High-temperature Neutron Diffraction Experiments Research Articles

High-pressure and high-temperature neutron-diffraction experiments using Kawai-type multi-anvil assemblies

ABSTRACT We applied Kawai-type multi-anvil assemblies (MA6-8) for time-of-flight neutron-diffraction experiments to achieve high-pressures and high-temperatures simultaneously. To achieve sufficient signal intensities, the angular access to the sample was enlarged using slits and tapers on the first-stage anvils. Using SiC-binder sintered diamond for the second-stage anvils that transmits neutrons, sufficient signal intensities were achieved at a high-pressure of ∼23.1 GPa. A high-temperature experiment was also conducted at 16.2 GPa and 973 K, validating the use of tungsten carbide for the second-stage anvils. The present study reveals the capability of the MA6-8 cells in neutron-diffraction experiments to attain pressures and temperatures beyond the limits of the conventional MA6-6 cells used in the high-pressure neutron diffractometer PLANET at the MLF, J-PARC.

Synthesis and Structural Determination of the Disordered Bixbyite Cu3-x Sb1+x O5.5+3x/2 with Spin-Glass Behaviour.

The ternary copper antimony oxide Cu3-x Sb1+x O5.5+3x/2 (x=0.23) has been synthesized under 0.8-1.3 MPa pO2 at 1022-1082 °C. Rietveld refinements of X-ray and neutron powder diffraction patterns concluded that the oxide adopts a bixbyite type structure, crystallising in the cubic space group Ia-3 with the unit cell parameter a=9.61164(4) Å at room temperature from powder neutron diffraction data. The cationic 8b and 24d sites were found to be occupationally disordered where both Cu and Sb could be found on both sites. This is supported by X-ray absorption spectroscopy experiments showing more than one possible Cu environment. There was a significant net deficiency of oxygen in the compound which was first inferred from observations of a thermochromic-like phenomena and also seen from in situ high temperature neutron diffraction experiments. Magnetic susceptibility and magnetization measurements show paramagnetic behaviour with spin-glass like transition below 6 K.

High-Temperature Neutron Diffraction Study on PbWO4 and CaWO4-Based Oxide Ion Conductors with Different Defect Structure

Introduction Oxide ion conductors have been attracted great interests for the application of electrolytes of solid oxide fuel cells (SOFC). Most of the oxide ion conductors employ oxide ion vacancies to perform ionic conduction, while very few possess oxide ion interstitials for the oxide ion migration. When lanthanum ions are partly substituted into lead site in scheelite-type structured PbWO4, oxide ion conduction appears at elevated temperatures [1,2]. In that case, oxide ion interstitials are formed as Pb1-xLaxWO4+x/2 for the oxide ion conduction. On the other hand, CaWO4 with the isostructure with PbWO4 does not show such an oxide ion conduction enhancement due to the lanthanum substation. Recently, we have reported that alkaline-ion substituted CaWO4 also exhibit high oxide ion conduction [3]. Although the defect structure of the latter system is oxide ion vacancy as Ca1-xKxWO4-x/2, activation energies of the oxide ion conduction for both systems are relatively close at high-temperature. In this study, oxide ion conduction paths of these two systems, Pb1-xLaxWO4+x/2 and Ca1-xKxWO4-x/2, are evaluated by means of high-temperature neutron diffraction experiments coupled with maximum entropy method and compared based on the defect structure. Experimental Pb1-xLaxWO4+x/2 and Ca1-xKxWO4-x/2 (x = 0.2 for both systems) were prepared by solid state reaction method from the starting oxides. Calcining and sintering temperatures are selected as 800oC and 900oC, respectively for Pb1-xLaxWO4+x/2, and 800oC and 1000oC for Ca1-xKxWO4-x/2. The phase singularities of the obtained compounds are confirmed by X-ray diffraction. About 10 g of the sample pellets were inserted into a vanadium holder through a thin quartz tube and neutron powder diffraction experiments were performed in the temperature range between 200°C and 800°C by using SHRPD diffractometer in J-PARC. After the Rietveld refinements using Z-Rietveld code, oxide ion conduction paths at high temperatures were estimated by using maximum entropy method (MEM) using Z-MEM. Typically, 50 reflections are served for the MEM analyses. Results and Discussions Figs.1 and 2 represent the nuclear density distribution for Pb1-xLaxWO4+x/2 and Ca1-xKxWO4-x/2, respectively, obtained by MEM based on the neutron diffraction data collected at 800oC. Regular atomic positions obtained by the Rietveld analysis are indicated by spheres in the figures. For Pb1-xLaxWO4+x/2, nuclear density seems to extend from regular site to interstitial ones, suggesting the oxide ion diffusion path occurs parallel to the a-b plane. On the other hand, Ca1-xKxWO4-x/2 without oxide ion interstitials did not show such diffusion path parallel to the a-b plane, but the neutron density distribution extends toward c-directions. Defect structure would largely attribute to the diffusion mechanism in these two scheelite systems.

