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At least 1,027 records · Page 57Linked to original sources
PubMed2003

Structural tuning of charge, orbital, and spin ordering in double-cell perovskite series between NdBaFe(2)O(5) and HoBaFe(2)O(5).

Charge, orbital, and magnetic ordering of NdBaFe(2)O(5) and HoBaFe(2)O(5), the two end-members of the double-cell perovskite series RBaFe(2)O(5), have been characterized over the temperature range 2-450 K, using differential scanning calorimetry, neutron thermodiffractometry and high-resolution neutron powder diffraction. Upon cooling, both compounds transform from a class-III mixed valence (MV) compound, where all iron atoms exist as equivalent MV Fe(2.5+) ions, through a "premonitory" charge ordering into a class-II MV compound, and finally to a class-I MV phase at low-temperature. The latter phase is characterized by Fe(2+)/Fe(3+) charge ordering as well as orbital ordering of the doubly occupied Fe(2+) d(xz) orbitals. The relative simplicity of the crystal and magnetic structure of the low-temperature charge-ordered state provide an unusual opportunity to fully characterize the classical Verwey transition, first observed in magnetite, Fe(3)O(4). Despite isotypism of the title compounds at high temperature, neutron diffraction analysis reveals striking differences in their phase transitions. In HoBaFe(2)O(5), the Verwey transition is accompanied by a reversal of the direct Fe-Fe magnetic coupling across the rare earth layer, from ferromagnetic in the class-II and -III MV phases to antiferromagnetic in the low-temperature class-I MV phase. In NdBaFe(2)O(5), the larger Nd(3+) ion increases the Fe-Fe distance, thereby weakening the Fe-Fe magnetic interaction. This decouples the charge and magnetic ordering so that the Fe-Fe interaction remains ferromagnetic to low temperature. Furthermore, the symmetry of the charge-ordered class-I MV phase is reduced from Pmma to P2(1)()ma and the magnitude of the orbital ordering is diminished. These changes destabilize the charge-ordered state and suppress the temperature at which the Verwey transition occurs. A comparison of the magnetic and structural features of RBaFe(2)O(5) compounds is included in order to illustrate how structural tuning, via changes in the radius of the rare-earth ion, can be used to alter the physical properties of these double-cell perovskites.

Journal Article↗
PubMed2003

Hydrothermal synthesis in the system Ni(OH)(2)-NiSO(4): nuclear and magnetic structures and magnetic properties of Ni(3)(OH)(2)(SO(4))(2)(H(2)O)(2).

We present the synthesis, characterization by DT-TGA and IR, single crystal X-ray nuclear structure at 300 K, nuclear and magnetic structure from neutron powder diffraction on a deuterated sample at 1.4 K, and magnetic properties as a function of temperature and magnetic field of Ni(3)(OH)(2)(SO(4))(2)(H(2)O)(2). The structure is formed of chains, parallel to the c-axis, of edge-sharing Ni(1)O(6) octahedra, connected by the corners of Ni(2)O(6) octahedra to form corrugated sheets along the bc-plane. The sheets are connected to one another by the sulfate groups to form the 3D network. The magnetic properties measured by ac and dc magnetization, isothermal magnetization at 2 K, and heat capacity are characterized by a transition from a paramagnet (C = 3.954 emu K/mol and theta = -31 K) to a canted antiferromagnet at T(N) = 29 K with an estimated canting angle of 0.2-0.3 degrees. Deduced from powder neutron diffraction data, the magnetic structure is modeled by alternate pairs of Ni(1) within a chain having their moments pointing along [010] and [010], respectively. The moments of Ni(2) atoms are oppositely oriented with respect to their adjacent pairs. The resulting structure is that of a compensated arrangement of moments within one layer, comprising one ferromagnetic and three antiferromagnetic superexchange pathways between the nickel atoms.

Journal Article↗
PubMed2004

Crystallographic, electronic, and magnetic studies of zeta(2)-GaM (M = Cr, Mn or Fe): trends in itinerant magnetism.

This study of the crystal structure, electronic structure, and magnetic properties of the zeta(2)-GaM (M = Cr, Mn or Fe) alloys is motivated by the recent reinvestigation of the crystallographic Al(8)Cr(5) structure type of zeta(2)-GaMn. The isostructural compounds zeta(2)-GaFe and zeta(2)-GaCr have been refined using X-ray powder diffraction as well as neutron powder diffraction for zeta(2)-GaFe. Their structures have been refined using the space group Rm, with cell parameters a = 12.625(8) A and c = 7.785(10) A for zeta(2)-GaCr and a = 12.4368(11) A and c = 7.7642(10) A for zeta(2)-GaFe. Band structure calculations using the self-consistent, spin-polarized TB-LMTO method were performed to understand their electronic structure and magnetic properties. Band calculations show that from GaCr to GaFe the magnetic interactions change from weakly antiferromagnetic coupling to ferromagnetic coupling. Magnetic measurements confirm ferromagnetism for GaFe and show a weak paramagnetic response for GaCr.

