High-pressure Neutron Diffraction Research Articles

Equation of state of lead from high-pressure neutron diffraction up to 8.9 GPa and its implication for the NaCl pressure scale

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 ${B}_{0}=41.2(2)$ GPa at 298 K to increase by $14%$ to ${B}_{0}$ = 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.

Phase transitions and hydrogen bonding in deuterated calcium hydroxide: High-pressure and high-temperature neutron diffraction measurements

In situ neutron diffraction measurements combined with the pulsed neutron source at the Japan Proton Accelerator Research Complex (J-PARC) were conducted on high-pressure polymorphs of deuterated portlandite (Ca(OD)2) using a Paris–Edinburgh cell and a multi-anvil press. The atomic positions including hydrogen for the unquenchable high-pressure phase at room temperature (phase II′) were first clarified. The bent hydrogen bonds under high pressure were consistent with results from Raman spectroscopy. The structure of the high-pressure and high-temperature phase (Phase II) was concordant with that observed previously by another group for a recovered sample. The observations elucidate the phase transition mechanism among the polymorphs, which involves the sliding of CaO polyhedral layers, position modulations of Ca atoms, and recombination of Ca–O bonds accompanied by the reorientation of hydrogen to form more stable hydrogen bonds.

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.

High pressure neutron and X-ray diffraction at low temperatures

This paper presents a review of techniques and considerations in the design and construction of high pressure, low temperature diffraction experiments. Also intended as an introductory text to new high pressure users, the crucial aspects of pressure cell design are covered. The general classification of common designs, and a discussion into the key beam interaction, mechanical, and thermal properties of commonly used materials is given. The advantages of different materials and high pressure cell classifications are discussed, and examples of designs developed for low temperature diffraction studies are presented, and compared.

Pressure dependence of Morin transition in α-Fe2O3 hematite

We report high-pressure neutron diffraction data on the Morin-transition TM in α-Fe2O3 hematite, up to 8 GPa and under hydrostatic conditions. We find a strictly linear pressure coefficient of with no evidence for intermediate magnetic states as indicated by all previous non-hydrostatic measurements. The behaviour of TM is highly regular and can well be explained within the framework of Artman et al.'s single-ion model if a change of the single-ion anisotropy energy by +1%/GPa is assumed. Such a value seems not to be unusual for iron oxides.

High-pressure neutron diffraction: a gentler way to study explosives

High-pressure neutron diffraction: a gentler way to study explosives

Phase-Transition Induced Elastic Softening and Band Gap Transition in Semiconducting PbS at High Pressure

We have investigated the crystal structure and phase stability, elastic incompressibility, and electronic properties of PbS based on high-pressure neutron diffraction, in-situ electrical resistance measurements, and first-principles calculations. The refinements show that the orthorhombic phase is structurally isotypic with indium iodide (InI) adopting a Cmcm structure (B33). The cubic-to-orthorhombic transition occurs at ∼2.1(1) GPa with a 3.8% volume collapse and a positive Clausius-Clapeyron slope. Phase-transition induced elastic softening is also observed, which is presumably attributed to the enhanced metallic bonding in the B33 phase. On the basis of band structure simulations, the cubic and orthorhombic phases are typical of direct and indirect semiconductors with band gaps of 0.47(1) and 1.04(1) eV, respectively, which supports electrical resistivity measurements of an abrupt jump at the structural transition. On the basis of the resolved structure for B33, the phase transition paths for B1→B33→B2 involve translation of a trigonal prism in B1 and motion of the next-nearest neighbor Pb atom into {SPb7} coordination and subsequent lattice distortion in the B33 phase.

Mechanisms of network collapse in GeO2 glass: high-pressure neutron diffraction with isotope substitution as arbitrator of competing models

The structure of the network forming glass GeO2 is investigated by making the first application of the method of in situ neutron diffraction with isotope substitution at pressures increasing from ambient to 8 GPa. Of the various models, the experimental results are in quantitative agreement only with molecular dynamics simulations made using interaction potentials that include dipole-polarization effects. When the reduced density ρ/ρ0 ≳ 1.16, where ρ0 is the value at ambient pressure, network collapse proceeds via an interplay between the predominance of distorted square pyramidal GeO5 units versus octahedral GeO6 units as they replace tetrahedral GeO4 units. This replacement necessitates the formation of threefold coordinated oxygen atoms and leads to an increase with density in the number of small rings, where a preference is shown for sixfold rings when ρ/ρ0 = 1 and fourfold rings when ρ/ρ0 = 1.64.

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.

Pressure-dependent spin fluctuations and magnetic structure in the topologically frustrated spin glass alloy Y(Mn0.95Al0.05)2

Longitudinal field (LF = 110 G) muon spin relaxation (mu SR) has been used to investigate the pressure dependence (P < 4.5 kbar) of paramagnetic spin fluctuations in the spin glass alloy Y(Mn0.95Al0.05)(2) via observation of the mu(+) spin depolarization. External mechanical force is seen to counteract the Al-induced chemical pressure, fully delocalizing the Mn moment and altering the nature of the spin fluctuation spectrum sensed by the muon. A qualitative change in the functional form of the mu(+) spin depolarization is observed. Complementary ambient and high-pressure neutron diffraction measurements suggest not only pressure-dependent structural transitions but also the instability of the localized manganese moment. The ambient and high-pressure mu(+) spin depolarization results from Y(Mn0.95Al0.05)(2) are likened to P = 0 results reported for other Y(Mn1-xAlx)(2) alloys. Finally, the possibility of using mu(+) spin depolarization rates to predict experimental inelastic neutron scattering (INS) line widths is considered; the muon having the potential to provide information equivalent to that obtained via INS but with greatly reduced data collection times.