Six-axis multi-anvil press for high-pressure, high-temperature neutron diffraction experiments.

We developed a six-axis multi-anvil press, ATSUHIME, for high-pressure and high-temperature in situ time-of-flight neutron powder diffraction experiments. The press has six orthogonally oriented hydraulic rams that operate individually to compress a cubic sample assembly. Experiments indicate that the press can generate pressures up to 9.3 GPa and temperatures up to 2000 K using a 6-6-type cell assembly, with available sample volume of about 50 mm(3). Using a 6-8-type cell assembly, the available conditions expand to 16 GPa and 1273 K. Because the six-axis press has no guide blocks, there is sufficient space around the sample to use the aperture for diffraction and place an incident slit, radial collimators, and a neutron imaging camera close to the sample. Combination of the six-axis press and the collimation devices realized high-quality diffraction pattern with no contamination from the heater or the sample container surrounding the sample. This press constitutes a new tool for using neutron diffraction to study the structures of crystals and liquids under high pressures and temperatures.

Thermal equations of state and melting of lithium deuteride under high pressure

Based on in situ high-pressure and high-temperature neutron diffraction experiments at pressures of up to 4.1 GPa and temperatures of up to 1280 K, thermoelastic parameters were derived by using a Birch–Murnaghan equation of state. With the pressure derivative of the bulk modulus, K0′, fixed at 4.0, we obtained the ambient bulk modulus K0=31.5±0.7 GPa, the temperature derivative of bulk modulus at constant pressure (∂K/∂T)P=−2.7×10−2 GPa/K, the volumetric thermal expansivities αT(K−1)=9.8±0.71×10−5+12.62±1.09×10−8T at atmospheric pressure and αT(K−1)=5.45±1.17×10−5+6.53±1.45×10−8T at 3.0 GPa, and the pressure derivative of thermal expansion (∂α/∂P)T=−2.72×10−5 GPa−1 K−1. Within the experimental uncertainties, the ambient bulk modulus and volumetric thermal expansion determined from this work are in good agreement with previous experimental results, whereas the derived (∂KT/∂T)P and (∂α/∂P)T values provide the thermoelastic equation-of-state parameters for LiD. We also determined the melting temperature of LiD under high pressure. Our results reveal a substantially increased thermal stability for crystalline LiD when compared to a previous theoretical prediction that used a combined technique of two-phase simulation and first-principles molecular dynamics.

