Showing papers on "Orthorhombic crystal system published in 2005"

Analysis of vapor-liquid-solid mechanism in Au-assisted GaAs nanowire growth

Abstract: GaAs nanowires were grown by molecular-beam epitaxy on (111)B oriented surfaces, after the deposition of Au nanoparticles. Different growth durations and different growth terminations were tested. After the growth of the nanowires, the structure and the composition of the metallic particles were analyzed by transmission electron microscopy and energy dispersive x-ray spectroscopy. We identified three different metallic compounds: the hexagonal β′Au7Ga2 structure, the orthorhombic AuGa structure, and an almost pure Au face centered cubic structure. We explain how these different solid phases are related to the growth history of the samples. It is concluded that during the wire growth, the metallic particles are liquid, in agreement with the generally accepted vapor-liquid-solid mechanism. In addition, the analysis of the wire morphology indicates that Ga adatoms migrate along the wire sidewalls with a mean length of about 3μm.

Raman Scattering Study of Piezoelectric (Na0.5K0.5)NbO3-LiNbO3 Ceramics

Abstract: A Raman scattering study of (Na0.5K0.5)NbO3 (NKN)-LiNbO3 (LN) lead-free piezoceramics has been carried out on nominal compositions of (1-x)NKN-xLN (0 ≤x ≤0.70). The Raman spectra demonstrated a variety of changes with x, mainly classified as lattice translations involving motions of the alkaline cations and internal modes of NbO6 octahedra. At 0.05 ≤x ≤0.07, the broadening of the scattering peaks corresponding to the internal modes of the NbO6 octahedra occurred preferably, which is consistent with the evidence for a morphotropic phase boundary (MPB). Furthermore, an abrupt shift was observed in the peak position of the symmetric stretching mode v1 to a higher frequency, resulting from the distortion of O-Nb-O angles caused by incorporating small Li ions into the perovskite units. This is considered to be a prologue for the structural transformation in the NKN-LN solid solution from orthorhombic to tetragonal symmetry. At compositions in which x increases above the MPB, the translational mode of the Li+ cation emerged clearly, and the overall scattering pattern gradually changed to complex patterns caused by the formation of a tungsten-bronze K3Li2Nb5O15 (KLN) secondary phase and an increase in the number of distorted LiNbO3-type crystal units.

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. Part I. Structural and thermo-electrical characterization

Abstract: With the purpose of fabricating ceramics where ferroelectric and magnetic order coexist, ceramics of Pb(Fe1∕2Nb1∕2)O3 have been prepared using the traditional ceramic method following three different routes. The first is a direct via starting from oxide reagents and the other two use different kinds of FeNbO4 precursors with either monoclinic or orthorhombic structures. Crystallographic and surface morphological studies were carried out by the powder x-ray diffraction and scanning microscopy techniques. The presence of Fe2+, detrimental to the ferroelectric and magnetic performance, was evaluated by x-ray photoelectron spectroscopy. The samples showed no structural differences, uniformly distributed grains, a ferro-paraelectric transition temperature at 110°C and a normal diffuse phase transition (nonrelaxor behavior). Differences in the degree of diffuseness, densities and grain size were observed depending on the kind of precursor. Measurements of dc and ac electrical resistivity, dielectric constant an...

Electric modulus approach to the analysis of electric relaxation in highly conducting (Na 0.75 Bi 0.25 )(Mn 0.25 Nb 0.75 )O 3 ceramics

Abstract: Broadband dielectric spectroscopy is applied to investigate the electrical properties of disordered perovskite-like ceramics in a wide temperature range. From the x-ray diffraction analysis it was found that the newly obtained (Na0.75Bi0.25) (Mn0.25Nb0.75)O3 ceramics consist of two chemically different phases. The major perovskite one has an orthorhombic structure described by the Pbcm space group (No 57, in yxz setting). The minor phase shows an orthorhombic symmetry, all-face-centred lattice F, with the lattice parameters a = 10.797(4) A, b = 7.601(3) A and c = 7.691(3) A. The electric modulus M* formalism used in the analysis enabled us to distinguish and separate the relaxation processes, dominated by marked conductivity in the e*(ω) representation. In the ceramics studied, the relaxation times are thermally activated and the dipole process has a clearly non-Debye behaviour. The relaxation process described with the use of the activation energy of approximately 0.4 eV and the characteristic relaxation time, τ0 = 1 × 10−11 s, was found to be related to oxygen vacancies. The low frequency relaxation shows Debye behaviour with a slightly lower activation energy and a longer characteristic time.

