Modified Bridgman anvils for high pressure synthesis and neutron scattering

ABSTRACT

A 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.     

Developments of nano-polycrystalline diamond anvil cells for neutron diffraction experiments

ABSTRACT

A new high pressure cell for neutron diffraction experiments using nano-polycrystalline anvils is presented. The cell design, off-line pressure generation tests and a gas-loading procedure for this cell are described. The performance is illustrated by powder neutron diffraction patterns of ice VII to ～82?GPa. We also demonstrate the feasibility of single crystal neutron diffraction experiments of Fe₃O₄ at ambient conditions using this cell and discuss the current limitation and future developments.     

High pressure neutron diffraction studies using the Paris-Edinburgh cell

Abstract

Neutron diffraction was until recently confined to pressures below ～ 3 GPa. This restricted range has limited the high-pressure structural information that is available for a wide range of phenomena for which neutron diffraction is the technique of choice. But now the recently-developed Paris-Edinburgh cell can achieve pressures up to ～ 30 GPa with a sample volume large enough to allow accurate structural studies with neutrons. After a period of development of the neutron scattering techniques needed to obtain the best possible results using the cell, a variety of successful structural studies have been performed. These illustrate the value of neutron diffraction in important areas such as locating hydrogen and other low-Z atoms in structures, the measurement of accurate structural pressure dependence and the examination of the changes in atomic thermal motion with pressure.     

Structural study and electrical properties of Zr-doped Nd<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore compounds

Nd₂Sn₂O₇ pyrochlores with the substitution of Zr⁴⁺ were prepared by conventional ceramic double sintering technique. The single-phase formation was confirmed by X-ray diffraction and neutron diffraction techniques. Relative intensity calculations for X-ray diffraction analysis were performed for oxygen positional parametersx = 0.331 and 0.375, while Rietveld refinements were employed for neutron diffraction data. The neutron diffraction study revealed that there are only two anion sites with 48f and 8b positions. This indicates that the 8a site, i.e. O(3) sublattice, is completely vacant and the structure is a perfect cubic pyrochlore with space group Fd3m (O _h ⁷ ). From the conductivity measurements, it is observed that the electronic conductivity dominates from room temperature up to about 525 K and forT > 525 K, the oxygen ion conduction dominates the charge transport in these compositions. Complex impedance spectroscopy indicates the existence of grain and grain boundary as two separate elements.     

A structural comparison of supercooled water and intermediate density amorphous ices

New data are presented on neutron diffraction in ultrapure bulk supercooled heavy water measured down to 262?K. The data are analysed in terms of the trends observed in the first sharp diffraction peak (FSDP) parameters, the feature which dominates the measured neutron spectra. The neutron FSDP position, height and width are compared to literature data for supercooled water, water under pressure and to the same parameters obtained for recently discovered intermediate density amorphous ices. It is found that the FSDP parameters in supercooled water and the amorphous ices generally exhibit a similar behaviour, suggesting a new structural regime may occur in deeply supercooled water below Q ₀?～?1.83?Å^?1 (T?～?251?K) associated with increased intermediate range ordering. It is argued that this structural regime may be linked to a similar trend in the density which appears when the density is plotted as a function of FSDP position. A detailed comparison of the neutron and X-ray structure factors for supercooled water and intermediate density amorphous ices with the same FSDP positions is also made. The diffraction data show that although the overall general structures are qualitatively very similar, the amorphous ice correlations are considerably sharper and extend to much higher radial distances.     

A nano-polycrystalline diamond anvil cell with bulk metallic glass cylinder for single-crystal neutron diffraction

ABSTRACT

A high-pressure cell for in-situ single-crystal neutron diffraction was developed. The cell uses nano-polycrystalline diamond anvils in a tubular load frame made of bulk metallic glass which is highly transparent to neutrons and does not produce Bragg reflections. Diffraction peaks from a sample can be measured from almost any direction and the simple geometry of the cell allows accurate attenuation corrections. We demonstrate the operation of the cell by ambient-pressure experiment using a single-crystal of NaCl on the D9 diffractometer at the Institute-Laue-Langevin. A high-pressure experiment was also carried out on a single crystal of ice VII at 2.35?GPa showing the potential to detect weak diffraction spots. The correct integration of weak reflections together with the simple attenuation correction will help to carry out precise structure analysis and address new scientific problems using neutron diffraction.     

