High-pressure Neutron Research Articles

Neutron diffraction study of the α- to β-phase transition in BaD2 under high pressure

We report high-pressure neutron diffraction, and Raman spectroscopy data from the α- (Pnma) to β- (P63/mmc) phase transition in BaD2. The transition was observed at 2.53(5) GPa on pressure increase, and 1.65(5) GPa on pressure decrease, with a relatively narrow 0.20(5) GPa region of phase coexistence. We report the isothermal equations of state for the α-phase at 300 K and the β-phase at both 300 K and 480 K, and have calculated the entropy change through the transition. The Raman data show that a low-energy Ba vibration is seen to soften with applied pressure; DFT calculations suggest that this is predominantly due to an instability in the Ba atoms prior to the first-order transition. The shift in D positions in the high-pressure phase suggest that the Wyckoff 4f-model (where one of the D atoms is split between two sites) is a better fit at lower pressures, but that the model tends towards the Wyckoff 2d-model (where D1 is localised on a single site) with increased pressure. These results are used to discuss implications on the reported increases in ionic-conductivity over the transition.

Anomalous hydrogen dynamics of the ice VII–VIII transition revealed by high-pressure neutron diffraction

Above 2 GPa the phase diagram of water simplifies considerably and exhibits only two solid phases up to 60 GPa, ice VII and ice VIII. The two phases are related to each other by hydrogen ordering, with the oxygen sublattice being essentially the same. Here we present neutron diffraction data to 15 GPa which reveal that the rate of hydrogen ordering at the ice VII-VIII transition decreases strongly with pressure to reach timescales of minutes at 10 GPa. Surprisingly, the ordering process becomes more rapid again upon further compression. We show that such an unusual change in transition rate can be explained by a slowing down of the rotational dynamics of water molecules with a simultaneous increase of translational motion of hydrogen under pressure, as previously suspected. The observed cross-over in the hydrogen dynamics in ice is likely the origin of various hitherto unexplained anomalies of ice VII in the 10-15 GPa range reported by Raman spectroscopy, X-ray diffraction, and proton conductivity.

Combined X-ray and neutron single-crystal diffraction in diamond anvil cells.

It is shown that it is possible to perform combined X-ray and neutron single-crystal studies in the same diamond anvil cell (DAC). A modified Merrill-Bassett DAC equipped with an inflatable membrane filled with He gas has been developed. It can be used on laboratory X-ray and synchrotron diffractometers as well as on neutron instruments. The data processing procedures and a joint structural refinement of the high-pressure synchrotron and neutron single-crystal data are presented and discussed for the first time.

Advances in the use of Paris-Edinburgh presses for high pressure neutron scattering

Advances in the use of Paris-Edinburgh presses for high pressure neutron scattering

In situ high pressure neutron diffraction and Raman spectroscopy of 20BaO-80TeO2 glass.

The short-range structure of 20BaO–80TeO2 glass was studied in situ by high pressure neutron diffraction and high pressure Raman spectroscopy. Neutron diffraction measurements were performed at the PEARL instrument of the ISIS spallation neutron source up to a maximum pressure of 9.0 ± 0.5 GPa. The diffraction data was analysed via reverse Monte Carlo simulations and the changes in the glass short-range structural properties, Ba–O, Te–O and O–O bond lengths and speciation were studied as a function of pressure. Te–O co-ordination increases from 3.51 ± 0.05 to 3.73 ± 0.05, Ba–O coordination from 6.24 ± 0.19 to 6.99 ± 0.34 and O–O coordination from 6.00 ± 0.05 to 6.69 ± 0.06 with an increase in pressure from ambient to 9.0 GPa. In situ high pressure Raman studies found that the ratio of intensities of the two bands at 668 cm−1 and 724 cm−1 increases from 0.99 to 1.18 on applying pressure up to 19.28 ± 0.01 GPa, and that these changes are due to the conversion of TeO3 into TeO4 structural units in the tellurite network. It is found that pressure causes densification of the tellurite network by the enhancement of co-ordination of cations, and an increase in distribution of Te–O and Ba–O bond lengths. The original glass structure is restored upon the release of pressure.

