High-pressure Neutron Diffraction Experiments Research Articles

Development of 0.5 mm gauge size radial collimators for high-pressure neutron diffraction experiments at PLANET in J-PARC

Radial collimators with a 0.5 mm gauge size (GS) were specially designed for high-pressure neutron diffraction experiments. The performance and efficacy of these collimators, along with the 1.5 mm and 3.0 mm GS radial collimators, were investigated. The radial collimators with nominal GS of 0.75 mm, 1.5 mm, and 3.0 mm effectively exhibited GS of 0.50 mm, 1.07 mm, and 2.78 mm, respectively. The transmissions of all three radial collimators were deemed equivalent. The assessment using a Paris–Edinburgh (PE) press equipped with double-toroidal anvils and a diamond anvil cell (DAC) tailored for neutron diffraction revealed that the scattering intensity from the anvils was considerably minimized using the radial collimators and the sample-to-anvil signal ratio, as defined by the Bragg peak intensity ratio (IMgO220/IDiamond220), reached values of 0.5 and 2.0 for the PE press and DAC, respectively, when using the 0.5 mm GS radial collimators. These results indicate that the 0.5 mm GS radial collimators have been fabricated as intended and exhibit efficacy for the high-pressure-neutron diffraction experiments, specifically those exceeding 30 GPa, which cannot be reached with a PE press equipped with the standard double-toroidal anvils of a 1.5 mm cup diameter. Among those ever manufactured for neutron scattering experiments, the radial collimators display the smallest GS.

Investigation of null-matrix alloy gaskets for a diamond-anvil-cell on high pressure neutron diffraction experiments

ABSTRACT To investigate a gasket of null-matrix alloys suitable for high pressure neutron diffraction measurements using diamond-anvil-cells (DACs), high pressure experiments were conducted with TiZr and Mn-Cu-Ni-Fe (M2052) alloys as gaskets. The M2052 gasket was retained at least up to 20 and 43 GPa using the flat surface anvils of 1.5 and 1.0 mm culet sizes, respectively, whereas the TiZr gasket was broken at a range of 15–20 GPa with a 1.5 mm culet size. Furthermore, the M2052 gasket behaved as a null-matrix alloy in high pressure neutron diffraction measurements, and we obtained a satisfactory diffraction profile with no diffraction peaks from the gasket. This finding of the M2052 gasket will lead to a more accurate structural analysis in neutron experiments using the DAC.

The neutron diffraction experiments under high pressure and high temperature on FENGHUANG diffractometer at CMRR

FENGHUANG diffractometer at CMRR, a high-intensity multi-section neutron powder diffractometer, has been constructed to in situ studies of materials under high pressure high temperature conditions. With development of anvils, high pressure high temperature cell assemblies, high pressure device and alignment system, we have been realized the angle-dispersive neutron diffraction data collect at the range of achievable pressures and temperatures of 0–30 GPa and 300–1500 K, with the sample size of millimeter scale. This paper describes the technical characteristics of diffractometer and related techniques for high pressure neutron diffraction experiments, and gives some scientific examples that have been recently performed on FENGHUANG. The experience gained experiments on FENGHUANG could help promote the development of high pressure techniques on neutron scattering facilities.

Construction of a hybrid clamped cell for high-pressure neutron-diffraction experiments with a large diamond window.

A hybrid pressure cell was fabricated from commercially available copper-beryllium and custom-made Ni-Cr-Al Russian alloy, tailored for usage as a reaction vessel supplying a volume of about 400 mm3. In order to directly (in situ) monitor pressure and chemical reactions within the chamber, a large diamond window suitable for spectroscopic sample analysis was implemented. The performance of the hybrid cell was validated from high-pressure neutron-diffraction measurements on carbon dioxide.

