Search

In a recent paper [C. E. Bertrand et al., J. Chem. Phys. 145, 014502 (2016)], we have shown that the collective dynamics of methanol shows a fast relaxation process related to the standard density-fluctuation heat mode and a slow non-Fickian mode originating from the hydrogen bonded molecular associates. Here we report on the length scale dependence of this slow relaxation process. Using quasielastic neutron scattering and molecular dynamics simulations, we show that the dynamics of the slow process is affected by the structuring of the associates, which is accessible through polarized neutron diffraction experiments. Using a series of partially deuterated samples, the dynamics of the associates is investigated and is found to have a similar time scale to the lifetime of hydrogen bonding in the system. Both the structural relaxation and the dynamics of the associates are thermally activated by the breaking of hydrogen bonding. [ABSTRACT FROM AUTHOR]

We report changes in the metric symmetry of unpoled and electrically poled single crystals of 0.26Pb(In1/2Nb1/2)O3-0.46Pb(Mg1/3Nb2/3)O3-0.28PbTiO3, as revealed by neutron diffraction in conjunction with electrical measurements. The unpoled crystals show relaxor characteristics and an average rhombohedral symmetry that persists from ambient temperature up to the Curie temperature of ∼165 °C. Poling along a [111] direction enhances the rhombohedral distortion away from cubic metric symmetry but appears not to induce any monoclinic phases. By contrast, the poled [001]-oriented crystal has orthorhombic (or monoclinic MC) metric symmetry at 25 °C. An intermediate transition to a metrically tetragonal phase around 120 °C is confirmed for both poled crystals. [ABSTRACT FROM AUTHOR]

Abstract Structure of binary Fe-19Ga and ternary Fe-19Ga-Tb alloys at room temperature are investigated by TEM and high-resolution neutron diffraction (ND). In contrast with as cast or water quenched samples with A2 structure, the D0 3 ordering was detected in slowly cooled or annealed samples by ND. According to TEM, the D0 3 ordering takes place in the form of ordered clusters with the size of about 5 nm embedded into an A2 matrix. Disordering of the D0 3 clusters at heating and ordering at cooling were investigated by three in situ techniques: ND, vibrating sample magnetometry (VSM), and internal friction (IF). Temperatures of ordering-disordering were estimated by VSM and IF to be close to 500 °C. The effect of Fe-Ga alloys doping with Tb is also analyzed. Graphical abstract Image 1 Highlights • Fe-19Ga alloys structure is examined at room temperature and up to 800 °C. • Different cooling regimes lead to formation of A2 or D0 3 structure. • D0 3 structure stability is examined at heating and cooling by several in situ methods. [ABSTRACT FROM AUTHOR]

Abstract The mechanical behavior of cold-rolled Mg AZ31B is studied during in-plane multiaxial loading and tension-tension strain path changes performed on cruciform samples using in situ neutron diffraction and EBSD. The results are compared with uniaxial tension loading of dogbone-shaped samples measured with in situ neutron diffraction and acoustic emission. The activity of slip and twinning mechanisms and the active twin variants are discussed for the different strain paths. It is shown that initial strains of 4–5% cause a strengthened yield stress during reload for strain path change angles of 90 and 135°. The strengthening is primarily due to dislocation accumulation during the initial load impeding dislocation motion during the reload. The twinning observed during the prestrain activates complex multivariant secondary twinning which may also contribute to the strengthening in the reload. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The investigation of crystal and magnetic structures of BaFe 12 - x Me x O 19 ( Me = In 3 + and Ga 3 + ; x = 0.1–1.2) solid solutions has been carried out by time-of-flight neutron diffraction method. It was determined the influence of diamagnetic In and Ga ions to unit cell volume of partially substituted barium ferrites. The electric field-induced polarization was observed in the BaFe 11.9 Me 0.1 O 19 (Me = In and Ga) solid solutions at room temperature. The refinement of crystal structure of barium ferrites has been carried out in frameworks of two possible space group: centrosymmetric P 6 3 / mmc (No. 194) and non- centrosymmetric P 6 3 mc (No. 186) that allowed to reveal reasons of appearance of nonzero dipole electric moment. For description of magnetic structure of BaFe 12 - x Me x O 19 (Me = In and Ga; x = 0.1–1.2) solid solutions was used the Gorter’s model where magnetic moments of iron ions are oriented along the hexagonal axis that is the axis of easy magnetization. [ABSTRACT FROM AUTHOR]