Optimal design and experimental verification of high-temperature and high-pressure assembly of neutron diffraction based on PE-type press

High-pressure and high-temperature(high-P-T) <i>in-situ</i> neutron diffraction detection method is a field of growing interest, in particular, for its numerous applications in the field of condensed matter physics, crystal chemistry, geophysics, materials science and engineering. In this work, we design and optimize a set of assembly for high-P-T <i>in-situ</i> neutron diffraction experiment in neutron source of China by using Paris-Edinburgh(PE)-type press. The high-P-T experiment is carried out with a high-pressure neutron diffraction spectrometer (Phoenix) of China Mianyang Research Reactor (CMRR). A 1500 KN uniaxial loading system and a 1500 W constant current source provides extreme conditions of high-P-T for PE press. The toroidal anvil we use is made of tungsten carbide. We use two types of gaskets: one is machined from the null-scattering TiZr alloy and the other is made from the thermal insulation ceramic material of ZrO<sub>2</sub>. High-temperature furnace is formed by graphite. First, a simplified simulation analyses of the pressure change rates in different areas of the entire assembly are carried out, and it is concluded that the gasket I, II, III areas are designed with a gradient decreasing method. The compression ratio of the sample chamber is significantly improved. Then when the gasket reaches the same compression ratio, the cell pressure will be higher than the pressure before optimization. After that, we conduct experimental verification on the optimized design. Through a series of optimization experiments for assembly on the rheological control of gasket, the improvement of thermal insulation performance and the maximization of effective sample volume under high-P-T, the key technical indicators and design scheme of the high-P-T <i>in-situ</i> neutron diffraction platform are verified. The temperature and pressure in the sample cavity are calibrated by using the MgO's high-P-T <i>in-situ</i> neutron diffraction spectrum and equation of state. The <i>in-situ</i> neutron diffraction sample cavity environment of the designed platform can reach the conditions of 11.4 GPa and 1773 K. The successful development of this assembly greatly improves the experimental conditions of CMRR high-P-T neutron diffraction platform. At the same time, it has important reference significance for further improving the high-P-T loading conditions of the PE-type press and expanding the application scope of the PE-type press.

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...

From Molecules to Carbon Materials—High Pressure Induced Polymerization and Bonding Mechanisms of Unsaturated Compounds

With the development of high-pressure apparatus, in situ characterization methods and theoretical calculations, high-pressure technology becomes a more and more important method to synthesize new compounds with unusual structures and properties. By compressing compounds containing unsaturated carbon atoms, novel poly-ionic polymers, graphanes and carbon nanothreads were obtained. Their compositions and structures were carefully studied by combining multiple cutting-edge technologies, like the in situ high-pressure X-ray and neutron diffraction, transmission electron microscopy, pair distribution function, solid-state nuclear magnetic resonance and gas chromatography-mass spectroscopy. The reaction mechanisms were investigated based on the crystal structure at the reaction threshold pressure (the pressure just before the reaction taking place), the long-range and short-range structure of the product, molecular structure of the intermediates, as well as the theoretical calculation. In this review, we will summarize the synthesis of carbon materials by compressing the unsaturated compounds and its reaction characteristics under extreme conditions. The topochemical reaction mechanism and related characterization methods of the molecular system will be highlighted. This review will provide a reference for designing chemical reaction and exploring novel carbon materials under high-pressure condition.

Suppression of the bulk high spin–low spin transition by doping the chiral magnet MnGe

In the MnGe chiral magnet, the helimagnetic order and local moment collapse in two steps, showing the succession of high spin (HS) and low spin (LS) states as pressure increases. Here, we use high-pressure neutron diffraction to study the doped compounds ${\mathrm{Mn}}_{0.86}{\mathrm{Co}}_{0.14}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.9}{\mathrm{Rh}}_{0.1}\mathrm{Ge}$, and show that the evolution of their microscopic magnetic properties is instead continuous. It means that the bulk HS-LS transition is a unique feature of pure MnGe, very sensitive to small changes of the band structure and easily suppressed by chemical substitution. On the other hand, the helimagnetic correlations appear to be strengthened by doping and survive up to larger pressures ($\ensuremath{\approx}19\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$, to be compared with $\ensuremath{\approx}13\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$). We discuss these results in the light of other disordered systems with remarkable properties, the so-called Invar alloys.

