fabricated by the solvothermal route in ethylene glycol (EG) system without any template. The diagram displays the variation of the phases and morphologies of products with different reaction time.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2010

Synthesis and structural characterisation using Rietveld and pair distribution function analysis of layered mixed titaniumzirconium phosphates

Crystalline metal (IV) phosphates with variable zirconium-to-titanium molar ratios of general formula (Ti 1-x Zr x )(HPO 4 ) 2 • H 2 O have been prepared by precipitation of soluble salts of the metals with phosphoric acid and heating the amorphous solids in 12 M H 3 PO 4 in an autoclave. The new materials are structurally characterised by Rietveld analysis of synchrotron X-ray powder diffraction data and pair distribution function (PDF) analysis of high energy synchrotron X-ray total scattering data. A broad range of zirconium-titanium phosphate solid solutions were formed showing isomorphous substitution of titanium by zirconium in the α-titanium phosphate lattice and vice versa for titanium substitution into the α-zirconium phosphate lattice. In both cases the solubility is partial with the coexistence of two substituted phases observed in samples with nominal compositions between the solubility limits.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Synthesis and structural characterization of Al{sub 4}Si{sub 2}C{sub 5}-homeotypic aluminum silicon oxycarbide, (Al{sub 6-x}Si{sub x})(O{sub y}C{sub 5-y}) (x{approx}0.8 and y{approx}1.6)

We have prepared a new layered oxycarbide, [Al{sub 5.25(5)}Si{sub 0.75(5)}][O{sub 1.60(7)}C{sub 3.40(7)}], by isothermal heating of (Al{sub 4.4}Si{sub 0.6})(O{sub 1.0}C{sub 3.0}) at 2273 K near the carbon-carbon monoxide buffer. The crystal structure was characterized using X-ray powder diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX). The title compound is trigonal with space group R3m (centrosymmetric), Z=3, and hexagonal cell dimensions a=0.32464(2) nm, c=4.00527(14) nm and V=0.36556(3) nm{sup 3}. The atom ratios Al:Si were determined by EDX, and the initial structural model was derived by the direct methods. The final structural model showed the positional disordering of one of the three types of Al/Si sites. The reliability indices were R{sub wp}=4.45% (S=1.30), R{sub p}=3.48%, R{sub B}=2.27% and R{sub F}=1.25%. The crystal is composed of three types of domains with nearly the same fraction, one of which has the crystal structure of space group R3-bar m. The crystal structure of the remaining two domains, which are related by pseudo-symmetry inversion, is noncentrosymmetric with space group R3m. - Graphical Abstract: A new aluminum silicon oxycarbide, (Al{sub 6-x}Si{sub x})(O{sub y}C{sub 5-y}) (x{approx}0.8 and y{approx}1.6). The crystal is composed of three types of domains (I, II and III), 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){sub 4}] unit layers and B-type [(Al,Si)(O,C){sub 2}] single layers.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Anisotropic compression of a synthetic potassium aluminogermanate zeolite with gismondine topology

Compression behavior of a potassium aluminogermanate with a gismondine framework topology (K-AlGe-GIS) was studied using in-situ high-pressure synchrotron X-ray powder diffraction. In contrast to the potassium gallosilicate analogue (K-GaSi-GIS), no elastic anomaly due to pressure-induced hydration and/or cation relocation was observed in K-AlGe-GIS. The Birch-Murnaghan fit to the pressure-volume data results in a bulk modulus of B{sub 0} = 31(1) GPa. The derived linear-axial compressibilities (i.e., {beta}{sub a} = 0.0065(5) GPa{sup -1}, {beta}{sub b} = 0.0196(4) GPa{sup -1}, {beta}{sub c} = 0.0081(7) GPa{sup -1}) indicate that the b-axis, normal to the 8-ring channels, is about three times more compressible than the a and c axes, parallel to the elliptical 8-ring channels. As a consequence a gradual flattening of the so-called 'double crankshaft' structural building units of the gismondine framework is observed. In K-AlGe-GIS, this flattening occurs almost linear with pressure, whereas it is nonlinear in the GaSi-analogue due to structural changes of the water-cation assembly under hydrostatic pressures.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Crystal structure and physical properties of quaternary clathrates Ba{sub 8}Zn{sub x}Ge{sub 46-x-y}Si{sub y}, Ba{sub 8}(Zn,Cu){sub x}Ge{sub 46-x} and Ba{sub 8}(Zn,Pd){sub x}Ge{sub 46-x}