High-pressure neutron diffraction techniques based on Paris-Edingburgh press

Since the 1990s, with the benefit of available large-volumed samples, wide detector windows, and portability, Paris-Edinburgh press has been widely used in neutron facilities to study the structures and physical properties of condensed matter under high-pressure extreme conditions. In the present study, We perform high-pressure neutron diffraction experiments in neutron source of China using the Paris-Edinburgh press. The experiments are carried out on a high-pressure neutron diffraction spectrometer (Fenghuang) at China Mianyang Research Reactor (CMRR). Fenghuang is a high-intensity and moderate-resolution diffractometer which has been upgraded from a neutron powder diffractometer and can be used under ambient and extreme conditions. A single cylinder pump with a max load of 200 MPa provides a loading pressure for Paris-Edinburgh press, and a precise mobile platform is used to hang and to locate the Paris-Edinburgh press. Using the tungsten-carbide (WC) toroidal anvils with TiZr gasket, we obtain the neutron diffraction spectra of Fe samples at different pressures successfully. We also obtain the neutron diffraction spectra respectively at pressures of 9.7 GPa and 10.7 GPa by using a WC single-toroidal anvil and a WC double-toroidal anvil under load 100 MPa. The TiZr gasket blows out before the load reaches 100 MPa in the WC single-toroidal anvil assembly, while it remains good in the WC double-toroidal anvil assembly under the same load. The WC single-toroidal anvil assembly becomes unstable under load about 80 MPa, and the WC double-toroidal anvil assembly is still stable under load 100 MPa. Thus, the stability of the double-toroidal anvil assembly is much higher than that of the single-toroidal anvil assembly. It is found that the thickness of the gasket edge is very important for the stability of the assembly during loading. The thicker the edge of the gasket, the more stable the assembly is. The main reason is that the groove of the double concave anvil can enhance the lateral support ability of the gasket, thereby making the double concave surface assembly more stable than the single concave surface assembly.

Hydrogen-rich Inclusion Compounds at High-pressure

Molecular hydrogen (H2) has been proposed as an alternative fuel source for vehicles. Though H2has many benefits, such as clean combustion and the highest known energy density by mass, there are issues in how to store it in a safe and cost effective way. One solution is to store hydrogen in a chemical compound, and gas clathrates (crystalline inclusion compounds) have shown promising results. Pressure provides a powerful means to tune the properties of such compounds and its effects on potential hydrogen storage materials are widely explored. We have recently developed a hydrogen-compatible gas loader for the Paris-Edinburgh press, which enables the loading of high density hydrogen into a clamp with a sample volume suitable for neutron diffraction experiments using the Paris-Edinburgh press [1]. Neutron diffraction is the technique of choice for such materials since it can reveal the location and occupancy of the hydrogen sites. We will present recent data from high-pressure neutron diffraction experiments on hydrogen hydrates as well as other clathrate forming systems like urea and hydroquinone.

Spin ice and soft spin ice ground state under high pressure: comparative study by neutron scattering

We performed high pressure neutron diffraction experiments in two geometrically frustrated pyrochlore magnets. The soft spin ice Tb2Sn2O7 was studied up to 4.6 GPa and down to 60 mK. Its ordered spin ice structure with k 0=0 propagation vector partly transforms under pressure into a magnetic structure with k 1=(0 0 1). The reduction of the total ordered Tb moment with respect to ambient pressure suggests that spin liquid fluctuations are also enhanced by pressure. The pressure-induced state is discussed by considering the influence of both isotropic pressure and stress. In contrast, in the true spin ice Ho2Ti2O7, no effect of pressure is observed up to 6 GPa and down to 1.4 K.

Feasibility Tests on Pressure-Transmitting Media for Single-Crystal Magnetic Neutron Diffraction under High Pressure

The hydrostaticity of pressure-transmitting media, such as graphite powder, a mixture of Fluorinert, Fomblin oil, silicone oil, Daphne7373 oil and glycerin, was investigated by high-pressure neutron diffraction experiments on the mosaic spread of NaCl single crystals. The width of the rocking curve of the NaCl single crystal in the medium other than glycerin increases rapidly with increasing pressure below 2.5 GPa, indicating that the medium solidifies below 2.5 GPa. On the other hand, glycerin transmits nearly hydrostatic pressure at least up to 7 GPa. The diffraction profiles of the media were also measured to evaluate the background intensities. Daphne7373 oil and glycerin generate low and flat diffraction profiles particularly at approximately 25°, where the principal magnetic diffraction signals are often present. Thus, glycerin is the most suitable pressure medium for single-crystal magnetic neutron scattering experiments beyond 3 GPa.

Pressure-Driven Phase Transitions in NaBH4: Theory and Experiments

We have investigated pressure-induced structural transitions in NaBH4 through density-functional theory calculations combined with X-ray and neutron diffraction experiments. Our calculations confirm that the cubic phase is stable up to 5.4 GPa and an orthorhombic phase occurs above 8.9 GPa, as observed in X-ray diffraction experiments. Both the calculations and X-ray diffraction measurements identify an intermediate tetragonal phase that appears between 6 and 8 GPa; that is, between the cubic and orthorhombic phases. This result is also confirmed by high-pressure neutron diffraction experiments performed on NaBD4. Our calculations and X-ray diffraction measurements show that the space group of the orthorhombic phase above 8.9 GPa is Pnma and the orthorhombic phase remains stable up to 30 GPa. The calculated equations of state are in excellent agreement with experiments.