Pressure Effects on Quadrupolar and Magnetic Ordering in HoB2C2Observed by Single-Crystal Neutron Diffraction

High-pressure single-crystal neutron diffraction experiments were carried out on a tetragonal HoB 2 C 2 where the antiferroquadrupolar (AFQ) order coexists with the antiferromagnetic (AFM) order competitively. The transition temperature T N increases monotonically with increasing pressure up to 9.8 GPa, whereas T Q decreases and vanishes at approximately 4 GPa as pressure increases, indicating that applying pressure enhances the AFM interaction and simultaneously suppresses the AFQ order. We presume that the volume shrinkage by applying pressure constrains the local atomic displacement of (B–C) layers and leads to the suppression of the AFQ order. We additionally found pressure-induced phases where the ordered state was considered to be antiferromagnetic.

High-pressure neutron diffraction at the SNS

High-pressure neutron diffraction at the SNS

High-pressure neutron diffraction study of Fe1.087Te

High-pressure neutron diffraction study of Fe_1.087Te

Pressure-induced change in the magnetic ordering of TbMnO3

We have studied TbMnO${}_{3}$ by high-pressure neutron diffraction up to 3.6 GPa and down to 1.5 K. The incommensurate spin structures of the Mn and Tb moments are strongly suppressed under pressure, and an $E$-type commensurate spin structure with $(0,1/2,0)$ propagation vector is stabilized for the Mn moments. The Mn and Tb incommensurate orders which persist at low pressures or low temperatures are also affected, with a change of phase of the modulation between neighboring Mn chains and a reorientation of the Mn moments along the $a$ axis. The results suggest both a change of the balance of the different Mn-Mn interactions and a strengthening of the Tb-Tb (or Tb-Mn) interactions under pressure.

High-pressure single-crystal neutron diffraction to 10 GPa by angle-dispersive techniques

Techniques have been developed that allow the measurement of accurate single-crystal neutron-diffraction data at pressures up to 10 GPa, using angle-dispersive methods. High-quality data have been collected up to 10 GPa, to a resolution of sinθ/λ ≃ 1.5 Å−1, from samples of size 3–4 mm^{3}. This article presents the methods developed to mount and centre the sample accurately on the instrument; to reduce the background and hence increase the precision of the measured reflection intensities; and to increase further the accessible region of reciprocal space with a single sample loading. Developments are also highlighted, with a view to increasing the range of both science and pressures that can be achieved at the Institut Laue–Langevin reactor source using single-crystal techniques.

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 synthesis and neutron diffraction investigation of the crystallographic and magnetic structure of TeNiO3 perovskite

TeNiO(3) has been prepared under moderate pressure conditions (3.5 GPa), starting from a reactive TeO(2) and Ni(OH)(2) mixture contained in a sealed platinum capsule under the reaction conditions (850 °C for 2 h). The sample has been studied by neutron powder diffraction (NPD) data and magnetization measurements. TeNiO(3) crystallizes in an orthorhombically-distorted perovskite structure (space group Pnma) with the unit cell parameters a = 5.9588(1) Å, b = 7.5028(1) Å and c = 5.2143(1) Å. The NiO(6) octahedral network is extremely tilted, shaping a trigonal-pyramidal environment for the Te, where it is effectively coordinated to three oxygen atoms at Te-O distances of 1.92 Å. Below T(N) ≈ 130 K, it experiences an antiferromagnetic ordering, as demonstrated by susceptibility and NPD measurements. Above the Néel temperature, a paramagnetic moment of 3.24(1) μ(B)/f.u. and θ(Weiss) = -199(1) K are obtained from the reciprocal susceptibility. Below T(N), the magnetic reflections observed in the neutron patterns can be indexed with a propagation vector k = 0. The magnetic structure corresponds to the magnetic mode G(y)F(z). The magnetic moments are oriented along the y-direction, with a canting along the z-axis. This ferromagnetic component explains the weak ferromagnetism observed in the magnetization isotherms; the infrequent shape of the magnetization cycles suggests a metamagnetic transition below 0.2 T. At T = 2.5 K, the ordered magnetic moment for the Ni(2+) ions is 1.88(5) μ(B) for the G(y) mode and 0.9(2) μ(B) for the F(x) mode.

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.

Temperature dependence of the pressure-induced amorphization of iceIhstudied by high-pressure neutron diffraction to 30 K

High-pressure neutron diffraction allowed the in situ observation of the pressure-induced amorphization of ordinary ice ${I}_{h}$ between 130 and 30 K, i.e., to lower temperatures than any other diffraction study before. We find that the pressure required for complete transformation into high-density amorphous ice (HDA) increases with decreasing temperature to $\ensuremath{\sim}80\text{ }\text{K}$ but remains approximately constant below. Our findings support earlier evidence of two distinct mechanisms responsible for the pressure-induced amorphization in ice ${I}_{h}$, namely, amorphization due to mechanical melting down to lowest temperatures, and amorphization due to thermal melting at elevated temperatures. Such scenario naturally explains why HDA prepared through compression at 77 K is structurally distinct from the form of HDA obtained by the compression of low-density amorphous ice (LDA) and hence cannot be associated with the hypothesized high-density proxy of liquid water in a two-state model.