Four-circle Neutron Diffractometer Research Articles

Neutron-diffraction study of the cubic-tetragonal phase structural transition in the single crystals of the solid solutions of zirconium and yttrium oxides

Objectives. The determination of the phase relations, crystallographic characteristics, microstructure features, and atomic crystal structure of zirconium oxide crystals that are partially and completely stabilized by yttrium oxide additives, and the identification of the crystallographic and crystal-chemical correlations with the physicochemical properties of single crystals.Methods. The neutron structure of the crystals was studied using the neutron time-of-flight and constant wavelength methods using a high-resolution Fourier diffractometer on the IBR-2 pulsed fast reactor and a four-circle neutron diffractometer “Syntex.” Single crystals were grown by directed crystallization from the melts of mixtures (1 − х)ZrO2 ∙хY2O3 , х = 0.03 and х = 0.12 with different growth rates (10 and 40 mm/h).Results. It was observed that when growing single crystals with x = 0.03–0.05, the crystal was stratified into cubic and tetragonal phases, and the ratio between the phases depended on the growth rate. At a growth rate of 40 mm/h, the content of the cubic phase was insignificant. In the crystals of partially stabilized zirconium dioxide (ZrO2) with the additions of 3 mol % Y2O3, the coherent coexistence of cubic and tetragonal phases was established, and the twin law for a tetragonal component (rotation of unit cell axis by 90° around the a (b) axis) that was observed during the phase transition from high-temperature cubic phase to tetragonal phase was determined. For the fully stabilized zirconium oxide of the cubic symmetry (with 12 mol % Y2O3), the 0.3 Å displacements of oxygen atoms from their partial structural positions in the directions [100] and [111] were determined. These displacements correlated with the directions of the ion transport.Conclusions. Previous studies have shown that the ratio between the cubic and tetragonal phases of the single crystals of the ZrO2 –Y2O3 system depends on the growth rate of the single crystals. The content of Y2O3 in the cubic and tetragonal phases of a single crystal was determined using the non-destructive neutronography method on the same volume sample of a solid solution of this system. Moreover, the displacements of oxygen atoms from the main position of the crystal were determined.

Four-circle single-crystal neutron diffractometer at the High Flux Isotope Reactor

A four-circle neutron diffractometer with a new multi-wafer 331 Si monochromator has been installed and commissioned on a thermal beamline at the High Flux Isotope Reactor at Oak Ridge National Laboratory. The instrument is well suited to studies of nuclear and magnetic structures as a function of composition and temperature, resolving symmetry changes (lattice distortions and local structural changes), mapping the evolution of complex magnetic phases, determining hydrogen bonding, analyzing nuclear and spin densities, mapping diffuse scattering, and exploring fiber diffraction. Three incident wavelengths are available, 1.000, 1.536 and 2.540 Å, with intensities of 2.5 × 106, 2.2 × 107and 8.0 × 106 neutrons cm−2 s−1, respectively. Either high-resolution or high-intensity modes are possible by horizontal bending of the monochromator. With increased bending of the monochromator, the incident flux on the sample passes through a maximum, increasing by ×2.0 for 1.000 Å, by ×3.5 for 1.536 Å and by ×3.5 for 2.540 Å, as compared to the flat condition. The flux increases because the lattice strain in the silicon crystals increases. The ω-scan peak width increases with monochromator curvature and this widthversusscattering angle flattens. Given these effects, the monochromator bending can be adjusted to deliver high intensity primarily for crystal structure refinements or high resolution for resolving symmetry changes. In addition to the traditional step-scanning mode, a more efficient continuous-scanning mode was developed, and both these are implemented through aLabView-based control program,i.e.a modified version of theSPICEsoftware package. A 4 K closed-cycle helium refrigerator is permanently mounted on the χ-circle of the goniometer to provide temperature control between 4 and 450 K.

A uniaxial pressure cell for neutron diffraction investigation and its use in studying the single-crystallineSr3Ru2O7 compound

The design and fundamental properties of a uniaxial pressure cell for use with a four-circleneutron diffractometer are reported. It is aimed at facilitating studies on the physical and/orstructural properties of solids. The results obtained for a uniaxial-pressure-induced ferromagnetSr3Ru2O7 showed that the unit cell volume increases upon applying uniaxial pressure along thec-axis.