Neutron Powder Diffraction Studies of Order-Disorder Phase Transitions and Kinetics

One of the major applications of neutron diffraction in mineral sciences has been in the study of order-disorder processes and phase transitions. Neutron scattering methods provide unique insights into the origins and mechanisms of these processes, and have enabled mineralogists to develop new models of phase transformation behavior. The time-temperature dependence of processes such as phase transformations, exsolution, cation ordering and disordering in minerals has considerable potential geophysical, geochemical and petrological importance. Let’s take atomic ordering as an example. Order-disorder transformations and similar structural phase transitions are typically some of the most efficient ways a mineral can adapt to changing temperature or chemical composition. Disorder of distinct species across different crystallographic sites at high temperature provides significant entropic stabilization of mineral phases relative to low-temperature ordered structures. Positional or orientational disordering can have similar drastic effects. For example, the calcite-aragonite phase boundary shows a significant curvature at high temperature due to disorder of CO3 groups within the calcite structure, associated with an orientation order-disorder phase transition (Redfern et al. 1989). This leads to an increased stability of calcite with respect to aragonite over that predicted by a simple Clausius-Clapeyron extrapolation of the low pressure-temperature thermochemical data. Understanding of the structural characteristics of the phase transition in calcite developed in tandem with studies of the analogous transition in nitratine, NaNO3, but it was not until Dove and Powell (1989) carried out high-temperature neutron diffraction experiments on powdered calcite that there was direct experimental evidence linking the thermodynamic and structural nature of the transition in CaCO3. The key to the success of their study was the fact that neutrons penetrate the entire volume of samples held at extreme conditions (in this case very close to the melting temperature and under a confining CO2 pressure). Cation ordering …

Temperature Dependence of Magnetic Moment Orientation in Co2Z-Type Hexaferrite Estimated by High-Temperature Neutron Diffraction

We investigated the correlation between the thermomagnetic curve of Co2Z-type hexagonal barium ferrite and its magnetic moment direction. We measured the thermomagnetic curve of Ba3Co1.8Fe24.2O41, prepared using the conventional solid-state reaction method, in the temperature range from 294 to 773 K with a vibrating sample magnetometer under 70 Oe. The curve shows two significant magnetization slumps at 540 K and 680 K. High-temperature XRD patterns show that no crystal transformation occurs in the temperature region from 294 to 773 K. High-temperature neutron diffraction experiments were performed to investigate the magnetic moment orientation at elevated temperatures. The Rietveld analyses of the neutron diffraction patterns indicate that the temperature rise from 523 to 573 K makes the magnetic moments turn to the c-axis from a direction parallel to the c-plane most significantly. The slump in magnetization at 540 K may be attributed to the change in easy magnetization direction from the c-plane to the c-axis. The change in average orientation of the magnetic moments must be induced by the disappearance of the contribution of cobalt to magnetism in this temperature range.

Neutron Diffraction Studies of Si-Mn Trip Steel In Situ upon Thermomechanical Processing

An excellent combination of high strength and formability can be obtained in a Si-Mn TRIP steels when processed by thermomechanical treatment consisting of high temperature deformation followed by isothermal holding in the bainite region and cooling to room temperature. Microstructure of these steels consists of ferrite, bainite and retained austenite. The combination of high strength and ductility is attributed to the transformation-induced plasticity (TRIP) effect resulting from the strain induced transformation of the retained austenite to martensite. These steels exhibit tensile strengths ranging 800-1000 MPa and elongation up to 30%. In the present paper, the influence of transformation conditions on volume fraction of ferrite and retained austenite in selected 0.2C1.9Si1.45Mn TRIP steel was investigated. Based on results of in situ high-temperature neutron diffraction experiments, the set of specimens was prepared and their transformation characteristics at room temperature were studied again by neu...