Three series of vacancy-free quaternary clathrates of type I, Ba{sub 8}Zn{sub x}Ge{sub 46-x-y}Si{sub y}, Ba{sub 8}(Zn,Cu){sub x}Ge{sub 46-x}, and Ba{sub 8}(Zn,Pd){sub x}Ge{sub 46-x}, have been prepared by reactions of elemental ingots in vacuum sealed quartz at 800 {sup o}C. In all cases cubic primitive symmetry (space group Pm3n, a{approx}1.1 nm) was confirmed for the clathrate phase by X-ray powder diffraction and X-ray single crystal analyses. The lattice parameters show a linear increase with increase in Ge for Ba{sub 8}Zn{sub x}Ge{sub 46-x-y}Si{sub y}. M atoms (Zn, Pd, Cu) preferably occupy the 6d site in random mixtures. No defects were observed for the 6d site. Site preference of Ge and Si in Ba{sub 8}Zn{sub x}Ge{sub 46-x-y}Si{sub y} has been elucidated from X-ray refinement: Ge atoms linearly substitute Si in the 24k site whilst a significant deviation from linearity is observed for occupation of the 16i site. A connectivity scheme for the phase equilibria in the 'Ba{sub 8}Ge{sub 46}' corner at 800 {sup o}C has been derived and a three-dimensional isothermal section at 800 {sup o}C is presented for the Ba-Pd-Zn-Ge system. Studies of transport properties carried out for Ba{sub 8{l_brace}}Cu,Pd,Zn{r_brace}{sub x}Ge{sub 46-x} and Ba{sub 8}Zn{sub x}Si{sub y}Ge{sub 46-x-y} evidenced predominantly electrons as charge carriers and the closeness of the systems to a metal-to-insulator transition, fine-tuned by substitution and mechanical processing of starting material Ba{sub 8}Ge{sub 43}. A promising figure of merit, ZT {approx}0.45 at 750 K, has been derived for Ba{sub 8}Zn{sub 7.4}Ge{sub 19.8}Si{sub 18.8}, where pricey germanium is exchanged by reasonably cheap silicon. - Graphical abstract: Quaternary phase diagram of Ba-Pd-Zn-Ge system at 800 {sup o}C.

36 MATERIALS SCIENCE↗
DOE OSTI2010

The anti-perovskite type hydride InPd{sub 3}H{sub 0.89}

Hydrogenation of tetragonal InPd{sub 3} in the ZrAl{sub 3} type structure (four-fold ccp superstructure) yields a hydride with a cubic AuCu{sub 3} type structure (one-fold ccp superstructure). Deuterium can be located by neutron powder diffraction in octahedral voids surrounded exclusively by palladium, [Pd{sub 6}], which are 88.5(6)% occupied in a statistical manner. The resulting deuteride InPd{sub 3}D{sub 0.89} thus crystallizes in a cubic anti-perovskite type structure (space group Pm3-bar m (no. 221), a=402.25(1) pm at 299(2) K). The Pd-D distance of 201.13(1) pm is typical for interstitial hydrides with palladium. Inelastic neutron scattering on the hydride InPd{sub 3}H{sub 0.89}, which shows a spectrum similar to that of binary palladium hydride, confirms the cubic site symmetry of hydrogen in [Pd{sub 6}] interstices. This is also confirmed by the absence of any quadrupole splitting in the {sup 2}D-NMR signal of the deuteride. {sup 1}H NMR spectra of InPd{sub 3}H{sub 0.89} do not show any motional narrowing. Values found for the H jump rate {tau}{sup -1} in InPd{sub 3}H{sub 0.89} remain below 10{sup 6} s{sup -1} in the studied temperature range 28-360 K, indicating a small hydrogen mobility in InPd{sub 3}H{sub 0.8} as compared with binary palladium hydride, PdH{sub {<=}1}. This can be attributed to the large spatial separation of the [Pd{sub 6}] sites. - Graphical abstract: Hydrogen induces a rearrangement in InPd{sub 3} from a ZrAl{sub 3} type structure to a cubic AuCu{sub 3} type structure, thus forming an anti-perovskite type hydride InPd{sub 3}H{sub 0.89}.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Synthesis and structural characterization of a new aluminum oxycarbonitride, Al{sub 5}(O, C, N){sub 4}