Investigations on the Structure of Fluid Methane by Neutron Diffraction Experiments

High pressure neutron diffraction experiments on a mixture of CH4 and CD4 as well as on pure deuterated methane CD4 were performed at three supercritical states. The structure factor was determined at three different densities at a temperature of T = 370K. The intra- and intermolecular structure was determined from the structure factor. The results from pure CD4 are in very good agreement with former neutron diffraction experiments. The experiments on the isotopic mixture allowed the direct determination of the pair correlation function of the molecular centres. The results of the diffraction experiments are presented in this paper.

Pressure-induced structural phase transition in NdAl x Ga 2-x

High-pressure neutron-diffraction experiments on the pseudo-ternary compound NdAlxGa2-x revealed a pressure-induced isostructural phase transition for x=1.00 and 1.08 above about 1.5 GPa at room temperature. Within its hexagonal AlB2 structure the compound changes its lattice ratio c/a discontinuously from 0.97 to 0.84 in analogy to chemical pressure for x>1.12. Pressures up to 5.2 GPa did not result in a pure high-pressure phase but rather in a mixture of both phases. The intermetallic MN2 phases of the AlB2 structure are known to appear in two distinct groups with different c/a ratios. The observed isostructural transition is discussed within the framework of near-neighbor diagrams.

The structure of a fluid mixture of deuterated ethane and deuterated methane by high-pressure neutron diffraction experiments

High-pressure neutron diffraction data on a mixture of 33.3 mole % deuterated ethane and 66.7 mole % deuterated methane at six supercritical states are presented. The density could be varied by a factor larger than 2. The density dependence of the intra- and intermolecular structure is determined. The resulting total atomic pair correlation functions are compared with the results of Monte Carlo computer simulations with effective pair potentials.

Neutron powder diffraction studies at high pressure using a pulsed neutron source

Abstract High-pressure neutron-diffraction experiments using the time-of-flight (TOF) method are presented. We first introduce the helium-gas pressure system at Argonne's Intense Pulsed Neutron Source (IPNS) with the Special Environment Powder Diffractometer (SEPD). The results for TIBa2CuO6+8, and La2−x, Srx, CuO4–8 are briefly reviewed. We then introduce the present situation in Japan. We have been developing a high-pressure system on high-resolution powder diffractometer (HRP) which was much improved recently and renamed VEGA at KEK. The best resolution of 0.45% was obtained at VEGA. The preliminary results for YBa2, Cu4O8, and HgBa2, CuO4+8, are presented and compared with the SEPD data.

Diamond anvil cells for neutron diffraction

Abstract Design and general description of diamond anvil cells, developed for neutron diffraction studies of powder and single crystal samples, are given. Some aspects of high pressure neutron diffraction experiments are discussed.

A neutron diffraction study of the high pressure structural phase transition in (CD3ND3)2MnCl4

Abstract High-pressure neutron diffraction experiments have been performed at room temperature on a powdered sample of the perovskite type-layer compound (CD3ND3)2MnCl4. A phase transition from the orthorhombic room-temperature phase (ORT) to a new high-pressure phase (HP) is demonstrated at 20.5 ± 0.2 kbars. A monoclinic unit cell with lattice parameters a = 6.824 (5) A; b = 7.409 (8) A c = 17.126 (12) A and β = 82.94(9)° has been inferred for the HP phase, consistent with a two-dimensional perovskite-type structure. The HP phase appears to be much more compact than ORT; it is characterized, in particular, by an important compression (≃10%) of the inter-layer distance. Space groups P2/c or P21/c consistent with the experimental data have been deduced for the HP phase, after group theoretical considerations based on shear transformation and order-disorder mechanisms.

Effects of hydrostatic pressure on the magnetic ordering in CeSb

High pressure neutron diffraction experiments have been performed on CeSb up to 21 kbar in the temperature range 3–32 K. T N changes from 16 K at P = 1 bar to 31 K at P = 21 kbar. AFP-phases observed at P = 1 bar disappear completely above P = 13 kbar. At P = 9 kbar, type I phase appears at T N. Two completely new phases with magnetic ordering sequence (++--+-) and (++--+--++-) are successively observed between type I and type IA phases.