Pulsed Neutron Powder Diffraction Research Articles

パルス中性子粉末回折による地球マントル高密度含水相の水素配置解析

パルス中性子粉末回折による地球マントル高密度含水相の水素配置解析

Hydrogen sites in the dense hydrous magnesian silicate phase E: a pulsed neutron powder diffraction study

Hydrogen site positions and occupancy in the crystal structure of dense hydrous magnesium silicate (DHMS) phase E were determined for the first time by pulsed neutron powder diffraction. A fully deuterated pure phase E powder sample, which had space group \(R\overline{3} m\) and lattice parameters of a = 2.97065(8) A and c = 13.9033(4) A, was synthesized at 15 GPa and 1100 °C. Through quantitative evaluation of refined structure parameters obtained with sufficient spatial resolution and very high signal-to-background ratio, we conclude that the O–D dipoles in the refined phase E structure are tilted by 24° from the direction normal to the layers of edge-shared MgO6 octahedra (octahedral layers). The tilted dipole structure of phase E is in remarkable contrast to that of brucite, Mg(OH)2, which has dipoles exactly normal to the octahedral layer. This contrast exists because the O–Si–O bonding unique in the phase E structure connects two adjacent octahedral layers and thereby reduces the interlayer O···O distance. This shrinkage of the interlayer distance induces the tilting of the O–D dipole and also generates unique O–D···O hydrogen bonding connecting all the layers in the phase E structure.

Structure refinement of sub-cubic-mm volume sample at high pressures by pulsed neutron powder diffraction: application to brucite in an opposed anvil cell

Neutron powder diffraction measurements of 0.9 mm3 of mixture of deuterated brucite and pressure medium were conducted at pressures to 2.8 GPa, using an opposed anvil cell and a medium-resolution diffractometer at Japan Proton Accelerator Research Complex pulsed neutron source. Spurious-free diffraction patterns were successfully obtained and refined to provide all structural parameters including Debye–Waller factors. Tilting of hydroxyl dipoles of brucite toward one of the three nearest-neighbor oxygen anions was confirmed to be substantial at pressure as low as 1.5 GPa. By this application, technical feasibility to analyze such a small sample has been newly established, which would be useful to extend the applications of neutron diffraction at high pressures.

Pulsed neutron powder diffraction at high pressure by a capacity-increased sapphire anvil cell

A new design of opposed anvil cell for time-of-flight neutron powder diffraction was prepared for use at advanced pulsed sources. A couple of single-crystal sapphire sphere anvils and a gasket of fully hardened Ti–Zr null alloy were combined to compress 35 mm3 of sample volume to 1 GPa and 11 mm3 to 2 GPa of pressures, respectively. A very high-quality powder diffraction pattern was obtained at Japan Proton Accelerator Research Complex for a controversial high pressure phase of methane hydrate. The counting statistics, resolution, absolute accuracy and d-value range of the pattern were all improved to be best suitable for precise structure refinement. The sample is optically accessible to be measured by Raman and fluorescence spectroscopy during and after compression. The current cell will be an alternative choice to study hydrogenous materials of complex structures that are stable at the described pressure regime.

Evidence of local disorder in the overdoped regime of Ba(Fe1−xCox)2As2

The appearance of superconductivity in the antiferromagnet of BaFe${}_{2}$As${}_{2}$ with Co doping coincides with a phase transition from an orthorhombic to a tetragonal structure where coexistence of the two is observed near the phase boundary. Overdoping brings suppression of ${T}_{C}$ near $x=0.12$ that is not accompanied by a transition back to the orthorhombic phase. Using pulsed neutron powder diffraction, the relation of superconductivity to the atomic structure in the overdoped regime has been investigated. The suppression of superconductivity is accompanied by a structural local distortion around Co. The distortion, described as a breathing mode with alternating two or four Fe ions displaced toward Co in the $\mathit{ab}$ plane, sets in above ${T}_{C}$ and remains in place below.

A cubic-anvil high-pressure device for pulsed neutron powder diffraction

A compact cubic-anvil high-pressure device was developed for in situ neutron powder diffraction studies. In this device, a cubic shaped pressure medium is compressed by six anvils, and neutron beams pass through gaps between the anvils. The first high-pressure experiment using this device was conducted at J-PARC and clearly showed the neutron diffraction patterns of Pb. Combining the cubic-anvil high-pressure device with a pulsed neutron source will prove to be a useful tool for neutron diffraction experiments.

Total scattering of disordered crystalline functional materials

There are disorders in some modern functional materials. As an example, the crystalline phase of an optical recording material has low thermal conductivity but high electrical conductivity, simultaneously. This contradiction is a challenge to material scientists in designing good functional materials, which should have at least two types of crystallographic sites. One site limits thermal conductivity while the other site carries electrons or holes with high mobility. This problem exists with not only optical recording materials but also thermoelectric materials. The periodic boundary condition gets lost in the disordered parts. This therefore, makes atomic pair distribution function (PDF) analysis with a wide range of real space suitable for investigating the form and size of crystalline parts as well as disordered parts in the material. Pulsed neutron powder diffraction is one of the best tools for use in this new type of emerging research, together with synchrotron X-ray powder diffraction and electron diffraction.