Probing Phonon Scattering Sites in the Thermoelectric Clathrate Eu8Ga16Ge30

The low thermal conductivity of the Thermoelectric Clathrates Eu₈Ga₁₆Ge₃₀ and Sr₈Ga₁₆Ge₃₀ has been attributed to rattling atoms (Eu or Sr) in the Eu2/Sr2 sites. The low Einstein temperature obtained using XAFS for the nearest neighbor Eu-Ga/Ge bonds at the Eu2 sites is consistent with the model. However, the comparable Einstein temperature for the Eu1 site indicates that Eu1 is also a rattler. The fit also confirms that the Eu2 is displaced along either the y or z axis ～ 0.45Å in agreement with neutron diffraction.     

Magnetic structure of Ce<Subscript>2</Subscript>Fe<Subscript>17 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compounds

The magnetic structure of intermetallic compounds Ce₂Fe_{17 − x} Mn _x (0 ≤ x ≤ 3) was studied using neutron diffraction. The neutron diffraction patterns measured at 4.2 K contain satellites indicating a modulated structure with the wave vector k = [0, 0, τ]. As the concentration x increases, the value of τ increases, while the average magnetic moment of Fe/Mn atoms decreases. A change in the magnitudes of the average magnetic moment and wave vector k is explained by competition between exchange interactions at distances of nearest neighbor transition element atoms.     

Possible observation of Kondo screening cloud in Yb14MnSb11 *

ABSTRACT

Single crystals of the underscreened Kondo ferromagnet Yb₁₄MnSb₁₁ were investigated using polarised neutron diffraction and magnetisation measurements. The magnitude and direction of the magnetisation at each of the 9 distinct crystallographic sites was measured using polarised neutron diffraction. A large moment of close to 4.2?μ_B was found at the unique Mn site. A compensating antiferromagnetic moment of ≈?0.5?μ_B was found to be distributed uniformly among the remaining Yb and Sb sites, which was not expected from a previous theoretical calculation. The compensating magnetisation, however, is consistent both in size and magnitude with the expected Kondo screening cloud.     

Precision measurement of the coherent neutron scattering length of 3He through neutron interferometry

Improved knowledge of the real part of the neutron scattering length of ³He is important for further development of nuclear few-body theory, as well as for a thorough understanding of neutron scattering off quantum liquids. The real part of the bound incoherent neutron scattering length b_i' has recently been measured directly with an experimental uncertainty of better than 1% by means of spin echo spectrometry. The uncertainty of the more fundamental bound multiplet scattering lengths b_±' is thus limited by today's 1.2% uncertainty of the spin-independent coherent part b_c'. Employing the skew-symmetric perfect crystal Si-interferometer at the S18 experimental site at ILL, Grenoble, we have re-measured the real part of the bound coherent neutron scattering length b_c' of ³He. Our result b_c' = 6.010(21)fm exhibits a significant deviation compared to the latest accepted value b_c' = 5.74(7)fm (H. Kaiser, H. Rauch, G. Badurek, W. Bauspiess, U. Bonse, Z. Phys. A 291, 231 (1979)). Including the known value of the incoherent neutron scattering length, we obtain new values for the real parts of the free singlet and triplet scattering lengths, a_-' = 7.573(30)fm and a₊' = 3.480(18)fm. Our result contravenes by more than 7 standard deviations the measurement of the same physical quantity that has recently been performed by a group at NIST in a very similar experiment (P.R. Huffman, D.L. Jacobson, K. Schoen, M. Arif, T.C. Black, W.M. Snow, S.A. Werner, Phys. Rev. C 70, 014004 (2004)) which yielded b_c' = 5.853(7)fm.     