Reinvestigation of Magnetic Structures for the Thermally Induced States of CuFe1-xAlxO2(x=0.00, 0.02 and 0.05) Using a Four-Circle Neutron Diffractometer

We have reinvestigated the effect of non-magnetic impurity on thermally-induced magnetic orderings in geometrically frustrated triangular lattice antiferromagnets CuFe 1- x Al x O 2 with x =0.00, 0.02 and 0.05, using a four-circle neutron diffractometer. Although we inferred in our previous work with a three-axis diffractometer that a proper screw type magnetic structure is realized in the intermediate-temperature (IM) phase of the samples with x =0.02 and x =0.05, in the present work we found that a sinusoidally amplitude-modulated magnetic structure with the magnetic moments canted by about 50° from the c axis toward the hexagonal direction is realized. These experimental findings suggest that the quasi-Ising magnetic orderings in CuFeO 2 are stabilized not with the strong Ising anisotropy expected from the Ising-like successive magnetic phase transitions in CuFeO 2 but with the small Ising anisotropy added to strongly competing exchange interactions.

特集 中性子散乱による最近の研究 高１軸応力下のマルテンサイト相のその場観察

Cu-Al-Ni alloys with composition close to Cu3Al show pseudoelasticity associated with transformations among four martensitic phases, γ1', β1' β1'', α1'. The crystallographic information was obtained by neutron diffraction techniques using the four-circle neutron diffractometer at Researcch Reactor Institute of Kyoto University, KUR, Japan. A small tensile jig with a load cell and an extensometer was attached for generating high uniaxial stress. Quadrant slit devices were inserted into the neutron beam path for accurate measurement of diffracting conditions. Strains as second rank tensors were described as a function of hysteresis and stress between 50 and 460 MPa.

A low-temperature attachment for a four-circle neutron diffractometer

A single-stage closed-cycle helium gas cryogenerator, capable of cooling between 77 K and 300 K, has been installed on the 4-circle neutron diffractometer at CIRUS. The cryo-tip assembly has been incorporated directly on the ϕ-circle of the full-circle crystal orienter in place of the goniometer head used in room temperature experiments. The salient features of this low-temperature attachment and its performance are discussed.

ATOMIC POSITIONS OF DEUTERIUM IN DTGS CRYSTAL DETERMINED BY NEUTRON DIFFRACTION

The crystal structure of the partly deuterized triglycine sulfate was studied by four-circle neutron diffractometer. The result is compared with TG-S data obtained by Kay et al. The distance between triglyeines groups increases hlightly by deuterization, The relative increases of lattice parameters a, b and c are 0.47%, 0.50%, 0.19%, respectively. The probability of deuterium substitution, P, on various hydrogen sites is obviously divided into two classes; P=0 for hydrogens locked in C-H bonds, while P= 0.84±0.03 for all the others.

Neutron diffraction structure determination of the high-temperature form of lithium tritantalate, H [sbnd]LiTa 3O 8

The crystal structure of H LiTa 3O 8 has been reexamined by electron and neutron diffraction techniques. Neutron Weissenberg and electron diffraction photographs show that the space group of the compound is Pmmn and not Pmma as determined previously by X-ray diffraction techniques. There are eight molecules in the unit cell of lattice parameters a = 16.718(2)A˚, b = 7.696(1)A˚, c = 8.931(1)A˚. These values show that the baxis of the new cell is doubled with respect to the parameter measured by X-rays. The structural refinement was based on 1074 independent reflections measured on a single crystal with a four-circle neutron diffractometer. The positions of all atoms, including the lithium atoms have been determined. The final R and wRfactors were 0.036 and 0.035, respectively. The eight lithium cations occupy two sets of4 fpositions (x,¼, z)of the Pmmn space group. The ordering of four lithium ions over two sets of possible positions (4 j) of space group Pmma is responsible for the doubling of the b axis. The other four Li + occupy two sets of positions (2 d) of space group Pmma. All lithium ions are surrounded by 12 oxygen atoms arranged as cuboctahedra. The large thermal vibrations found for the lithium atoms and the ionic conductivity of H LiTa 3O 8 at high temperatures are consistent with weak Li O bonding.

Lithium ordering in H [sbnd]LiTa 3O 8

It has been found by electron and neutron diffraction techniques that the b axis of the orthorhombic unit cell of H LiTa 3O 8, determined by X-ray difraction, must be doubled in order to expalin all the reflections. The new space group is Pmmn with eight molecules per unit cell of dimensions :a = 16.718, b = 7.696, c = 8.931 A. A structural refinement, based on reflections measured by a four-circle neutron diffractometer on a single crystal, gave the position of all atoms, including the lithium ones. The ordering of the latter atoms is responsible for the doubling of the b axis. They do not contribute much to the stability of the structure, but are mainly needed to balance the electrostatic charge. The weak Li O bonding is responsible for the large thermal vibrations of the lithium atoms and for the ionic conductivity of H LiTa 3O 8 at higher temperatures.