Two-Dimensional Sodium Fluctuation at High Temperatures in High-Temperature Thermoelectric Material γ-Na0.7CoO2

A thermoelectric material, γ-Na0.7CoO2, with a high figure of merit, ZT>1, at high temperatures has been studied using the atomic pair distribution function (PDF) analysis of pulsed neutron powder diffraction data. We have found significant PDF peak broadening in the correlation between sodium and oxygen atoms at high temperatures. The broad peak indicates that a sodium atom frequently leaves a NaO6 prism. In other words, sodium atoms hop in a two-dimensional layer at high temperatures. The PDF intensity of the Na–O correlation below a critical length, where a sodium atom is leaving a prism, is inversely proportional to thermal conductivity. This suggests that the short phonon lifetime originates from the frequent sodium hopping in the two-dimensional layer, leading to high-performance thermoelectric generation.

Structural study on optical recording materials Ge 2Sb 2+xTe 5 and GeBi 2Te 4

Crystal structures of optical recording materials Ge 2Sb 2+ x Te 5 and GeBi 2Te 4 have been studied in terms of the fast phase transition mechanism by both real- and reciprocal-space Rietveld analyses of pulsed neutron powder diffraction data. These crystalline phases had large local lattice distortions. In addition, the crystallite size of GeBi 2Te 4 was very small (about 150 Å). These results suggest that the similarity of entropy between crystalline and amorphous phases plays an important role in terms of the fast phase transition mechanism.

Large displacement of germanium atoms in crystalline Ge2Sb2Te5

Local structure of NaCl-type crystalline Ge2Sb2Te5 has been studied by the atomic pair distribution function analysis of pulsed neutron powder diffraction data. We have found the large displacement of germanium atoms in this crystalline phase. Usually, such a large lattice distortion has disadvantages for the electric conductivity. The electronic band structure, however, implies that this significant distortion in addition to the lattice defect at Na site reduces only thermal conductivity, while the highly ordered tellurium atoms at Cl site keep high electric conductivity.

―高圧力と中性子― ＴＯＦ粉末中性子回折法

Neutron diffraction is a powerful technique to clarify the relationship between the crystal structures and the properties of functional materials. Since neutron atomic scattering factors do not depend on atomic numbers, neutron diffraction gives more reliable structural information than X-ray to detect light elements among heavy ones. In 2007, the proton accelerator project, J-PARC, will be completed and start delivering the most intense pulsed neutron in the world. Instrument proposals are being reviewed in the viewpoint of science, instrumental designs, and manpower, etc. This article was written to introduce the technique of pulsed neutron powder diffraction to promote high-pressure science in J-PARC.

Influence of Pressure and Temperature on the Crystal Structure of Deuterated Ammonium Copper Tutton Salt, (ND4)2[Cu(D2O)6](SO4)2

The influence of pressure on the unit cell parameters of deuterated ammonium copper(II) sulfate hexahydrate at various temperatures from 50 to 325 K obtained from pulsed neutron powder diffraction is reported. Application of pressure causes the structure to switch to a packing arrangement like that observed for the corresponding hydrogenous compound over the complete temperature range, with a higher pressure being required at lower temperatures. A pressure hysteresis occurs upon the release of pressure in the temperature range 303−275 K and below ∼295 K the compound remains in the high pressure (high density) modification at 1 bar. As the temperature is raised from ∼296 to 298 K, the structure reverts progressively to the low-pressure modification. Using the Clapeyron equation, an enthalpy ΔH of 259 cm-1 at 260 K was evaluated for the transition. With zero applied pressure, a negative temperature expansion coefficient is observed above 230 K, apparently due to an increasing thermal population of the high-density packing arrangement at higher temperatures.

Phonons from neutron powder diffraction

The spherically averaged structure function $\soq$ obtained from pulsed neutron powder diffraction contains both elastic and inelastic scattering via an integral over energy. The Fourier transformation of $\soq$ to real space, as is done in the pair density function (PDF) analysis, regularizes the data, i.e. it accentuates the diffuse scattering. We present a technique which enables the extraction of off-center phonon information from powder diffraction experiments by comparing the experimental PDF with theoretical calculations based on standard interatomic potentials and the crystal symmetry. This procedure (dynamics from powder diffraction(DPD)) has been successfully implemented for two systems, a simple metal, fcc Ni, and an ionic crystal, CaF$_{2}$. Although computationally intensive, this data analysis allows for a phonon based modeling of the PDF, and additionally provides off-center phonon information from powder neutron diffraction.