Neutron scattering studies of structural transformations and vibrational spectra of ice after high pressure treatment

Abstract

The transitions of the recovered high-pressure phase ice VIII first to high-density amorphous (hda) and low-density amorphous ices, and finally to cubic Ic, and hexagonal Ih ice were observed at heating using real-time neutron diffraction. Inelastic incoherent neutron scattering measurements on the hdu ice, ice Ih and high-pressure phase ice VI revealed similarity between the amorphous phase and crystalline ice VI and led to the new proposition that hda ice consists of two interpenetrating hydrogen-bounded networks with no hydrogen bonds between “sublattices”.     

Neutron scattering study of short-range correlations and ionic diffusion in copper selenide

The paper reports the results of a neutron scattering study of Cu_{2 - d}Se {\hbox{C}}{{\hbox{u}}_{{2 - \delta }}}{\hbox{Se}} superionic compounds. The crystallographic model fitted to the diffraction data shows the occupation of 8c and 32f sites by Cu atoms. Observed diffuse background is related to correlated thermal vibrations of Se and Cu atoms, with Se↔Cu (8c,32f) and Cu (8c)↔Cu (8c) correlations being most important. The quasi-elastic neutron experiments show the decrease of the self-diffusion coefficient with the deviation from the stoichiometry due to the longer residence time of Cu ions between diffusion hops. Combination of neutron diffraction, diffuse scattering and quasi-elastic scattering experimental data suggests that the Cu atoms diffuse between the nearest 8c sites through the 32f sites.     

Powder neutron diffraction of wüstite (Fe0.93O) to 12 GPa using large moissanite anvils

High-pressure powder neutron diffraction of wüstite-Fe_0.93O has been achieved to 12 GPa using a large gem-moissanite (SiC) anvil cell. The moissanite anvils are weakly absorbing and provide greater neutron fluxes to the sample than is possible with tungsten carbide anvils. There is minimal diffraction overlap from the single-crystal moissanite anvils compared to tungsten carbide or synthetic diamond anvils, providing cleaner background profiles. The required sample volume for high-pressure neutron diffraction is dramatically reduced to several cubic millimeters. High-quality powder diffraction patterns of wüstite were recorded at 90 min exposure times on the HIPPO diffractometer at LANSCE when the sample volume was in the range of ～10 mm³. This is about two orders of magnitude smaller than the necessary sample volume (～1.0 cm³) for the same kind of experiment with other high-pressure cells and nominal neutron fluxes.     

Phase coexistence in proton glass

Abstract

Proton glasses are crystals of composition M_1?x(NW₄)_xW₂A0₄, where M = K,Rb,Cs, W = H,D, A = P, As. For x = 0 there is a ferroelectric (FE) transition, while for x-1 there is an antiferroelectric (AFE) transition. In both cases, the transition is from a paraelectric (PE) state of tetragonal structure with dynamically disordered hydrogen bonds to an ordered state of orthorhombic structure. For an intermediate x range there is no transition, but the hydrogen rearrangements slow down, and eventually display nonergodic behavior characteristic of glasses. We and others have shown from spontaneous polarization, dielectric permittivity, nuclear magnetic resonance, and neutron diffraction experiments that for smaller x there is coexistence of ferroelectric and paraelectric phases, and for larger × there is coexistence of antiferroelectric and paraelectric phases. We present a method for analytically describing this coexistence, and the degree to which this coexistence is spatial and/or temporal. We discuss also the experimental determination of these coexistence parameters.     