A new aluminum oxycarbonitride, Al{sub 5}(O{sub x}C{sub y}N{sub 4-x-y}) (x{approx}1.4 and y{approx}2.1), has been synthesized and characterized by X-ray powder diffraction, transmission electron microscopy and electron energy loss spectroscopy (EELS). The title compound was found to be hexagonal with space group P6{sub 3}/mmc, Z=2, and unit-cell dimensions a=0.328455(6) nm, c=2.15998(3) nm and V=0.201805(6) nm{sup 3}. The atom ratios O:C:N were determined by EELS. The final structural model, which is isomorphous with that of (Al{sub 4.4}Si{sub 0.6})(O{sub 1.0}C{sub 3.0}), showed the positional disordering of one of the three types of Al sites. The maximum-entropy method-based pattern fitting (MPF) method was used to confirm the validity of the split-atom model, in which conventional structure bias caused by assuming intensity partitioning was minimized. The reliability indices calculated from the MPF were R{sub wp}=6.94% (S=1.22), R{sub p}=5.34%, R{sub B}=1.35% and R{sub F}=0.76%. The crystal was an inversion twin. Each twin-related individual was isostructural with Al{sub 5}C{sub 3}N (space group P6{sub 3}mc, Z=2). - Graphical abstract: A new oxycarbonitride discovered in the Al-O-C-N system, Al{sub 5}(O{sub 1.4}C{sub 2.1}N{sub 0.5}). The crystal is an inversion twin, and hence the structure is represented by a split-atom model. The three-dimensional electron density distributions are determined by the maximum-entropy methods-based pattern fitting, being consistent with the disordered structural model. Display Omitted

36 MATERIALS SCIENCE↗
DOE OSTI2010

NdBaFe{sub 2}O{sub 5+w} and steric effect of Nd on valence mixing and ordering of Fe

NdBaFe{sub 2}O{sub 5} above and below Verwey transition is studied by synchrotron X-ray powder diffraction and Moessbauer spectroscopy and compared with GdBaFe{sub 2}O{sub 5} that adopts a higher-symmetry charge-ordered structure typical of the Sm-Ho variants of the title phase. Differences are investigated by Moessbauer spectroscopy accounting for iron valence states at their local magnetic and ionic environments. In the charge-ordered state, the orientation of the electric-field gradient (EFG) versus the internal magnetic field (B) agrees with experiment only when contribution from charges of the ordered d{sub xz} orbitals of Fe{sup 2+} is included, proving thus the orbital ordering. The EFG magnitude indicates that only some 60% of the orbital order occurring in the Sm-Ho variants is achieved in NdBaFe{sub 2}O{sub 5}. The consequent diminishing of the orbit contribution (of opposite sign) to the field B at the Fe{sup 2+} nucleus explains why B is larger than for the Sm-Ho variants. The decreased orbital ordering in NdBaFe{sub 2}O{sub 5} causes a corresponding decrease in charge ordering, which is achieved by decreasing both the amount of the charge-ordered iron states in the sample and their fractional valence separation as seen by the Moessbauer isomer shift. The charge ordering in NdBaFe{sub 2}O{sub 5+w} is more easily suppressed by the oxygen nonstoichiometry (w) than in the Sm-Ho variants. Also the valence mixing into Fe{sup 2.5+} is destabilized by the large size of Nd. The orientation of the EFG around this valence-mixed iron can only be accounted for when the valence-mixing electron is included in the electrostatic ligand field. This proves that the valence mixing occurs between the two iron atoms facing each other across the structural plane of the rare-earth atoms. -- Graphical Abstract: Moessbauer spectrum detects ordering of d{sub xz} orbitals of Fe{sup II}O{sub 5} via the electric-field gradient (EFG) of the orbital, which makes the main component of the total EFG parallel with the magnetic moment B. Display Omitted

36 MATERIALS SCIENCE↗
DOE OSTI2010

Magnetic properties of Fe{sub 2}P-type Tb{sub 6}FeTe{sub 2}, Tb{sub 6}CoTe{sub 2}, Tb{sub 6}NiTe{sub 2} and Er{sub 6}FeTe{sub 2} compounds

