High-pressure Neutron Diffraction Study Research Articles

High pressure neutron diffraction study of magnetic ordering in erbium

We have studied magnetic ordering in polycrystalline erbium at high pressures up to 32 GPa and low temperatures down to 10 K using neutron diffraction techniques at the Spallation Neutron Source at Oak Ridge National Laboratory, USA. For the hexagonal close-packed (hcp) phase, strong nuclear and magnetic satellite intensities permit a simultaneous refinement of the nuclear and magnetic structures. At 1 GPa of applied pressure, a modulation vector q=γc∗ with γ≈2/7 for the c-axis modulated and cycloidal phases is consistent with prior single-crystal studies at low pressures. At 6.7 GPa in the hcp phase, we find γ≈0.31, indicating a reduction in the period of the magnetic structure with respect to the crystal lattice. The magnetic ordering temperature at 6.7GPa is slightly above 60 K. At 32 GPa in the double hexagonal close-packed phase, the magnetic scattering constrains the magnetic ordering temperature to 25±5 K. Our neutron diffraction study demonstrates that the magnetic ordering persists in the high-pressure double hexagonal close-packed phase of erbiumto the highest pressure of 32 GPa.

High-pressure neutron diffraction study of magnetite, Fe3O4, nanoparticles

We use in situ high-pressure neutron powder diffraction to study elastic properties of Fe3O4 magnetite nanoparticles of different sizes. It is found that nanoparticles are elastically softer than the bulk. Apart from the smallest nanoparticle of diameter 8 nm, the atomic and magnetic structures do not change significantly with nanoparticle size or pressure. The 8 nm sample appears to take a disordered spinel structure instead of the inverse spinel structure of the bulk and larger nanoparticles, as seen in bond lengths and magnetic structures. Synchrotron x-ray total scattering was used to support this interpretation. Furthermore, this study suggests that the influence of magnetic disorder at the nanoparticle surface is significant for the size of 8 nm.

High-pressure neutron diffraction study of RCo2 compounds

The crystal structures and magnetic properties of RCo2 (R = Tb, Dy, Ho and Er) compounds are investigated in the temperature range 4 K ≤ T ≤ 300 K and pressures ≤5.6 GPa. There is a strong correlation between evolution with pressure of structural parameters and magnetic properties. Although the magnetic moments of R atoms are little affected by pressure, those of Co change sensitively, with higher rates at lower pressures (<2 GPa). Depending on magnitude of the exchange interactions between R and Co sublattices, their magnetisations can be decoupled or collapse at the same Curie temperature. The first-order transitions are correlated with the collapse of Co moments when the internal fields acting on Co atoms are below a critical value. The experimental data are correlated with the presence of the internal fields as determined by the pressure and temperature as well as of external fields.

Novel data analysis method for obtaining better performance from a complex 3D-printed collimator

Additively manufactured scattered beam collimators are increasingly being employed to boost the sample to cell peak signal ratio in high pressure neutron diffraction studies because of manufacturing versatility and performance improvements. We study how the measured diffraction pattern is affected by the presence of a collimator downstream of the sample, and develop a novel protocol that provides more effective background rejection. This protocol takes into account critical performance-determinants that were identified in this study, namely: (i) effectively identifying the collimator pattern on the detector; (ii) understanding the dependence of this pattern on sample and cell composition; and (iii) accurately identifying and differentiating the different regions of the pattern on the detector based on the dependency of the cell or sample and finally (iv) resolving the intensities at regions of the detector where neutrons scattered from the sample are preferentially represented, in order to boost the sample to cell peak signal ratio. Application of this novel analysis protocol is shown to increase the collimator performance over the traditional method.

High-Pressure Neutron Diffraction Study of the Crystal and Magnetic Structure of Materials at the Pulsed Reactor IBR-2: Current Opportunities and Prospects

The experimental capabilities and results of recent studies of the crystal and magnetic structure of functional materials on the diffractometers DN-12 and DN-6 of upgraded high-flux pulsed reactor IBR-2 in wide ranges of high pressures and temperatures are reviewed. The combination of high neutron flux on a sample, wide-aperture multidetector systems, and focusing devices has made it possible to implement diffraction experiments on samples with record-low volumes under ultrahigh pressures (up to 35 GPa) in the temperature range of 5–300 K. The results of recent studies of the structural and magnetic phase transitions in ferroelectrics, multiferroics, low-dimensional magnets, and other materials are presented.