Use of a single-stage, closed-cycle cryorefrigerator for routine measurements at moderately low temperatures on a four-circle neutron diffractometer

A single-stage closed-cycle helium cryorefrigerator has been installed on a conventional four-circle neutron diffractometer. The cold head together with a specially designed support and alignment unit are sufficiently compact to enable it to be mounted directly onto the tp rotation table within the X circle in place of the normal goniometer head used for room-temperature experiments. The device has a temperature range of 50 to 300 K, is extremely simple to use, and has a low operation cost as no cryogenic liquids are consumed. There are additional restrictions on the diffractometer angles but, with care, it is possible to measure at least half the reciprocal sphere.

Neutron structure refinement of orthorhombic hexachloroethane

The structure of orthorhombic hexachloroethane has been refined at 294 and 140 K. At room temperature 983 symmetry-independent reflections were measured on a four-circle neutron diffractometer. The final R(F) was 0..066 including an isotropic extinction parameter. At 140 K 793 symmetry-independent reflections refined to R(F) = 0-056. The temperature parameters have been analyzed in terms of the tensors T, L and S describing the rigid-body movement of the C2C16 molecule. The temperature variation of the T and L tensors can be understood with the assumption of strong anharmonic contributions. There is, however, no indication of rotational disorder in the orthorhombic phase.

A new algorithm for rapid and precise centering of reflections in computer-controlled neutron diffractometers

A rapid and unique method for centering reflections on a computer-controlled, four-circle neutron diffractometer is presented. The method is based on numeric differentiation using the three-point form of Stirling's interpolation formula and its most outstanding features are the speed of operation and the high precision of the derived diffractometer coordinates.

An on-line TDC-312 computer-controlled neutron diffractometer

The design and fabrication of an indigenous on-line computer controlled four-circle neutron diffractometer at the CIRUS reactor in Trombay are described. The diffractometer has an 18 in dia full-circle crystal-orienter which is sturdy enough to carry a cryostat. Three crystal orientation angles χ, φ and ω and the detector angle 2θ can be set to an accuracy of 0·01°. The four angle shafts are driven through precision worm-gears by SCR-controlled DC motors and their instantaneous positions sensed by optical digitizers. The diffractometer is interfaced to an ECIL TDC-312 computer system consisting of the CPU with 4K-memory, ASR-35 teletype, X-Y plotter and the digital input/output system (DIOS). The DIOS which operates under program control is a real-time peripheral device used to exchange information in digital form between the computer and the diffractometer. A software package consisting of over 40 useroriented teletype commands has been developed for on-line control and automatic data-acquisition.

Helium temperatures in a four-circle neutron diffractometer

The cooling of single-crystal samples to helium temperatures within a four-circle neutron diffractometer has been achieved by a novel continuous-flow system. The system relies solely on helium for its cooling power with two flow circuits being used. The first delivers liquid to a reservoir cryostat of capacity 1–4 l located close to the Eulerian cradle of the diffractometer and the second draws the refrigerant from this cryostat via a flexible transfer tube into the sample region of a compact cooling unit mounted off the cradle. This arrangement allows unhindered motion of the diffractometer circles for sample temperature operation in the range 3.9–300 K. The system requires a total helium flow of 1.0 l h−1 to maintain a temperature of 20±0.1 K, decreasing to 0.5 l h−1 at higher temperatures.

Crystal structure refinement of ferroelectric rochelle salt

The crystal structure of Rochelle salt has been refined at 0°C, where the crystal is ferroelectric. Intensity data were collected on a fully automatic four-circle neutron diffractometer. An external dc field of 90 V was applied along the ferroelectric a-axis of the specimen during the course of measurement. The R-factor of 0.102 were obtained.

A computer-controlled neutron diffractometer

A computer-controlled four-circle neutron diffractometer is described. The axes are driven by pulsed stepping motors and all operations are under on-line control of a PDP–8 computer. Details of preliminary operating experience are given.

Geometrical properties of a four-circle neutron diffractometer for measuring intensities at an `optimum' azimuth of the reflecting planes

Geometrical properties of a four-circle neutron diffractometer for measuring intensities at an `optimum' azimuth of the reflecting planes