The magnetic ordering of the Fe{sub 2}P-type Tb{sub 6}FeTe{sub 2}, Tb{sub 6}CoTe{sub 2} Tb{sub 6}NiTe{sub 2} and Er{sub 6}FeTe{sub 2} phases (space group P6-bar 2m) has been investigated through magnetization measurement and neutron powder diffraction. Tb{sub 6}FeTe{sub 2}, Tb{sub 6}CoTe{sub 2} and Tb{sub 6}NiTe{sub 2} demonstrate high-temperature ferromagnetic and low-temperature spin reorientation transitions, whereas Er{sub 6}FeTe{sub 2} shows antiferromagnetic transition, only. The Tb{sub 6}FeTe{sub 2} and Tb{sub 6}NiTe{sub 2} phases show same high-temperature collinear ferromagnetic structure, whereas Tb{sub 6}FeTe{sub 2} is the commensurate non-collinear ferromagnet and Tb{sub 6}NiTe{sub 2} is the canted ferromagnetic cone with K{sub 1}=[0, 0, {+-}3/10] and K{sub 2}=[{+-}2/9, {+-}2/9, 0] wave vectors at 2 K. The magnetic structure of Er{sub 6}FeTe{sub 2} is a flat spiral with K{sub 1}=[0, 0, {+-}1/10] at 2 K. The magnetic entropy change for Tb{sub 6}NiTe{sub 2} is {Delta}S{sub m}=-4.86 J/kg K at 229 K for the field change {Delta}{mu}{sub 0}H=0-5 T. In addition, novel Fe{sub 2}P-type Gd{sub 6}FeTe{sub 2}, Zr{sub 6}FeTe{sub 2}, Hf{sub 6}FeTe{sub 2}, Dy{sub 6}NiTe{sub 2}, Zr{sub 6}NiTe{sub 2} and Hf{sub 6}NiTe{sub 2} phases have been obtained. -- Graphical abstract: The novel Fe{sub 2}P-type Tb{sub 6{l_brace}}Fe, Co, Te{r_brace}Te{sub 2} and Er{sub 6}FeTe{sub 2} phases (space group P6-bar 2m) show the complex magnetic ordering below 18-228 K. The magnetocaloric effect for Tb{sub 6}NiTe{sub 2} in terms of the isothermal entropy change, -{Delta}S{sub m}, has the maximum value of 4.86 J/kg K at 229 K for the 0-5 T field change.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Fully reversible hydrogen absorption and desorption reactions with Sc(Al{sub 1-x}Mg{sub x}), x=0.0, 0.15, 0.20

The hydrogen storage properties of Sc(Al{sub 1-x}Mg{sub x}), x=0.0, 0.15, 0.20, have been studied by X-ray powder diffraction, thermal desorption spectroscopy, pressure-composition-isotherms and scanning electron microscopy techniques. Hydrogen is absorbed from the gas phase at 70 kPa and 400 {sup o}C under the formation of ScH{sub 2} and aluminium with magnesium in solid solution. The reaction is fully reversible in vacuum at 500 {sup o}C and shows the hydrogenation-disproportionation-desorption-recombination (HDDR) behaviour. The activation energy of desorption was determined by the Kissinger method to 185 kJ/mol. The material is stable up to at least six absorption-desorption cycles and there is no change in particle size during cycling. -- Graphical abstract: XRD pattern of Sc(Al{sub 1-x}Mg{sub x}). From the top: x=0, 0.15, 0.20. The hydrogen absorption properties were studied by thermal desorption spectroscopy, pressure-composition-isotherms and scanning electron microscopy techniques. Display Omitted

36 MATERIALS SCIENCE↗
DOE OSTI2011

Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La{sub 2}Co{sub 1+z}(Mg{sub x}Ti{sub 1-x}){sub 1-z}O{sub 6}

Perovskite-type cobaltates in the system La{sub 2}Co{sub 1+z}(Mg{sub x}Ti{sub 1-x}){sub 1-z}O{sub 6} were studied for z=0{<=}x{<=}0.6 and 0{<=}x<0.9, using X-ray and neutron powder diffraction, electron diffraction (ED), magnetic susceptibility measurements and X-ray absorption near-edge structure (XANES) spectroscopy. The samples were synthesised using the citrate route in air at 1350 {sup o}C. The space group symmetry of the structure changes from P2{sub 1}/n via Pbnm to R3-bar c with both increasing Mg content and increasing Co content. The La{sub 2}Co(Mg{sub x}Ti{sub 1-x})O{sub 6} (z=0) compounds show anti-ferromagnetic couplings of the magnetic moments for the Co below 15 K for x=0, 0.1 and 0.2. XANES spectra show for the compositions 0{<=}x{<=}0.5 a linear decrease in the L{sub 3}/(L{sub 3}+L{sub 2}) Co-L{sub 2,3} edge branching ratio with x, in agreement with a decrease of the average Co ion spin-state, from a high-spin to a lower-spin-state, with decreasing nominal Co{sup 2+} ion content. -- Graphical abstract: XRPD patterns for perovskite compounds along the lines La{sub 2}Co(Mg{sub x}Ti{sub 1-x})O{sub 6} and La{sub 2}Co{sub 1+z}(Mg{sub 0.5}Ti{sub 0.5}){sub 1-z}O{sub 6}. Display Omitted Research Highlights: {yields}Tuning of the oxidation state of Co in the perovskite system La{sub 2}Co{sub 1+z}(Mg{sub x}Ti{sub 1-x}){sub 1-z}O{sub 6}, z=0{<=}x{<=}0.6 and 0{<=}x<0.9. {yields}XANES study of the oxidation state of Co in the perovskite system La{sub 2}Co{sub 1+z}(Mg{sub x}Ti{sub 1-x}){sub 1-z}O{sub 6}, z=0{<=}x{<=}0.6 and 0{<=}x<0.9. {yields}Phase relations in the perovskite system La{sub 2}Co{sub 1+z}(Mg{sub x}Ti{sub 1-x}){sub 1-z}O{sub 6}, z=0{<=}x{<=}0.6 and 0{<=}x<0.9. {yields}Magnetic properties in the perovskite system La{sub 2}Co{sub 1+z}(Mg{sub x}Ti{sub 1-x}){sub 1-z}O{sub 6}, z=0{<=}x{<=}0.6 and 0{<=}x<0.9.