Journal Article↗
PubMed2002

Synthesis and crystal structure of Sr(2)AlH(7): a new structural type of alkaline earth aluminum hydride.

The title hydride and its deuteride were successfully synthesized. The heavy atom structure and hydrogen positions were determined respectively by X-ray powder diffraction and time-of-flight neutron powder diffraction. They crystallize with a new monoclinic structure in space group I2 (No. 5); cell parameters: a = 12.575(1) A, b = 9.799(1) A, c = 7.9911(8) A, beta = 100.270(4) degrees (hydride), a = 12.552(1) A, b = 9.7826(8) A, c = 7.9816(7) A, beta = 100.286(4) degrees (deuteride), Z = 8. Sr(2)AlH(7) is the first example that consists of isolated [AlH(6)] units and infinite one-dimensional twisted chains of edge-sharing [HSr(4)] tetrahedra along the crystallographic c axis.

Journal Article↗
PubMed2003

Polymorphism of heptalithium nitridovanadate(V) Li7[VN4].

The system Li-V-N was studied by means of X-ray and neutron powder diffraction, thermal and chemical analyses, and XAS spectroscopy at the vanadium K-edge. Three polymorphs of Li(7)[VN(4)] have been established from X-ray and neutron powder diffraction (gamma-Li(7)[VN(4)], space group Pfourmacr;3n, No. 218, a = 960.90(4) pm, V = 887.23(6) x 10(6) pm(3), Z = 8; beta-Li(7)[VN(4)], space group Pathremacr;, No. 205, a = 959.48(3) pm, V = 883.31(5) x 10(6) pm(3), Z = 8; alpha-Li(7)[VN(4)], P4(2)/nmc, No. 137, a = 675.90(2) pm, c = 488.34(2) pm, V = 223.09(1) x 10(6) pm(3), Z = 2). Crystallographic and phase relations are discussed. All three modifications are diamagnetic, indicating vanadium in the oxidation state +5. The V-K XAS spectra support the oxidation state assignment, the non-centrosymmetric coordination (tetrahedral), and the nearly identical second coordination sphere of vanadium, made up from Li in all three phases. The 3d-related features of the spectra display strongly localized properties. The phase transitions appear to be reconstructive; no direct group-subgroup symmetry relations of the crystal structures exist. The formation of solid solutions between Li(2)O and beta-Li(7)[VN(4)] with the general formula Li(1.75)((V(0.25(1)(-)(x))Li(0.25)(x))(N(1)(-)(x)O(x)())) with 0 <or= x <or= 1 leads to increasing substitution of V by Li. At the approximate composition with x = 0.125 the Li-V-disorder seems to be complete: X-ray diffraction lines give rise to only one-half of the original unit cell dimensions, a' = 476.47(3) pm approximately (1/2)a(beta-Li(7)[VN(4)]).

Journal Article↗
PubMed2003

The membrane bound N-terminal domain of human adenosine diphosphate ribosylation factor-1 (ARF1).

The small G protein adenosine diphosphate ribosylation factor-1 (ARF1) is activated by cell membrane binding of a self-folding N-terminal domain. We present a model of the human ARF1 N-terminal peptide in planar lipid bilayers, determined from neutron lamellar diffraction and circular dichroism data with molecular modelling. This amphipathic domain lies at a shallow membrane depth, ideal for regulation of the ARF1 bio-timer by rapid, reversible membrane binding. The helical region does not elongate upon membrane binding, leaving the connecting flexible linker region's length unchanged.

ADP-Ribosylation Factor 1↗
PubMed2005

Anti-KSbF6 structure of CaTbF6 and CdTbF6: a confirmation of the singular crystal chemistry of Tb4+ in fluorides.

The crystal structures of two new tetravalent terbium fluorides, CaTbF6 and CdTbF6, have been determined from X-ray and neutron powder diffraction data. The title compounds exhibit an anti-KSbF6 structure, the three-dimensional framework of which is built of [TbF6]2- chains of edge-sharing dodecahedra further linked, by sharing corners, to isolated [MF6]4- octahedra (M=Ca, Cd). The mechanism of the anionic sublattice rearrangement when going from KSbF6 to CaTbF6 is described and related to a simple cubic fluoride-ion packing. Comparison with the crystal structures of beta-BaTbF6 and other representatives of the M(II)M('IV)F6 family allows the singular crystal-chemical properties of some fluoroterbates to be emphasized.

Cadmium↗
PubMed2005

Combined structural refinement of Bi3.5La0.5Ti3O12 using neutron and X-ray powder diffraction data.

A combined structural refinement of Bi3.5La0.5Ti3O12 against both neutron and X-ray diffraction data was performed at 298 K on the basis of the Raman study. The upshift of Raman peaks suggested that the substitution sites of La atoms in Bi3.5La0.5Ti3O12 were only the Bi sites in the perovskite units. Of the two crystal structural models (orthorhombic and monoclinic systems) considered for the crystal structural system of Bi3.5La0.5Ti3O12, the weighted R factor, Rwp, and goodness-of-fit indicator, S (=Rwp/Re), of the monoclinic system were lower than those of the orthorhombic one. The final Rwp and S values based on the monoclinic system were 7.04% (6.34 and 7.76% for the neutron data and the X-ray data, respectively) and 1.45, respectively. The lattice parameters obtained from the combined structural refinement were a = 5.4321(1) A, b = 5.4161(1) A, and c = 32.8614(3) A. The beta angle was 89.95(4) degrees . Spontaneous polarizations calculated from the refined structural parameters were 27.0 microC/cm2 for the monoclinic system and 1.8 microC/cm2 for the orthorhombic one.

