T. Hansen

The D20 instrument at the ILL: a versatile high-intensity two-axis neutron diffractometer

D20 is a medium to high resolution two-axis diffractometer capable of producing a neutron flux of 108 s−1 cm−2 at the sample position. The 1536 detection cells of its curved linear position sensitive detector (PSD) cover a continuous 2θ range of 153.6° over a total solid angle of 0.27 sr. This combination of a high incident neutron flux and a large detector solid angle provides D20 with the fastest counting rate, at a given resolution, of any reactor-based neutron diffractometer. Different monochromators and take-off angles, plus optional Soller collimators and secondary slits, permit a wide choice in the Q-space range, wavelength, resolution and flux. A high-resolution configuration offers Δd/d ~ 2 × 10−3. Fast modern counting electronics allow in situ time-resolved experiments at the timescale of a few tens of milliseconds. In addition, a variety of sample environments, including an optional radial oscillating collimator for suppressing parasitic scattering, contribute to a rich scientific programme.

Electromagnetic levitation apparatus for diffraction investigations on the short-range order of undercooled metallic melts

One of the most fundamental questions in the research field of undercooled melts is concerned with the structural short-range order of melts, because the short-range order decisively influences the physical properties and the solidification behaviour of undercooled liquids. Following the pioneering work of Frank in 1952, an icosahedral short-range order should be energetically favoured in undercooled metallic melts. Although this hypothesis is nearly 50 years old, direct experimental information on the short-range order prevailing in undercooled metallic liquids has only been available for a short time. This is mainly due to the fact that undercooled melts are a metastable state of matter which is difficult to conserve for times long enough to perform diffraction experiments of good quality. In this paper an apparatus is described that allows us to investigate the short-range order of undercooled metallic melts by combining the undercooling technique of electromagnetic levitation with the diffraction techniques of elastic neutron scattering and energy dispersive x-ray diffraction (EDXD) with synchrotron radiation. The diffraction experiments were performed with this device at the Institut Laue-Langevin (ILL) and at the European Synchrotron Radiation Facility (ESRF). These experiments provided the first direct experimental proof of an icosahedral short-range order prevailing in a great variety of undercooled melts of pure metals and alloys forming quasicrystalline or polytetrahedral phases. The icosahedral short-range order exists already at temperatures, T, above the melting temperature, TL, and becomes more pronounced if the melts are undercooled.

Operation of sealed microstrip gas chambers at the ILL

Abstract Microstrip Gas Counters (MSGCs) were introduced at the ILL as a response to the problem of fabricating the large area neutron detector of the D20 neutron powder diffractometer. This banana-like detector consists of 48 MSGCs, each comprising 32 counting cells. It was in operation during 18 months before being stopped due to the progressive deterioration of the anode strips. In order to increase its lifetime, significant modifications were introduced in the recently assembled new version. Another instrument, D4C, was recently equipped with a modular detector made of nine MSGCs, each of them in an individual gas vessel. Besides the unidimensional individual readout MSGC of D20 and D4C, the ILL has developed bidimensional MSGCs with a charge division readout. All these detectors employ sealed vessels containing a gas mixture at a pressure which can be as high as 15xa0bar, necessitating very clean conditions. This paper describes the experience acquired at the ILL in the fabrication and operation of these detectors.

D20 high-flux two-axis neutron diffractometer

Abstract The monochromatic beam of D20 has a high flux of up to 6 x 107 n cm−2 s−1. The two monochromators, HOPG (0 0 2) plus graphite filters and Cu (2 0 0), are vertically focusing and give wavelengths from λ = 0.8 to 2.5 A . The position-sensitive detector (PSD) has an aperture of 160° with 10 cells per degree (2θ) with ‘microstrip’ detection electrodes. The 1600 cells are connected to 1600 amplifiers, followed by anti-coincidence logics and a fast data acquisition system (DAS) with parallel input. This parallelism allows counting rates up to 50 000 s−1 per cell. High flux and the stationary large PSD permit numerous short-time measurements as used for investigation of phase transitions (thermodiffractometry), kinetic studies, etc. The DAS allows stroboscopic measurements. The time resolution is about 15–40 μs. Due to the high counting rate, a high precision of intensity is obtained by long measurements as used for studying disordered systems and physisorption.

Texture Measurements of Hydroxyapatite Crystallites at Bone-Implant Interfaces in Sheep Tibia

The preferred orientation of hydroxyapatite Ca10(PO4)6(OH)2 (HAp) crystallites at the interface bone-implant in sheep tibia bones has been measured with the neutron 2-axis diffractometer D20 at the Institut Max von Laue-Paul Langevin, extracted 60 days after implantation. The implant has two faces, one coated and one non-coated with plasma-sprayed HAp (80 µm). We probed the samples with a spatial resolution of 0.5 mm started from the interface in order to inspect the reorganisation of the HAp crystallite distribution after implantation.