Atomic Structure of PbZrO3 Determined by Pulsed Neutron Diffraction

The atomic structure of lead zirconate, PbZrO3 (PZ), was studied using Rietveld refinement and atomic pair distribution function analysis of pulsed neutron powder diffraction data for the antiferroelectric, intermediate and paraelectric phases. The symmetry of PZ at T = 20 K in the antiferroelectric phase was determined to be Pbam. The structure was characterized by distortions of the ZrO6 octahedra which are smaller than in previous studies. Locally correlated displacements of Pb in the c direction develop with increasing temperature. The average magnitude was 0.06 Å at room temperature, 0.14 Å at T = 473 K and 0.20 Å in the intermediate phase at T = 508 K. The intermediate phase was characterized by in-plane antiferroelectric Pb displacements which produce 1\over 2{110} superlattice diffraction peaks. Above 473 K the local structure of PZ remains largely unchanged, in spite of the transitions in the long-range order from the antiferroelectric to the intermediate and to the paraelectric phases.

Re-examination of the second-order structural phase transition in La 2− xA xCuO 4 (A = Ba,Sr)

In the high temperature tetragonal phase of La 1.85Ba 0.15CuO 4 (LBCO) and La 1.875Sr 0.125CuO 4 (LSCO), the average octahedral tilts disappear at high temperature. However, we show that, locallu, tilts remain at temperatures well above the orthorhombic to tetragonal phase transition. We have used full profile refinement of the real-space atomic pair distribution function (PDF) obtained from pulsed neutron powder diffraction measurements. Our modeling indicates that order-disorder models of the octahedral tilts give significantly better agreement with the data than displacive models.

Nonuniform metallic state in manganites and cuprates

Local lattice distortions suggesting nonuniform charge distributions were found to be a common feature of manganites and cuprates in the metallic state by neutron scattering studies. The atomic pair-density function (PDF) determined by pulsed neutron powder diffraction showed that in La1-xSrxMnO3 doped holes form lattice polarons which persist even in the metallic state. In La1.85Sr0.15CuO4 the energy width of the LO phonons around (π,0) determined by inelastic neutron scattering reflects magnetic satellites indicative of spin charge stripe formation. The implications of such nonuniform charge distribution for metal-insulator transition and superconductivity are discussed.

Evidence of local lattice distortions in La1−xSrxMnO3 provided by pulsed neutron diffraction

The pulsed neutron powder diffraction study of La1−xSrxMnO3 reveals that the local atomic structure is significantly different from the long range structure, and these differences are important in understanding their properties. Even though the crystal structure suggests that the Jahn–Teller (JT) distortion disappears quickly with Sr doping, the analysis by the pair distribution function technique indicates that locally JT distortions are always present. Doped charge carriers induce local anti-JT distortions and disrupt the long range JT distortion. In the paramagnetic phase, such anti-JT distortions are represented by Mn4+ ions, corresponding to small polarons. Below Tc charge carriers are delocalized, but still induce local lattice distortions, giving rise to an inhomogeneous metallic state. The effects of these local distortions are discussed.

Short and intermediate range structural and chemical order in the relaxor ferroelectric lead magnesium niobate

We have obtained a model of short and intermediate range atomic structure in the relaxor ferroelectric Pb(Mg1/3,Nb2/3)O3 (PMN) by pair-density function (PDF) analysis of pulsed neutron and synchrotron x-ray powder diffraction data. All atomic species are found to be displaced from their ideal cubic perovskite lattice sites in a manner consistent with an eight site crystallographic disorder model. Use of the pair-density function in analyzing the structure of PMN has allowed short and intermediate range displacement correlations to be explored. Our structural model shows strong local displacement correlations resulting in two structurally distinct regions. Here we show that this model is in agreement with observed electron diffraction patterns and high resolution transmission electron microscopy images, and examine how it may be used in interpreting them. Differences in the local structure of the two regions may be important in explaining the relaxor behavior.

The crystal structure analysis of deuterated benzene and deuterated nitromethane by pulsed-neutron powder diffraction: a comparison with single crystal neutron diffraction analysis

The results of time-of-flight neutron powder diffraction analyses at 4.2 K are compared with those from steady-state neutron single-crystal analyses at 15 K for deuterated benzene, C 6D 6, and deuteronitromethane, CD 3NO 2.

Measurement of Atomic Elastic Constants by Pulsed Neutron Powder Diffraction

AbstractWe have developed a technique for determining the atomic elastic constants from measurements of the Debye-Waller factors. The Debye-Waller factors are obtained by Rietveld refinement of time-of-flight neutron diffraction data and interpreted in terms of an atomic Debye-Waller temperature. The method is applicable to powders and to materials that must be encapsulated for safety or environmental reasons. We will illustrate our technique with applications to actinide metals, to metallic hydrides and to high-temperature superconductors.