Synthesis and characterization, by high-pressure neutron and X-ray powder diffraction, of hybrid perovskites containing helium as a structural component

Synthesis and characterization, by high-pressure neutron and X-ray powder diffraction, of hybrid perovskites containing helium as a structural component

Development of cubic anvil type high pressure apparatus for neutron diffraction

High-pressure neutron diffraction (HPND) experiments in extended pressure and temperature ranges can provide invaluable information for understanding many pressure-induced emergent phenomena, such as unusual phase transitions and quantum critical behavior involving spin, orbital, charge and structural degrees of freedom, in strongly correlated materials. Many apparatuses for different purposes of HPND experiments have been developed in several laboratories. Recently, a clamp-type cubic anvil high pressure cell that can generate pressure over 7 GPa at 3 K was developed for low-temperature HPND measurements. In this paper, characteristics of the clamp-type cubic anvil high pressure cell are presented and its performances are demonstrated by measuring magnetic neutron scattering under pressure on MnP single crystal samples.

Coexistence of Eu antiferromagnetism and pressure-induced superconductivity in single-crystal EuFe2As2

By performing high-pressure single-crystal neutron diffraction measurements, the evolution of structure and magnetic ordering in EuFe2As2 under hydrostatic pressure were investigated. Both the tetragonal-toorthorhombic structural transition and the Fe spin-density-wave (SDW) transition are gradually suppressed and become decoupled with increasing pressure. The antiferromagnetic order of the Eu sublattice is, however, robust against the applied pressure up to 24.7 kbar, without showing any change of the ordering temperature. Under the pressure of 24.7 kbar, the lattice parameters of EuFe2As2 display clear anomalies at 27(3) K, well consistent with the superconducting transition observed in previous high-pressure resistivity measurements. Such an anomalous thermal expansion around Tc strongly suggests the appearance of bulk superconductivity and strong electron-lattice coupling in EuFe2As2 induced by the hydrostatic pressure. The coexistence of long-range ordered Eu-antiferromagnetism and pressure-induced superconductivity is quite rare in the EuFe2As2-based iron pnictides.

Equation of state and strain-induced stabilization of δ-phase stabilized plutonium alloys

An in-situ high-pressure neutron diffraction experiment was conducted on a δ-phase stabilized plutonium alloy with isotope 242Pu. Upon room-temperature compression, neither of the previously reported pressure-induced transformation paths, δ → α′ or δ → γ’ → α′, was observed up to 1.2 GPa. Instead, a drastic reduction in the diffraction intensity of the δ phase was observed when the pressure was above 0.8 GPa. At the highest pressure of the experiment (1.2 GPa), the diffraction data appear to be characteristic of an amorphous state, manifested by the diminishing intensities of all diffraction lines. In addition, no evidence was found to support the transformation to a body-centered tetragonal structure (δ’), which was previously inferred from the diffraction line broadening during initial compression to 0.1 GPa. The discrepancies between the present and previous experiments suggest a substantial strain-induced stabilization of the δ-phase and is presumably attributed to the different stress states in the high-pressure environments. From the pressure - volume measurements, the determined isothermal bulk modulus for the δ-phase is in the range of 31.9 ± 1.3–34.8 ± 1.8 GPa using different pressure scales, comparable to those obtained from the resonant ultrasound spectroscopy measurements of the alloys of similar composition. The pressure-induced elastic softening is neither convinced in the present work, nor can it be resolved from the diffraction experiments if it is intrinsically weak.

Modified Bridgman anvils for high pressure synthesis and neutron scattering

ABSTRACTA simple modified Bridgman design for large volume pressure anvils usable in the Paris-Edinburgh (PE) press has been demonstrated at Oak Ridge National Laboratory Spallation Neutron Source. The design shows advantages over the toroidal anvils typically used in the PE press, mainly rapid compression/decompression rates, complete absence of blow-outs upon drastic phase transitions, simplified cooling, high reliability, and relative low loads (∼40 tons) corresponding to relatively high pressures (∼20 GPa). It also shows advantages over existing large-volume diamond cells as sample volumes of ∼2–3 mm3 can be easily and rapidly synthesized. The anvils thus allow sample sizes sufficient for in situ neutron diffraction as well as rapid synthesis of adequate amounts of new materials for ex situ analysis via total neutron scattering and neutron spectroscopy.

Development of a technique for high pressure neutron diffraction at 40 GPa with a Paris-Edinburgh press

ABSTRACTWe have developed a technique for neutron diffraction experiments at pressures up to 40 GPa using a Paris-Edinburgh press at the PLANET beamline in J-PARC. To increase the maximum accessible pressure, the diameter of the dimple for sample chamber at the top of the sintered diamond anvils is sequentially reduced from 4.0 mm to 1.0 mm. As a result, the maximum pressure increased and finally reached 40 GPa. By combining this technique with the beam optics which defines the gauge volume, diffraction patterns sufficient for full-structure refinements are obtainable at such pressures.