Phase stability of multiferroic GaFeO3 up to 1368 K from in situ neutron diffraction

We report a detailed high-temperature powder neutron diffraction investigation of the structural behavior of the multiferroic GaFeO3 between 296 and 1368u2009K. Temperature dependent neutron diffraction patterns do not show any appreciable change either in intensity or appearance/disappearance of the observed peaks up to 1368u2009K, ruling out any structural transition in the entire temperature range. Evolution of the distortion of the oxygen polyhedra around Ga1, Ga2, Fe1, and Fe2 cations sites suggest that the Ga1-O tetrahedron is least distorted and Fe1-O is most distorted. Structural features regarding the distortion of polyhedral units would be crucial to understand the temperature dependence of the microscopic origin of polarizations. The electric polarization has been estimated using a simple ionic model and its value is found to decrease with increasing temperature.

Structural aspects of glass-formation in Ni-Nb melts

We report on investigations of the static structure factors of glass-forming Ni59.5Nb40.5 alloy melts by combination of the containerless processing technique of electrostatic levitation with neutron diffraction. By application of the isotopic substitution method, the full set of partial structure factors was determined. The short-range order in liquid Ni59.5Nb40.5 is characterized by a large nearest neighbor coordination number of Z(NN) = 14.3 and a chemical short-range order with an affinity for the formation of heterogeneous Nb-Ni nearest neighbors. The structure factors observed here in the liquid state closely resemble those reported for amorphous Nb-Ni solids. The comparison with earlier results on the short-range structure in Zr-based glass-forming melts suggests that a large local density of packing, chemical order, and structural frustration are, amongst others, common structural properties of these metallic glass-forming systems, which favor glass-formation

Helical order and multiferroicity in the S = 1/2 quasi-kagome system KCu3As2O7(OD)3

Several Cu2+ hydroxide minerals have been recently identified as candidate realizations of the S=1/2 kagome Heisenberg model. In this context, we have studied the distorted system KCu3As2O7(OD)3 using neutron scattering and bulk measurements. Although the distortion favors magnetic order over a spin liquid ground state, refinement of the magnetic diffraction pattern below TN1=7.05(5) K yields a complex helical structure with k=(0.77,0,0.11). This structure, as well as the spin excitation spectrum, are well described by a classical Heisenberg model with ferromagnetic nearest neighbor couplings. Multiferroicity is observed below TN1, with an unusual crossover between improper and pseudoproper behavior occurring at TN2=5.5 K. The polarization at T=2 K is P=1.5μCm−2. The properties of KCu3As2O7(OD)3 highlight the variety of physics which arise from the interplay of spin and orbital degrees of freedom in Cu2+ kagome systems.

Neutron diffraction on mercury : Density dependence of the static structure factor

We report on a new neutron diffraction determination of the static structure factor of liquid mercury at room temperature. By applying pressure on the sample up to 2 kbar (giving a density change of about 0.8%), it has been possible to measure, for the first time for mercury, the isothermal density derivative of the static structure factor. This quantity, in the case of simple insulating fluids, has already been shown to be more sensitive than the structure factor itself to the details of the interatomic potential.

New insights into the compressibility and high-pressure stability of Ni(CN)2: a combined study of neutron diffraction, Raman spectroscopy, and inelastic neutron scattering.

Nickel cyanide is a layered material showing markedly anisotropic behaviour. High-pressure neutron diffraction measurements show that at pressures up to 20.1 kbar, compressibility is much higher in the direction perpendicular to the layers, c, than in the plane of the strongly chemically bonded metal-cyanide sheets. Detailed examination of the behaviour of the tetragonal lattice parameters, a and c, as a function of pressure reveal regions in which large changes in slope occur, for example, in c(P) at 1 kbar. The experimental pressure dependence of the volume data is fitted to a bulk modulus, B0, of 1050 (20) kbar over the pressure range 0-1 kbar, and to 124 (2) kbar over the range 1-20.1 kbar. Raman spectroscopy measurements yield additional information on how the structure and bonding in the Ni(CN)2 layers change with pressure and show that a phase change occurs at about 1 kbar. The new high-pressure phase, (Phase PII), has ordered cyanide groups with sheets of D4h symmetry containing Ni(CN)4 and Ni(NC)4 groups. The Raman spectrum of phase PII closely resembles that of the related layered compound, Cu1/2Ni1/2(CN)2, which has previously been shown to contain ordered C≡N groups. The phase change, PI to PII, is also observed in inelastic neutron scattering studies which show significant changes occurring in the phonon spectra as the pressure is raised from 0.3 to 1.5 kbar. These changes reflect the large reduction in the interlayer spacing which occurs as Phase PI transforms to Phase PII and the consequent increase in difficulty for out-of-plane atomic motions. Unlike other cyanide materials e.g. Zn(CN)2 and Ag3Co(CN)6, which show an amorphization and/or a decomposition at much lower pressures (~100 kbar), Ni(CN)2 can be recovered after pressurising to 200 kbar, albeit in a more ordered form.