The understanding of the structure and the stability of high entropy alloys is still incomplete and the mechanisms behind the composition-property relationships are unclear. One reason is that few systematic and accurate determinations of their composition-dependent structure on atomic level have been made. In this paper some results on the structure obtained by X-ray and neutron diffraction of the CoCrFeNi alloy, to which Pd, Sn and Cu have been added in different amounts, are reported. The investigations make it obvious that none of the alloys is completely homogeneous, as has earlier been suggested, and that they do not form a perfect solid solution. [ABSTRACT FROM AUTHOR]

An air- and moisture-stable nanoscale polyhydrido copper cluster [Cu32(H)20{S2P(O iPr)2}12] ( 1H) was synthesized and structurally characterized. The molecular structure of 1H exhibits a hexacapped pseudo-rhombohedral core of 14 Cu atoms sandwiched between two nestlike triangular cupola fragments of (2×9) Cu atoms in an elongated triangular gyrobicupola polyhedron. The discrete Cu32 cluster is stabilized by 12 dithiophosphate ligands and a record number of 20 hydride ligands, which were found by high-resolution neutron diffraction to exhibit tri-, tetra-, and pentacoordinated hydrides in capping and interstitial modes. This result was further supported by a density functional theory investigation on the simplified model [Cu32(H)20(S2PH2)12]. [ABSTRACT FROM AUTHOR]

The ground state of Mn3C, a metastable but nonetheless fundamental phase in the context of high-manganese steels, is still a matter of debate, and even the atomic structure of Mn3C has never been refined in the past. Moreover, Mn3C has been described as being nonmagnetic, antiferromagnetic, and also ferrimagnetic. Herein, Mn3C's crystal structure and its magnetic ground state were probed for the first time by high-resolution neutron diffraction between 3.5 and 300 K using well-crystalline samples. Although the neutron-diffraction quality is perfectly able to detect even the magnetic states of the impurities MnO and γ-Mn, there is no sign of a magnetic ordering for Mn3C down to 3.5 K. Hence, it is justified to characterize Mn3C as paramagnetic at the time being. [ABSTRACT FROM AUTHOR]

We developed a six-axis multi-anvil press, ATSUHIME, for high-pressure and high-temperature in situ time-of-flight neutron powder diffraction experiments. The press has six orthogonally oriented hydraulic rams that operate individually to compress a cubic sample assembly. Experiments indicate that the press can generate pressures up to 9.3 GPa and temperatures up to 2000 K using a 6-6-type cell assembly, with available sample volume of about 50 mm³ . Using a 6-8-type cell assembly, the available conditions expand to 16 GPa and 1273 K. Because the six-axis press has no guide blocks, there is sufficient space around the sample to use the aperture for diffraction and place an incident slit, radial collimators, and a neutron imaging camera close to the sample. Combination of the six-axis press and the collimation devices realized high-quality diffraction pattern with no contamination from the heater or the sample container surrounding the sample. This press constitutes a new tool for using neutron diffraction to study the structures of crystals and liquids under high pressures and temperatures. [ABSTRACT FROM AUTHOR]

The article focuses on the use of neutron multiple diffraction (NMD) as a tool for structural studies. Multiple diffraction (MD) occurs where there is more than one reciprocal lattice point on the sphere of reflection, and it may occur intentionally or accidentally. Information on mirror symmetries in MD patterns is offered.

We present polarized-neutron diffraction measurements of the Ising-type spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature TN. Below TN, a clear evolution from a pure spin-density wave (SDW) structure to a mixture of SDW and commensurate antiferromagnet (CAFM) structures is observed on cooling. For a rapidly cooled sample, the majority phase at low temperature is the SDW, while if the cooling is performed sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5 and 10 K. Above TN, we use Monte Carlo methods to analyze the magnetic diffuse scattering data. We show that both intrachain and interchain correlations persist above TN, but are essentially decoupled. Intrachain correlations resemble the one-dimensional ferromagnetic Ising model, while interchain correlations resemble the frustrated triangular-lattice antiferromagnet. Using previously published bulk property measurements and our neutron diffraction data, we obtain values of the ferromagnetic and antiferromagnetic exchange interactions and the single-ion anisotropy. [ABSTRACT FROM AUTHOR]