Unresolved problems of ice polymorphs: approach from high-pressure neutron diffraction studies

Unresolved problems of ice polymorphs: approach from high-pressure neutron diffraction studies

Phase stability of the layered oxide, Ca2Mn3O8; probing interlayer shearing at high pressure

We have performed high-pressure neutron diffraction studies on the layered oxide, Ca2Mn3O8.

High-Pressure Study of Two Polymorphs of 2,4,6-Trinitrotoluene Using Neutron Powder Diffraction and Density Functional Theory Methods

A high-pressure neutron diffraction study was conducted on polycrystalline samples of the two known polymorphs of 2,4,6-trinitrotoluene [monoclinic (m) and orthorhombic (o) TNT] under hydrostatic c...

Hydrogen bonding in ethanol – a high pressure neutron diffraction study

ABSTRACTWe have determined the high pressure structure of perdeuterated ethanol by using neutron powder diffraction in the pressure region ∼2.5–4.0 GPa and compare it to that previously measured by single-crystal X-ray diffraction and the one suggested by ab initio structure calculations. In contrast to previous X-ray single-crystal studies, we see no evidence for a disordered proton around the hydroxyl group, but confirm the monoclinic symmetry in the measured pressure region. We also present the compression behaviour of the unit cell in this pressure range.

High-pressure neutron diffraction study of Pd3Fe

High-pressure neutron diffraction data from powder and single-crystal samples of atomically disordered (Fm3¯m) and ordered (Pm3¯m) Pd3Fe were collected up to pressures of 15 GPa, and high-pressure SQUID magnetometry data were collected up to 6 GPa. The data show a subtle decrease in the magnetic moment with applied pressure, resulting in a transition to a paramagnetic state by approximately 8 GPa at 300 K. Diffraction results have been used to determine the equation of state, resulting in a bulk modulus of 176.78(9) GPa for the disordered powder and 187.96(7) GPa for the ordered single-crystal samples, approximately 20% more compressible than previously reported from X-ray measurements. High-temperature superconducting quantum interference device magnetometry was used to confirm the ambient pressure Curie temperature of the sample (545 K), which was further investigated using high-temperature single-crystal neutron diffraction at ambient pressure.

Use of a miniature diamond-anvil cell in high-pressure single-crystal neutron Laue diffraction.

The first high-pressure neutron diffraction study in a miniature diamond-anvil cell of a single crystal of size typical for X-ray diffraction is reported. This is made possible by modern Laue diffraction using a large solid-angle image-plate detector. An unexpected finding is that even reflections whose diffracted beams pass through the cell body are reliably observed, albeit with some attenuation. The cell body does limit the range of usable incident angles, but the crystallographic completeness for a high-symmetry unit cell is only slightly less than for a data collection without the cell. Data collections for two sizes of hexamine single crystals, with and without the pressure cell, and at 300 and 150 K, show that sample size and temperature are the most important factors that influence data quality. Despite the smaller crystal size and dominant parasitic scattering from the diamond-anvil cell, the data collected allow a full anisotropic refinement of hexamine with bond lengths and angles that agree with literature data within experimental error. This technique is shown to be suitable for low-symmetry crystals, and in these cases the transmission of diffracted beams through the cell body results in much higher completeness values than are possible with X-rays. The way is now open for joint X-ray and neutron studies on the same sample under identical conditions.

Urea and deuterium mixtures at high pressures.

Urea, like many network forming compounds, has long been known to form inclusion (guest-host) compounds. Unlike other network formers like water, urea is not known to form such inclusion compounds with simple molecules like hydrogen. Such compounds if they existed would be of interest both for the fundamental insight they provide into molecular bonding and as potential gas storage systems. Urea has been proposed as a potential hydrogen storage material [T. A. Strobel et al., Chem. Phys. Lett. 478, 97 (2009)]. Here, we report the results of high-pressure neutron diffraction studies of urea and D2 mixtures that indicate no inclusion compound forms up to 3.7 GPa.

A high pressure neutron study of colossal magnetoresistant NdMnAsO0.95F0.05

A high pressure neutron diffraction study of the oxypnictide NdMnAsO0.95F0.05 has been performed at temperatures of 290–383 K and pressures up to 8.59 GPa. The results demonstrate that the antiferromagnetic order of the Mn spins is robust to pressures of up to 8.59 GPa. TN is enhanced from 360 to 383 K upon applying an external pressure of 4.97 GPa, a rate of 4.63 K GPa−1. NdMnAsO0.95F0.05 is shown to violate Bloch's rule which would suggest that NdMnAsO0.95F0.05 is on the verge of a localized to itinerant transition. There is no evidence of a structural transition but applied pressure tends to result in more regular As–Mn–As and Nd–O–Nd tetrahedra. The unit cell is significantly more compressible along the c-axis than the a-axis, as the interlayer coupling is weaker than the intrinsic bonds contained within NdO and MnAs slabs.