Stress‐induced Curie temperature increase in the Fe64Ni36 invar alloy

Structural and magnetic changes on invar Fe₆₄Ni₃₆ alloy (T_C = 500 K) produced by mechanical milling followed by heating up to 1073 K, were investigated by neutron diffraction, magnetization measurements, X‐ray diffraction under high pressures and X‐ray absorption at both Fe and Ni K‐edges. We argue that the strain induced in the Fe₆₄Ni₃₆ material after this treatment mainly affects the Fe sites due to the magnetovolume coupling, the most notorious feature being the increase of the Curie temperature (ΔT_C = 70 K). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First measurements of dtμ cycle characteristics in liquid H/D/T mixture

Muon catalyzed fusion in a dense triple mixture of hydrogen isotopes was investigated for the first time. The experimental method is based on the registration of neutrons from dtμ fusion by a full absorption detector in 4π geometry. The measurements were performed in H/D/T mixture at T=21 K and φ≃ 1.1 LHD using four sets of isotope concentrations. The basic parameters of the dtμ cycle (neutron yield, cycling rate and total sticking) in H/D/T mixtures obtained from neutron time distributions are presented and discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Defect recovery behavior of neutron irradiated molybdenum and tungsten of two purity levels

Abstract

The recovery behavior of radiation-induced defects during post-irradiation annealing was studied on molybdenum and tungsten specimens of two different purity levels. An electrical resistivity measurement technique at liquid nitrogen boiling temperature (～77°K) was used. Irradiation of both materials was conducted in Oak Ridge Research reactor at reactor ambient temperature (～70°C). The accumulative neutron fluence received was 7.3E+19 neutrons cm^?2 (E_n>l MeV) and 5.1E+20 neutrons cm^?2 (thermal). It was found that the number of recovery stages appeared to be independent of either the material or the impurity content. The stages are then believed to be due to the recovery of intrinsic defects and the recovery mechanisms are most likely the same for molybdenum and tungsten on the homologous temperature scale.     

Neutron scattering studies of the D-O and D-12 cyclohexane under high pressure

Abstract

Preliminary results of neutron diffraction and inelastic neutron scattering studies under high pressure of normal (D-O) and deuterated (D-12) cyclohexane are presented.     

Incoherent structure factor of hydrogen in NbH0.11

We show that the measurement of the quasielastic energy spectrumS(Q,) of incoherently scattered neutrons provides a direct andprecise method to determine the location of the H atom diffusing on interstitial sites in a metal lattice. At the reciprocal lattice points the spectrum contains a sharp elastic line with a relative intensity which is equal to the structure factor of the occupied sites. In a neutron scattering experiment performed on a NbH_0.11 single crystal, we have measured the structure factor at the (110), (200) and (222) reflection at elevated temperatures (148°C and 280°C). Our results give clear evidence of tetrahedral site occupancy and exclude an appreciable partial occupation of octahedral (3%) and triangular sites (7%) at 280°C. These findings support the basic assumption of the Chudley Elliott theory in the study of jump diffusion models of H assuming a negligible flight time in the jump process between neighbouring sites.     

Helimagnetism of CuxZn1−xCr2Se4 (for 0.0 ≤ × ≤ 0.1)

Abstract

The magnetization, the susceptibility and the magnetic anisotropy field of Cu _x Zn_1?xCr₂Se₄ compounds have been studied at low temperatures (down to 2.9 K) in: high magnetic stationary fields (up to 14 T), high pulsed magnetic fields (up to 25 T), medium magnetic stationary fields (up to 0.6 T). The magnetic structure of these spinels was studied by neutron powder diffraction.

The magnetic properties of Cu_xZn_1?xCr₂Se₄ are explained in terms of the molecular field approximation assuming the existence of 90° exchange interactions, ferromagnetic for Cr-Se-Cr between the nearest Cr ions and antiferromagnetic for Cr-Se-Se-Cr between the second-nearest Cr ions. The exchange parameters and integrals for the whole series under consideration are calculated. Taking into account the three magnetic phase transitions observed in these spinels (Juszczyk, Krok, Okońska-Koz?owska, Broda, Warczewski, Byszewski, 1981) and the neutron diffraction studies a modification of the simple spin spiral forced by a strong magnetic field is described.