Structures, Phase Transformations, and Dielectric Properties of Pyrochlores Containing Bismuth

Abstract: The structures, phase transformations, and dielectric properties of pyrochlore ceramics containing bismuth were studied. The relation between the orthorhombic and cubic pyrochlores in the Bi2O3-ZnO-Nb2O5 (BZN) system has been investigated. The effect of bismuth in distorted cubic pyrochlore structures is discussed. BZN compositions with pyrochlore structures have excellent dielectric properties, very low dielectric loss, and high dielectric constants with stable frequency and temperature dependence.

Crystal structure of the orthorhombic U2-Al4Mg4Si4 precipitate in the Al–Mg–Si alloy system and its relation to the β′ and β″ phases

Abstract: The atomic structure of a common precipitate in the Al–Mg–Si system has been determined. It is isotypic with TiNiSi (space group Pnma) and contains four units of MgAlSi in a unit cell of size a = 0.675 nm, b = 0.405 nm, c = 0.794 nm. EDS analyses support the composition. A model was based on the atomic structure of the β′ precipitate, electron diffraction and high-resolution transmission electron microscopy (HRTEM) images. A quantum mechanical refinement of the model removed discrepancies between simulated and experimental diffraction intensities. Finally, a multi-slice least square refinement confirmed the structure. The structural relation with β″ is investigated. A similar Mg–Si plane also existing in β″ and β′, can explain most coherency relations between the precipitate phases and with matrix.

Three-dimensional 3d-4f heterometallic coordination polymers: synthesis, structures, and magnetic properties.

Abstract: Three new 3d-4f heterometallic coordination polymers, [Ln 2 (H 2 O) 4 M 2 (H 2 O) 2 (QA) 5 ].nH 2 O (H 2 QA = quinolinic acid; Ln = Gd, M = Ni, n = 7 (1); Ln = Gd, M = Co, n = 6.5 (2); Ln = Dy, M = Co, n = 6.5 (3)), have been synthesized through hydrothermal pretreatment and cooling-down crystallization. These compounds possess the isostructural 3D frameworks with 1 D chairlike channels along the c axis, which are occupied by noncoordinating water molecules. Crystal data: for 1, C 3 5 H 4 1 Gd 2 Ni 2 N 5 O 3 3 , orthorhombic, space group Pna2 1 , with a = 28.567(6) A, b = 14.498(3) A, c = 12.250(2) A, and Z= 4; for 2, C 3 5 H 4 0 Gd 2 Co 2 N 5 O 3 2 . 5 , orthorhombic, space group Pna2 1 , with a = 28.843(3) A, b = 14.4325(13) A, c = 12.2275(9) A, and Z = 4; for 3, C 3 5 H 4 0 Dy 2 Co 2 N 5 O 3 2 . 5 , orthorhombic, space group Pna2 1 , with a = 28.8471(14) A, b = 14.4534(10) A, c = 12.2520(7) A, and Z = 4. The magnetic behaviors for the three compounds have been investigated.

Thin films of hard cubic Zr3N4 stabilized by stress

Abstract: Hard, refractory thin films consisting of group IVB element mono-nitrides deposited using various chemical and physical vapour-deposition techniques are widely used in wear-resistant applications. As the demand for performance exceeds the capabilities of existing materials, new materials with superior properties must be developed. Here we report the realization and characterization of hard cubic Zr3N4 (c-Zr3N4) thin films. The films, deposited using a novel but industrially viable modified filtered cathodic arc (FCA) method, undergo a phase transformation from orthorhombic to cubic above a critical stress level of 9 GPa as determined by X-ray diffraction and Raman spectroscopy. The c-Zr3N4 films are significantly harder (approximately 36 GPa) than both the orthorhombic Zr3N4 (o-Zr3N4) and ZrN films (approximately 27 GPa). The ability to deposit this material directly onto components as a thin film will allow its use in wear- and oxidation-resistant applications.

Syntheses, Structures, and Magnetic Properties of Diphenoxo-Bridged MIILnIII Complexes Derived from N,N‘-Ethylenebis(3-ethoxysalicylaldiimine) (M = Cu or Ni; Ln = Ce−Yb): Observation of Surprisingly Strong Exchange Interactions

Abstract: A series of heterodinuclear CuIILnIII and NiIILnIII complexes, [MIIL1LnIII(NO3)3] (M = Cu or Ni; Ln = Ce−Yb), with the hexadentate Schiff base compartmental ligand N,N‘-ethylenebis(3-ethoxysalicylaldiimine) (H2L1) have been synthesized and characterized. The X-ray crystal structure determinations of 13 of these compounds reveal that they are all isostructural. All of these complexes crystallize with the same orthorhombic P212121 space group with closely similar unit cell parameters. Typically, the structure consists of a diphenoxo-bridged 3d−4f dinuclear core, self-assembled to two dimensions due to the intermolecular nitrate···copper(II) or nitrate···nickel(II) semicoordination and weak C−H···O hydrogen bonds. Despite that, the metal centers of the neighboring units are well separated (the ranges of the shortest intermolecular contacts (A) are (M···M) 7.46−7.60, (Ln···Ln) 8.56−8.69, and (M···Ln) 6.12−6.20). Variable-temperature (5−300 K) magnetic susceptibility measurements of all the complexes have been...