Journal Article↗
PubMed2004

Phonons from powder diffraction: a quantitative model-independent evaluation.

We describe a model-independent approach for the extraction of detailed lattice dynamics information from neutron powder diffraction data, based on a statistical analysis of atomistic configurations generated using reverse Monte Carlo structural refinement. Phonon dispersion curves for MgO extracted in this way are shown to reproduce many of the important features found in those determined independently using neutron triple-axis spectroscopy. By means of molecular dynamics simulations, we quantify the extent to which the diffraction data are sensitive to lattice dynamics in this system.

Journal Article↗
PubMed2004

Role of the lattice in the gamma-->alpha phase transition of Ce: a high-pressure neutron and x-ray diffraction study.

The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the gamma-->alpha phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the gamma-->alpha phase transition, DeltaS(gamma-alpha)(vib) approximately (0.75+/-0.15)k(B), is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence that is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the gamma-->alpha phase transition in elemental Ce.

Journal Article↗
PubMed2002

Direct observation of eta 2-imine formation through beta-H abstraction between amide ligands. Neutron and X-ray diffraction structure of a dihydride imine ditantalum complex.

Reactions of Ta(NMe2)5 with D2SiR'Ph (R' = Me, Ph) were found to give a dideuteride eta 2-imine complex (Me2N)3Ta(mu-D)2(mu-N-eta 2-N,C-CH2NMe)Ta(NMe2)3(1-d2) through C-H activation of an amide ligand via beta-H abstraction, and the structure of 1 was confirmed by single crystal neutron and X-ray diffraction.

Journal Article↗
PubMed2005

Neutron and X-ray diffraction study of the broken symmetry phase transition in solid deuterium.

The solid hydrogen compounds D2, HD and H2 remain quantum molecular solids up to pressures in the 100 GPa range. A remarkable macroscopic consequence is the existence of a pressure-induced broken symmetry phase transition, in which the molecules go from a spherical rotational state to an anisotropic rotational state. Theoretical understanding of the broken symmetry phase structure remains controversial, despite numerous studies. Some open questions concern the existence of long- or short-range orientational order; whether a strong isotopic shift on the transition pressure should be assigned to the nuclear zero-point motion or to quantum localization; and whether the structures are cubic, hexagonal or orthorhombic. Here we present experimental data on the structure of the broken symmetry phase in solid D2, obtained by a combination of neutron and X-ray diffraction up to 60 GPa. Our data are incompatible with orthorhombic structures predicted by recent theoretical works. We find that the broken symmetry phase structure is incommensurate with local orientational order, being similar to that found in metastable cubic para-D2.

Journal Article↗
PubMed1976

Investigation of biological macromolecular systems with a pulsed neutron source--a review.

The conclusion that can be drawn on the basis of the above considerations is that investigation of biological macromolecules and crystalline structures by SAS and diffraction of neutrons with the TOF method is feasible. The main difficulties of the TOF method (the wavelength dependence of the incident beam, resolution power, and detector efficiency; the need for their determination and up-to-date values) are compensated for by its advantages. Both methods allow a high data accumulation rate and optimal employment of the incident neutron spectrum. The latter has been achieved by utilizing a dominant part of the Maxwellian spectrum and by a more uniform distribution of statistical accuracy over the most informative measuring range. Another advantage is the high degree of monochronatization of the incident neutron beam by the TOF method. The rigid requirements concerning the data accumulation rate and the capacity of the on-line system computer memory are technical problems but not basic ones.

Collagen↗
PubMed1997

Structure, crystal chemistry and density of enamel apatites.

The apatitic calcium phosphate crystals in dental enamel are too small for single crystal diffraction studies so the only possible direct structure determination must use whole-pattern-fitting Rietveld analysis of X-ray and neutron powder diffraction patterns. As a result, aspects of the structure are not known in detail. Further structural information can be obtained by consideration of published chemical analyses and infrared studies, taking into account studies of the crystal chemistry of synthetic apatitic analogues of enamel apatite. The apatitic constitutional water and total water content of enamel are particularly important, but there are difficulties in their determination. Making reasonable assumptions, a number of models of the unit cell can be derived. The weight per cent (including constitutional water) and density of the enamel apatite crystals for the most probable model are about 98 wt.% and 3.0 g cm-3, respectively. The apatite volume per cent calculated from these values is about 96%. The weight per cent and volume per cent of enamel apatite are higher than normally accepted values because of inclusion of constitutional water and use of a density for enamel apatite that takes into account its known lattice expansion over hydroxyapatite and probable lattice vacancies.

Animals↗