36 MATERIALS SCIENCE↗
DOE OSTI2011

High-pressure high-temperature synthesis and crystal structure of the isotypic rare earth (RE)-thioborate-sulfides RE{sub 9}[BS{sub 3}]{sub 2}[BS{sub 4}]{sub 3}S{sub 3}, (RE=Dy-Lu)

Application of high-pressure high-temperature conditions (3.5 GPa at 1673 K for 5 h) to mixtures of the elements (RE:B:S=1:3:6) yielded crystalline samples of the isotypic rare earth-thioborate-sulfides RE{sub 9}[BS{sub 3}]{sub 2}[BS{sub 4}]{sub 3}S{sub 3}, (RE=Dy-Lu), which crystallize in space group P6{sub 3} (Z=2/3) and adopt the Ce{sub 6}Al{sub 3.33}S{sub 14} structure type. The crystal structures were refined from X-ray powder diffraction data by applying the Rietveld method. Dy: a=9.4044(2) A, c=5.8855(3) A; Ho: a=9.3703(1) A, c=5.8826(1) A; Er: a=9.3279(12) A, c=5.8793(8) A; Tm: a=9.2869(3) A, c=5.8781(3) A; Yb: a=9.2514(5) A, c=5.8805(6) A; Lu: a=9.2162(3) A, c=5.8911(3) A. The crystal structure is characterized by the presence of two isolated complex ions [BS{sub 3}]{sup 3-} and [BS{sub 4}]{sup 5-} as well as [{open_square}(S{sup 2-}){sub 3}] units. -- Graphical abstract: Isotypic rare earth-thioborate-sulfides RE{sub 9}[BS{sub 3}]{sub 2}[BS{sub 4}]{sub 3}S{sub 3}, (RE=Dy-Lu) were prepared by application of high-pressure high-temperature conditions to mixtures of the elements. Their crystal structures are characterized by presence of the two isolated complex ions [BS{sub 3}]{sup 3-} and [BS{sub 4}]{sup 5-} as well as [{open_square}(S{sup 2-}){sub 3}] units. Quantum mechanical calculations revealed the arrangement of the intrinsic vacancies. Display Omitted Research Highlights: {yields} Application of high-pressure high-temperature conditions to mixtures of the elements yields crystalline samples of the isotypic rare earth-thioborate-sulfides RE{sub 9}[BS{sub 3}]{sub 2}[BS{sub 4}]{sub 3}S{sub 3}, (RE=Dy-Lu). {yields} RE{sub 9}[BS{sub 3}]{sub 2}[BS{sub 4}]{sub 3}S{sub 3}, (RE=Dy-Lu) crystallize in space group P6{sub 3} (Z=2/3) and adopt the Ce{sub 6}Al{sub 3.33}S{sub 14} structure type. {yields} The crystal structure of RE{sub 9}[BS{sub 3}]{sub 2}[BS{sub 4}]{sub 3}S{sub 3}, (RE=Dy-Lu) is characterized by the presence of two isolated complex ions [BS{sub 3}]{sup 3-} and [BS{sub 4}]{sup 5-} as well as [{open_square}(S{sup 2-}){sub 3}] units. {yields} The vacancies in Lu{sub 9}[BS{sub 3}]{sub 2}[BS{sub 4}]{sub 3}S{sub 3} are located at the trigonal planar coordinated boron site with preferred ordering -B2-B2-{open_square}-B2-B2-{open_square}- along [001].

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