Microstructural Characterization of Commercial Hot‐Pressed Boron Carbide Ceramics

Abstract: Two commercial hot-pressed boron carbide ceramics were investigated by transmission electron microscopy. Atomic-scale observations suggest that the grain boundaries of the two materials are free of grain-boundary films. Two triple-junction phases were found and characterized to be rhombohedral Fe 2 B 1 0 3 and orthorhombic Ti 3 B 4 . In addition, intra-granular precipitates, AIN, Mo 2 (C, B) and graphite, were identified and found to have coherent relationships with the boron carbide matrix. Micron-scale inclusions were also observed and most of them were determined to be graphite. The formation mechanisms of the secondary phases and their possible influence on mechanical properties are also discussed.

Observation of a wurtzite form of gallium arsenide.

Abstract: After a pressure decrease to ambient, the high-pressure SC16 phase of GaAs is found to transform to the hexagonal wurtzite structure. This has been suggested for GaAs in calculations but never previously observed experimentally. Wurtzite-GaAs is found to be stable at ambient pressures at temperatures up to 473 K, with a structure that is only slightly distorted from ideal. On recompression, the ratio is constant with pressure and wurtzite-GaAs transforms to the orthorhombic phase at 18.7(9) GPa.

Polymorphism in micro-, submicro-, and nanocrystalline NaNbO3.

Abstract: NaNbO3 powders with various particle sizes (ranging from 30 nm to several microns) and well-controlled stoichiometry were obtained through microemulsion-mediated synthesis. The effect of particle size on the phase transformation of the prepared NaNbO3 Powders was studied using X-ray powder diffraction, Raman spectroscopy, and nuclear site group analysis based on these spectroscopic data. Coarsened particles exhibit an orthorhombic Pbcm (D-2h(11), no. 57) structure corresponding to the bulk structure, as observed for single crystals or powders prepared by conventional solid-state reaction. The crystal symmetry of submicron powders was refined with the space group Pmc2(1) (C-2v(2), no. 26). The reduced perovskite cell volumes of these submicron powders were most expanded compared to all the other structures. Fine particles with a diameter of less than 70 nm as measured from SEM observations showed an orthorhombic Pmma (D-2h(5), no. 51) crystal symmetry. The perovskite formula cell of this structure was pseudocubic and was the most compact one. A possible mechanism of the phase transformation is suggested.

Modulated phases in NaNbO3: Raman scattering, synchrotron x-ray diffraction, and dielectric investigations

Abstract: Raman spectra of sodium niobate (NaNbO3) were obtained in all phases and revealed a significant disorder in the high-temperature U, T2 and T1 phases and ac omplicated folding of the Brillouin zone at the transitions into modulated S, R, P and N phases associated with the competitive zone-boundary soft mode s( in-phase and out-of phase octahedral tilts) along the M–T–R line. An extensive Raman study combined with x-ray diffraction (XRD) and dielectric measurements confirmed the presence of the incommensurate (INC) phase in sodium niobate. XRD experiments revealed the invar effect in the temperature interval 410–460 K corresponding to the INC phase associated with rotations of the NbO6 octahedra modulated along the b-direction. Our experiments suggest that the phase P consists of three phases: monoclinic (Pm) between 250 and 410 K, INC between 410 and 460 K, and orthorhombic (Po) between 460 and 633 K. At the low-temperature transition to the ferroelectric rhombohedral N phase all folded modes originating from the M- and T-points of the Brillouin zone abruptly disappear, Raman spectra in the N phase become much simpler and all peaks were assigned.

Universal octahedral-site distortion in orthorhombic perovskite oxides.

Abstract: Lattice parameters of the orthorhombic perovskites RMO3 (R=rare earth, M=Ti, V, ..., Ni, and Ga) have been simulated based on the ionic M-O bond length and rigid MO6/2 octahedra. Comparison with experimental data shows that the long-standing lattice-parameter anomaly generally found for the larger R3+ ions in these families is caused by a structural feature that is not revealed by the geometric tolerance factor widely used for the perovskites.

Structure of the intermediate phase of PbTe at high pressure.