From high-resolution neutron-diffraction experiments we present equation-of-state (EOS) data of elemental lead as functions of both temperature (80-298 K) and applied pressure (up to 8.9 GPa), with the pressure values derived from the NaCl pressure gauge. Based on Brown's 1999 NaCl EOS we find the bulk modulus of lead of B0 = 41.2(2) GPa at 298 K to increase by 14% to B0 = 47.0(5) GPa by 80 K. The ambient temperature value coincides within 0.35 GPa (1.7%) of the value determined from published ultrasonic data. The good agreement between the neutron and ultrasonic data deteriorates if pressure values are derived from Decker's 1971 scale. Our results hence lend significant support to Brown's revised NaCl pressure standard. Furthermore, the experimental results are compared with T = 0 K first-principles calculations. [ABSTRACT FROM AUTHOR]

Deformation behavior of a high-entropy alloy (HEA) was investigated by in situ tensile deformation with neutron diffraction. It was found that the face-centered cubic (FCC) HEA alloy showed strong crystal elastic and plastic anisotropy, and the evolution of its lattice strains and textures were similar to those observed in conventional FCC metals and alloys. Our results demonstrated that, in spite of chemical complexity, the multi-component HEA behaved like a simple FCC metal and the deformation was caused by the motion of mixed dislocations. [ABSTRACT FROM AUTHOR]

We used x-ray and neutron diffraction to determine the low-temperature structure of IrTe2. A structural modulation was observed with a wave vector of k = (1/5,0,1/5) below Ts ≈ 285 K, accompanied by a structural transition from a trigonal to a triclinic lattice. We also performed first-principles calculations for high-temperature and low-temperature structures, which elucidate the nature of the phase transition and the low-temperature structure. A local bonding instability associated with the Te 5p states is likely the origin of the structural phase transition in IrTe2. [ABSTRACT FROM AUTHOR]

A paramagnetic metal to nonmagnetic insulator transition at TMIT = 210 K is reported for the β-pyrochlore oxide CsW2O6, accompanied by a first order structural transition that creates (110> oriented chains in the pyrochlore lattice. Comparison of CsW2O6, which has 1 electron per 2 W sites, to the fully d0 analog CsTaWO6 shows that the transitions are electronically driven. Corefinement of high resolution synchrotron x-ray and neutron diffraction data shows that the structural distortion that creates the W chains cannot be attributed to simple charge or orbital ordering. Density functional theory calculations suggest that the phase transition is driven by a sharply peaked electronic density of states near the Fermi energy in the cubic β-pyrochlore phase. A further electronic instability is required to create the insulating ground state. [ABSTRACT FROM AUTHOR]

The article reports on the highlights of the colloquium held in honor of Alan Hewat, former member of the Institut Laue-Langevin, held on October 26, 2012. Hewat retired after several years of contributing to the areas of neutron science, instrumentation and management. He focused on neutron powder diffraction and the structural determination of new materials. Presentations were made by Sir Brian Fender, Roland Tellgren, and Peter Fischer.

Ho2CrSbO7 is found to form an 'ordered spin-ice structure' below 13 K where the magnetic moments are constrained along the 111 axes, but with a ferromagnetic moment in one direction. [ABSTRACT FROM AUTHOR]

The ferroic phase transition in LaEr(MoO4)3 has been analyzed for the first time. It has been confirmed that this compound undergoes a phase transition from a tetragonal system (paraelectric-paraelastic phase), with space group P-421m [β-Gd2(MoO4)3 averaged phase] to an orthorhombic system (ferroelectric-ferroelastic phase), with space group Pba2 [β'-Gd2(MoO4)3 phase] in a reversible process. This phenomenon, together with the observed demixing at high temperature has been studied using different techniques. LaEr(MoO4)3 samples have been obtained by the conventional solid-state synthesis. The thermal dependence of the crystal structure was studied by powder X-ray and neutron diffraction, following a new refining procedure in which the symmetry modes of atomic displacements from the paraelectric-paraelastic structure were analyzed. Dielectric spectroscopy measurements have confirmed the structural results, showing a very smooth phase transition. Finally, calculations within the framework of Density Functional Theory show a behavior of the lattice parameters similar to that observed in our experiments. [ABSTRACT FROM PUBLISHER]