Explosives at Extreme Conditions: Polymorphism of 2,4-Dinitroanisole

2,4-dinitroanisole (DNAN) is an energetic material, developed as an insensitive replacement for TNT in melt-cast explosive formulations. While DNAN-based formulations demonstrate greatly reduced sensitivity to accidental initiation compared to those using TNT, issues remain with the replacement of TNT with DNAN. For instance, DNAN based formulations have demonstrated catastrophic levels of irreversible growth during heat-cycling, with volume increases of up to 15% reported. [1] In order to investigate the role of polymorphism in the irreversible growth of DNAN, high-pressure and variable-temperature neutron and x-ray diffraction studies have been performed. Two polymorphs of DNAN have been found to exist at ambient temperature and pressure, the thermodynamic form, DNAN-I, and the kinetic form, DNAN-II.[2,3] The phase diagrams of both form-I and -II of DNAN have been explored for the first time. In the case of DNAN-II, two high-pressure phase transitions were found. DNAN-II initially transformed to DNAN-III, which at higher pressures transformed to DNAN-IV. In addition, variable temperature studies demonstrated that the DNAN-II to DNAN-III transition also occurs when DNAN-II is cooled below room temperature. The thermal expansion of the DNAN-II/III lattice was investigated from 150K to 363K, demonstrating that an abrupt change in the thermal behaviour of lattice parameters occurs at the DNAN-II/III transition. From these combined crystallographic studies, the structure of DNAN-III has been solved, showing it is closely related to DNAN-II. In the case of DNAN-I, high-pressure neutron powder diffraction studies demonstrated that it transforms to a new form (DNAN-V) that is distinct from DNAN-II,-III or -IV. Rietveld refinement of the high-pressure DNAN-I data also determined that the material exhibits negative linear compressibility, which is of interest given the use of DNAN as a shock-insensitive energetic material. Comparison of the behaviour of DNAN-I and –II under variable temperature and high-pressure conditions indicates that the kinetic form, DNAN-II, is the denser phase under all conditions studied. This work highlights the importance of crystallographic techniques in order to understand the polymorphism of energetic materials.

A high-pressure neutron diffraction study of the ferroelastic phase transition in RbCaF3

The fluoroperovskite phase RbCaF3 has been investigated using high-pressure neutron powder diffraction in the pressure range ~0–7.9 GPa at room temperature. It has been found to undergo a first-order high-pressure structural phase transition at ~2.8 GPa from the cubic aristotype phase to a hettotype phase in the tetragonal space group I4/mcm. This transition, which also occurs at ~200 K at ambient pressure, is characterised by a linear phase boundary and a Clapeyron slope of 2.96 × 10−5 GPa K−1, which is in excellent agreement with earlier, low-pressure EPR investigations. The bulk modulus of the high-pressure phase (49.1 GPa) is very close to that determined for the low-pressure phase (50.0 GPa), and both are comparable with those determined for the aristotype phases of CsCdF3, TlCdF3, RbCdF3, and KCaF3. The evolution of the order parameter with pressure is consistent with recent modifications to Landau theory and, in conjunction with polynomial approximations to the pressure dependence of the lattice parameters, permits the pressure variation of the bond lengths and angles to be predicted. On entering the high-pressure phase, the Rb–F bond lengths decrease from their extrapolated values based on a third-order Birch–Murnaghan fit to the aristotype equation of state. By contrast, the Ca–F bond lengths behave atypically by exhibiting an increase from their extrapolated magnitudes, resulting in the volume and the effective bulk modulus of the CaF6 octahedron being larger than the cubic phase. The bulk moduli for the two component polyhedra in the tetragonal phase are comparable with those determined for the constituent binary fluorides, RbF and CaF2.