Abstract: The evolution of PbTe with pressure has been reexamined using synchrotron x-ray diffraction. The phase transition at 6 GPa is not to the GeS (B16) or TlI (B33) type structures, as previously reported, but to an orthorhombic Pnma structure, with cell parameters a=8.157(1), b=4.492(1), and c=6.294(1)A at 6.7 GPa. This structure corresponds to a distortion of the low-pressure NaCl structure with a coordination intermediate between the sixfold B1 (NaCl) and the eightfold B2 (CsCl) structure. We discuss the stability of this new structure with respect to other proposed phases using numerical methods. These results may modify the admitted paths of phase transitions between the B1 and B2 structures.

The structure, electrical and ethanol-sensing properties of La1−xPbxFeO3 perovskite ceramics with x≤0.3

Abstract: The nanocrystalline La1−xPbxFeO3 ceramics (x=0–0.3) can be prepared by the citric method. All the compounds crystallize as perovskite phase with orthorhombic structure. The Pb-doping restrains the growth of the grain size. The conductivity and gas sensitivity of La1−xPbxFeO3-based sensor were investigated. Results demonstrate that Pb-doping can enhance the conductance and improve the sensitivity, selectivity and response time of LaFeO3. Among the materials investigated, La0.8Pb0.2FeO3 shows the best sensitivity to ethanol at an operating temperature of 140 °C.

Effect of pressure on the crystal structure of l-serine-I and the crystal structure of l-serine-II at 5.4 GPa

Abstract: The crystal structure of l-serine has been determined at room temperature at pressures between 0.3 and 4.8 GPa. The structure of this phase (hereafter termed l-serine-I), which consists of the molecules in their zwitterionic tautomer, is orthorhombic, space group P212121. The least compressible cell dimension (c), corresponds to chains of head-to-tail NH⋯carboxylate hydrogen bonds. The most compressible direction is along b, and the pressure-induced distortion in this direction takes the form of closing up voids in the middle of R-type hydrogen-bonded ring motifs. This occurs by a change in the geometry of hydrogen-bonded chains connecting the hydroxyl groups of the —CH2OH side chains. These hydrogen bonds are the longest conventional hydrogen bonds in the system at ambient pressure, having an O⋯O separation of 2.918 (4) A and an O⋯O⋯O angle of 148.5 (2)°; at 4.8 GPa these parameters are 2.781 (11) and 158.5 (7)°. Elsewhere in the structure one NH⋯O interaction reaches an N⋯O separation of 2.691 (13) A at 4.8 GPa. This is amongst the shortest of this type of interaction to have been observed in an amino acid crystal structure. Above 4.8 GPa the structure undergoes a single-crystal-to-single-crystal phase transition to a hitherto uncharacterized polymorph, which we designate l-serine-II. The OH⋯OH hydrogen-bonded chains of l-serine-I are replaced in l-serine-II by shorter OH⋯carboxyl interactions, which have an O⋯O separation of 2.62 (2) A. This phase transition occurs via a change from a gauche to an anti conformation of the OH group, and a change in the NCαCO torsion angle from −178.1 (2)° at 4.8 GPa to −156.3 (10)° at 5.4 GPa. Thus, the same topology appears in both crystal forms, which explains why it occurs from one single-crystal form to another. The transition to l-serine-II is also characterized by the closing-up of voids which occur in the centres of other R-type motifs elsewhere in the structure. There is a marked increase in CH⋯O hydrogen bonding in both phases relative to l-serine-I at ambient pressure.

La0.8Sr0.2Co0.2Fe0.8O3−δ as a potential oxygen carrier in a chemical looping type reactor, an in-situ powder X-ray diffraction study

Abstract: Perovskite type materials which can change oxygen content quickly under changing oxygen partial pressures are believed to be suitable materials for chemical looping of natural gas. In order to investigate the oxidation/reduction of these materials in-situ powder X-ray diffraction was used to study the La1−xSrxCo1−yFeyO3−δ system. It was found that for samples with high iron content a hexagonal/orthorhombic to cubic phase transition was observed upon heating in a reductive atmosphere and that the cationic lattice is maintained upon reduction. On the other hand, the samples with high cobalt content were unstable with respect to heating in a reductive atmosphere as they decomposed to form La2O3, Co and at least one additional phase. Once a suitable candidate had been obtained a chemical looping experiment was carried out using H2 as a model fuel, again using in-situ powder X-ray diffraction.