Acceleration of δ- to β-HMX-D8 Phase Retransformation with D2O and Intergranular Strain Evolution in a HMX-Based Polymer-Bonded Explosive

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is one of the most widely used explosives. Problems about phase state and unrecoverable damage are significant in the application of HMX and HMX-based polymer-bonded explosive (PBX). Recently, the acceleration of δ- to β-HMX retransformation in humid atmosphere was reported. However, the function of water was not well understood. In this paper, we studied the mixed phase during δ- to β-HMX-d8 retransformation with neutron diffraction. Evidence of γ-HMX-d8 is specially focused to estimate whether the hydrated phase is part of the δ- to β- phase retransformation. Besides, in situ high-pressure neutron diffraction of HMX-based PBX was performed to study the evolution of intergranular strains in special structures such as twin plane, cleavage planes, and slip systems. We observed an abnormal release of strain on certain planes under applied pressure which had not been reported. This abnormal phenomenon was considered to explain the formation of microcrack...

Pressure-Effects and Adaptation Mechanisms of Ambient and Deep-Sea Bacterial Enzymes

Submersible expeditions into the deepest parts of Earth's oceans have led to discoveries of organisms thriving in the extremes, raising questions on the limits of life and the functioning of their central biological macromolecules. Questions arise as to how life would be able to survive under such “harsh” conditions, and what adaptations may be present in these organisms’ proteins. Our major focus is on understanding biophysical effects of extreme pressure, which is a relatively underexplored condition for protein solutions. Investigating the effects of high hydrostatic pressure (HHP) on the biophysical properties of proteins, in conjunction with structural adaptations utilized by known piezophilic organisms to maintain proper function, provide better insight into how organisms are affected by and survive at such depths. The results gathered from these data can impact our use of pressure for sterilization of food products, our understanding of piezophilic adaptations, and enhance our understanding of the origins of life. Graphics processing unit (GPU)-accelerated OpenMM molecular dynamics simulations were used to explore pressure-effects on and pressure-adaptations within a central metabolic enzyme, dihydrofolate reductase (DHFR). In-silico comparisons of hydrogen bonding, collective motion, among other phenomena, were made between bacterial DHFRs of the mesophile (ambient pressure) Escherichia coli, the deep-sea piezophiles Moritella profunda and Moritella yayanosii, as well as mutants containing specific residues identified to be important in pressure-sensitivity. Furthermore, high-pressure small-angle neutron and X-ray scattering, fluorescence and other high-pressure experimental techniques conducted up to 3.5 kbar further probed the biophysical effects of HHP on E. coli and M. profunda DHFRs. The combined results from in-silico and in-situ techniques provide enhanced insight into the interplay between the sequence, structure, and hydration of proteins underpinning life inhabiting extreme pressures.

Permanent densification of silica glass for pressure calibration between 9 and 20 GPa at ambient temperature

ABSTRACTPermanent density increase of silica glass was used to calibrate pressure generation delivered by cupped sintered diamond anvils (‘dimple anvils’) [Haberl B, Molaison JJ, Neuefeind JC, et al. Simple modified Bridgman anvil design for high pressure synthesis and neutron scattering. High Press. Res. submitted] within the Paris-Edinburgh press between approximately 9 and 20 GPa. Raman spectral changes of recovered silica glass with increased density were used to determine the maximum pressure reached by following an established calibration curve [Deschamps T, Kassir-Bodon A, Sonneville C, et al. Permanent densification of compressed silica glass: a Raman-density calibration curve. J. Phys. Condens. Matter. 2013;25:025402]. The monotonic Raman shift of the Main Band spectral region (∼200–700 cm−1) of silica glass recovered from 9 to 20 GPa allows for continuous pressure calibration and is applicable to all presses that operate within this pressure range. Radial & axial Raman profiles were conducted to determine the pressure distribution within the sample chamber. This technique has been verified by in situ resistance measurements of the insulator-to-metal phase transition of ZnS near 15 GPa.

On intensities in high pressure neutron powder diffraction using single and polycrystalline diamond anvils: small versus large sample volumes

ABSTRACTWe report a quantitative comparison of measured intensities of neutron powder diffraction data collected in a single-crystal diamond anvil cell and in large-volume sintered diamond anvils. As expected from the difference in sample volumes, the latter provides 1–2 orders of magnitude higher intensities, depending on the anvil material. The remaining differences are due to effects of absorption and angular aperture.