Density-driven structural transformations in network forming glasses: a high-pressure neutron diffraction study of GeO2 glass up to 17.5 GPa

The structure of GeO2 glass was investigated at pressures up to 17.5(5) GPa using in situ time-of-flight neutron diffraction with a Paris–Edinburgh press employing sintered diamond anvils. A new methodology and data correction procedure were developed, enabling a reliable measurement of structure factors that are largely free from diamond Bragg peaks. Calibration curves, which are important for neutron diffraction work on disordered materials, were constructed for pressure as a function of applied load for both single and double toroid anvil geometries. The diffraction data are compared to new molecular-dynamics simulations made using transferrable interaction potentials that include dipole-polarization effects. The results, when taken together with those from other experimental methods, are consistent with four densification mechanisms. The first, at pressures up to ≃ 5 GPa, is associated with a reorganization of GeO4 units. The second, extending over the range from ≃ 5 to 10 GPa, corresponds to a regime where GeO4 units are replaced predominantly by GeO5 units. In the third, as the pressure increases beyond ∼10 GPa, appreciable concentrations of GeO6 units begin to form and there is a decrease in the rate of change of the intermediate-range order as measured by the pressure dependence of the position of the first sharp diffraction peak. In the fourth, at about 30 GPa, the transformation to a predominantly octahedral glass is achieved and further densification proceeds via compression of the Ge–O bonds. The observed changes in the measured diffraction patterns for GeO2 occur at similar dimensionless number densities to those found for SiO2, indicating similar densification mechanisms for both glasses. This implies a regime from about 15 to 24 GPa where SiO4 units are replaced predominantly by SiO5 units, and a regime beyond ∼24 GPa where appreciable concentrations of SiO6 units begin to form.

High pressure neutron and synchrotron X-ray diffraction studies of tetragonal LaFeAsO0.9F0.1

We studied polycrystalline tetragonal LaFeAsO0.9F0.1 up to 9 GPa using time-of-flight neutron and synchrotron X-ray diffraction. Consistent with previous studies, the layer-structured LaFeAsO0.9F0.1 has a bulk modulus of 74–79 GPa, with the c−axis being twice as compressible as the a−axis. The refined structural parameters under pressure show non-monotonic variation with kink points at 4 GPa, well correlating with a maximum superconducting temperature (Tc) of 43 K previously reported for LaFeAsO0.9F0.1. At this pressure, however, both the As–Fe–As bond angle and anion height from the Fe layers deviate substantially from the respective ideal values of 109.47° and 1.38 Å for a regular FeAs4 tetrahedron. These findings indicate that the correlation between the maximum Tc and the geometry of conductive Fe–As layers is material dependent and may also be sensitive to atomic doping in the parent Fe-based superconductors.

High-pressure neutron diffraction study of Fe1.087Te

High-pressure neutron diffraction study of Fe_1.087Te

High-pressure powder neutron diffraction study on lead scandium niobate

The structural evolution of PbSc0.5Nb0.5O3 (PSN) under pressure has been studied by in situ powder neutron diffraction. Rietveld refinementsto the data show that the continuous phase transition detected by x-ray diffraction atpc = 4.1 GPa (Maier et al 2010 Phys. Rev. B 81 174116) is associated with long-range ordering ofantiphase octahedral tilts and local ordering of ferroic Pb displacements. Similar toPbSc0.5Ta0.5O3 (PST) (Maier et al 2010 Acta Crystallogr. 66 280–91), antiphase octahedraltilting even exists below the critical pressure in a regime in which thestructure retains a cubic metric. In contrast to PST, in which the Pb atomicdisplacement parameters (DPs) form ellipsoids elongated along the cubic⟨111⟩ directions, the atomic DPs of Pb in PSN form flattened discs parallel to the cubic{111}-planes. This indicates that in PST the Pb displacements are along the cubic⟨111⟩ directions, whereas in PSN the local Pb displacements are randomly distributed along the cubic⟨110⟩ directions. The latter can be explained by the abundance of underbonded oxygen atomsassociated with the chemical B-site disorder.

High-pressure neutron diffraction study of BaFe2As2

The crystal structure of BaFe2As2 was studied by high-pressure neutron powder diffraction in the pressure range from ambient to 6.5 GPa as well as in the temperature range from 12 K to 293 K at 4.4 GPa and no pressure or temperature induced phase changes were observed. The compression mechanism of BaFe2As2 was found to be anisotropic as the a- and c-axes are reduced by 2.49 and 3.66%, respectively at 6.5 GPa. Within the FeAs layers the Fe-As and Fe-Fe bonds decrease by 2.49 and 3.66%, respectively. The Ba-As distance decreases by 3.70% while the As-As inter-atomic distance along the c-axis exhibits a complex pressure dependence. The bulk modulus B 0 and its pressure derivative B 0' were determined to be B 0 = 59(2) GPa and B 0' = 6.1(7) at ambient temperature.