Ferroelectric domain morphologies of (001) PbZr1−xTixO3 epitaxial thin films

Abstract: Ferroelectric domain morphologies in (001) PbZr1−xTixO3 epitaxial thin films were studied using the phase-field approach. The film is assumed to have a stress-free top surface and is subject to a biaxial substrate constraint. Both the electrostatic open-circuit and short-circuit boundary conditions on the film surfaces were considered. The phase-field simulations indicated that in addition to the known tetragonal and rhombohedral phases, an orthorhombic phase becomes stable in films under large tensile constraints. The orthorhombic domain structure contains (100) and (010) 90° domain walls and (110) and (1–10) 180° domain walls. For the rhombohedral phase in a thin film, the domain walls are found to be along {101}, (100), and (010) of the prototypical cubic cell. It is shown that the short-circuit boundary condition and compressive substrate constraint enhance the out-of-plane polarization component while the open-circuit boundary condition and tensile substrate constraint suppress it. It is also shown t...

Self-Assembly of a Fluorescent Chiral Zinc(II) Complex That Leads to Supramolecular Helices

Abstract: The chiral Zn(II) complex [ZnLCI 2 ], 1 {L = 4-methyl-2,6-di[(S)-(+)-1-phenylethyliminomethyl] phenol}, self-assembles via C-HCl hydrogen bonding into supramolecular helices Complex 1 exhibits emission in solution at room temperature in the visible range Crystal data for 1: orthorhombic space group P2 1 2 1 2 1 , a = 9614(2) A, b = 13825(3) A, c = 18667(3) A, V = 24811(8) A 3 , Z = 4

Melilite-type borates Bi2ZnB2O7 and CaBiGaB2O7

Abstract: The bismuth borates Bi2ZnB2O7 and CaBiGaB2O7 have been synthesized by solid-state reactions at temperatures in the 650−825 °C range at 1 atm pressure. These compounds are the only synthetic diborate members of the melilite family, A2XZ2O7, in which layers of A cations alternate with XZ2O7 tetrahedral layers. Except for CdBiGaB2O7, the synthesis of other substituted bismuth borate melilites has been unsuccessful. The crystal structures of Bi2ZnB2O7 and CaBiGaB2O7 have been determined by powder X-ray diffraction and refined by the Rietveld method using powder neutron diffraction data. CaBiGaB2O7 adopts the regular tetragonal melilite structure (P421m space group, Z = 2) containing B2O7 tetrahedral dimers. The refinement of split eight-coordinated sites for the Ca2+ and Bi3+ interlayer cations suggests the presence of additional disorder. Bi2ZnB2O7 adopts a unique orthorhombic melilite superstructure (Pba2 space group, Z = 4) containing both tetrahedral B2O7 and triangular B2O5 dimers. The Bi3+ cations occu...

A study of the manganites La0.5M0.5MnO3(M = Ca, Sr, Ba) prepared by hydrothermal synthesis

Abstract: We present results of the first systematic study of the hydrothermal synthesis of the manganites La0.5M0.5MnO3 (M = Ca, Sr, Ba), and a detailed characterisation of the products using a number of experimental methods. All three materials can be produced as phase-pure, polycrystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at temperatures of 240 °C for M = Sr or Ba and 270 °C for M = Ca. Iodometric titration and Mn K-edge X-ray absorption near edge spectroscopy (XANES) combined with elemental analysis for metals confirms the average manganese oxidation state as 3.5 in each of the materials. Scanning electron microscopy shows that the materials are made up of cube-shaped particles 1 µm in dimension for La0.5Ba0.5MnO3, 10 µm on average for La0.5Sr0.5MnO3 and 20 µm for La0.5Ca0.5MnO3. La0.5Ba0.5MnO3 is shown to adopt an A-site ordered perovskite structure (tetragonal, P4/mmm, a = 3.9160(1) A, c = 7.8054(2) A); this is determined using powder neutron diffraction, thermogravimetric analysis and magnetisation measurements. High-resolution powder neutron diffraction data from La0.5Sr0.5MnO3 show that the material adopts a distorted perovskite structure (tetragonal, I4/mcm, a = 5.44778(40) A, c = 7.7353(10) A) similar to that previously reported for materials of the same composition prepared by a solid-state route. Hydrothermal La0.5Ca0.5MnO3 adopts an orthorhombic perovskite structure (Pnma, a = 5.410(1) A, b = 7.604(1) A, c = 5.443(2) A). Finally, we examine poorly ordered precursors of La0.5Ba0.5MnO3, formed prior to the crystallisation of the perovskite, using Mn K-edge X-ray absorption fine structure spectroscopy. This shows that at the earliest stage of the reaction the manganese is found in an oxidation state of 3.5 in edge-shared octahedral sites, consistent with the formation of a K0.5MnO2·nH2O, birnessite-like phase before the formation of the perovskite.