Premelting transition in uranium dioxide

Thermal analysis of the cooling curves of small samples of UO2±x —initially laser heated (in a high-pressure autoclave to inhibit evaporation) on a subsecond time scale to temperatures just below their melting points [T m(x)]—reveals, in the case of nominally stoichiometric UO2.00, a significant, λ-like, heat capacity [C p(T)] peak near 2670 K; the cooling curves of samples exposed to a reducing environment, on the other hand, exhibit undercooling, characteristic of a first-order phase transition, while under oxidizing conditions it is found that the premelting transition readily disappears. These findings confirm Bredig's original prediction of a premelting transition in this material, in common with that found in other (nonactinide) fluorites near 0.85T m. A simple model is presented in terms of which the observed behavior can be rationalized. The model is based on the hypothesis that the premelting transition is due to Frenkel disordering of the oxygen sublattice—a process which is rendered cooperative by attractive interactions between complementary Frenkel defects (oxygen interstitials and vacancies); these interactions are treated in a “mean-field” approximation. The quantitative degree of maximum disorder (realized just above the transition) is, on the other hand, controlled by repulsive interactions between like defects—the inclusion of which, solely through their effect on the configurational entropy, satisfactorily reproduces the values inferred from recent high-temperature neutron diffraction experiments. Assuming that the phase transition in stoichiometric UO2.00 is of second order, the model predicts a divergent heat capacity, C v, which approximates well to the experimental (λ-like) C p peak. Crucial to reproducing the observed behavior away from stoichiometry is the introduction of a (linear) dependence of the nonconfigurational partial entropy of formation on the prevailing concentration of intrinsic Frenkel defects in UO2±x; interestingly, it is found that the line of calculated (but unrealized) second-order transitions in UO2+x intersects the U4O9 phase boundary near to where a high-temperature diffuse order-disorder transition has been observed in the oxygen superlattice, suggesting that the second-order, λ-transition in UO2.00 is the stoichiometric counterpart of this transition in U4O9.

Oxidation enthalpy of YBa2Cu3O6+y in relation with phase behaviour and stoichiometry

Abstract Oxidation enthalpies for YBa2Cu3O6+y are derived from continuous equilibrium partial oxygen pressure curves, obtained with the constant-stoichiometry cooling technique in the temperature range of 350–950°C. Accurate stoichiometry data for high-purity single-phase YBa2Cu3O6+y samples in equilibrium with atmosphere O2 were redetermined by thermogravimetric and neutron diffraction techniques. Phase behaviour was checked by means of high-temperature neutron diffraction experiments. Structural data for YBa2Cu3O6+y at 800°C in atmospheric O2 are compared with those for a sample with the same stoichiometry at room temperature. It is found that the oxidation enthalpy of orthorhombic YBa2Cu3O6+y is independent of stoichiometry and temperature. For tetragonal YBa2Cu3O6+y a slight temperature dependence of the oxidation enthalpy is found, which is probably connected with a gradual ordering of oxygen atoms at decreasing temperatures. The oxidation enthalpy data in their relation with stoichiometry, temperature and phase behaviour give support for a localized description of the charge distribution in YBa2Cu3O6+y.

High temperature neutron diffraction study of the order-disorder transition in TiC 1− x

High temperature neutron diffraction experiments on TiC 1− x samples have allowed to determine from the thermal dependence of the (1/2 1/2 1/2), (3/2 1/2 1/2) and (3/2 3/2 1/2) superlattice reflexions the transition temperatures due to long-range ordering of carbon vacancies: 765° ± 5° C for TiC 0.58. 785° ± 5° C for TiC 0.63 and 770° ± 5° C for TiC 0.67. For the first composition, the transition is practically of second order type with a critical coefficient β = 0.26 ± 0.03, suggesting predominant short-range interactions between carbons and vacancies. A preliminary study of the kinetics of longrange ordering was performed between 650° and 750°C for TiC 0.63: at 735°C, the average ordered domain size D increases with time t as D 3.24 − D 3024 0 ∼- Ct ; the process is thermally activated with an apparent activation energy of 3.7± 0.2 eV. The kinetics of ordering decrease rapidly with increasing carbon content. The results are discussed in terms of recent theories of order-disorder and antiphase domain coarsening in alloys.

High temperature neutron diffraction study of Fe1-xO

Accurate high temperature neutron diffraction experiments have been carried out between 800 degrees C and 1200 degrees C on polycrystalline Fe1-xO at compositions between Fe0.947O and Fe0.906O. The ratio of octahedral iron vacancies to tetrahedral iron interstitials is close to the value of 3.25 which would be expected if Koch-Cohen clusters persist in the high temperature equilibrium region.