Volume 218, Issue 2 - Neutron Diffraction in Structure Research

An overview is given over recent developments and trends in single-crystal neutron diffractometry. Special emphasis is put on instrumental developments which have considerably enhanced the efficiency of neutron scattering experiments over the last years by improving monochromator and/or detector systems. Not discussed are developments in the field of polarised neutrons. Examples from the Berlin Neutron Scattering Center (BENSC) are given for the measurement of short-range magnetic order using the flat-cone diffractometer E2 of BENSC and of the study of long-range magnetic order making use of a high-field cryomagnet at BENSC. The perspectives of neutron diffractometry within the next 5 to 10 years are discussed with view on the new powerful spallation sources SNS (under construction in the United States), JSNS (under construction in Japan) and ESS, the European Spallation Source, planned to be constructed in Europe.

Sample environment equipment, cryostats, furnaces, pressure cells etc. are an essential adjunct to most neutron scattering experiments to induce the sample being studied into a phase or state of particular interest. He we give a brief overview of the diverse range of Sample Environment equipment that is currently in use, a background to its development and also some recent trends.

Neutron diffraction provides an experimental method of directly locating hydrogen atoms in proteins, and the development of the neutron imaging plate (NIP) became a breakthrough event in neutron protein crystallography. The general features of the NIP are reviewed. A high resolution neutron diffractometer dedicated to biological macromolecules (BIX-3) with the NIP has been constructed at Japan Atomic Energy Research Institute and this has enabled 1.5 Å resolution structural analyses of several proteins to be carried out. The specifications of BIX-3 and LADI (a quasi-Laue type diffractometer installed in the Institut Laue-Langevin) are compared. The crystal structures of myoglobin, wild type rubredoxin and a mutant of rubredoxin have been carried out using BIX-3. From these studies, several topics, such as the location of hydrogen bonds and certain acidic hydrogen atoms, the identification of methyl hydrogen atoms, details of H/D exchange and dynamical behavior of hydration structures have been investigated, and important information has been extracted from the structural results. Finally, a systematic procedure to grow large single crystals of proteins or nucleic acids is described.

Solid-state metal hydrides display hydrogen densities close to that of liquid hydrogen and thus provide a safe and efficient way of storing hydrogen. As a result of recent neutron and synchrotron diffraction work some novel metal hydrides have been characterized that shed new light on the nature of metal-hydrogen interactions. While hydrogen appears as an anion surrounded by a large inventory of cation configurations in ionic hydrides such as Ca 4 Mg 3 H 14 , Ca 19 Mg 8 H 54 , Eu 2 MgD 6 , Eu 6 Mg 7 D 26 and Eu 2 Mg 3 D 10 , it acts as a terminal ligand in covalently bonded hydride complexes based on p -elements such as [BH 4 ] – and d -elements such as [IrH 5 ] 4– and [IrH 4 ] 5– in the complex hydrides LiBH 4 and Mg 6 Ir 2 H 11 , respectively. Surprisingly, hydride complexes and hydride anions can also be discerned in typically metallic (interstitial) hydrides such as NdMgNi 4 H 4 (= Nd 3+ Mg +2 ·[Ni 4 H 4 ] 5– ) and LaMg 2 NiD 7 (= La 3+ Mg +2 2 ·[NiH 4 ] 4– ·3H – ). Some hydrides also reveal other interesting features such as a hydrogenation induced Ce 4+ → Ce 3+ valence change in CeMn 1.8 Al 0.2 H 4.4 at room temperature that is accompanied by a Mn/Al metal atom exchange over distances of ~2.6 Å, and a hydrogen induced metal-to-nonmetal transition near ambient conditions that leads from the metallic compound Mg 3 Ir to the red colored hydride Mg 6 Ir 2 H 11 . In this article recent work and some methodological aspects are highlighted.

We have studied by in-situ neutron diffraction the recrystallisation behaviour of HDA ice in the pressure range 0.3–3.9 GPa, i.e. the entire stability range of HDA. We report quantitative and detailed structural information on the various high pressure ice phases formed metastably at low temperatures. We find that between ~0.4–0.7 GPa, HDA transforms at 175 K to mainly phases IV and V, and XII, and at 1–1.2 GPa to a mixture of ice VI and XII. On isothermal compression at 100 K, HDA recrystallises to an ice VII-like structure which is either partially ordered or mixed with ice VIII. Full structural data obtained by Rietveld refinements are reported for all these phases at low temperatures. Phases IV, V and XII are fully hydrogen disordered when obtained from HDA. The transformation behaviour in the 0.5–1.2 GPa range is in good agreement with the picture reported from quenched-recovered samples, although some differences persist in the recrystallisation to IV/XII mixtures. The recrystallisation behaviour of HDA over the entire pressure range appears to follow closely that of liquid water under pressure.

Atomic probability densities as derived from a careful analysis of the Debye-Waller factors contain important information about the average static and dynamic disorder in crystals. Although the inclusion of anharmonic terms often requires single crystal data, in favourable cases, in particular at high temperatures, it is possible or even necessary to rely on neutron powder experiments too. This is illustrated by several examples including the anharmonic motion of the atoms in ZnS, the anionic diffusion and the static disorder in zirconia based super-ionic conductors, and the critical behaviour at the structural phase transition in PbTiO 3 . Anharmonic and split models are compared on the example of Zr—Mg—O—N.

Modern materials and their properties are often characterized by varying degrees of disorder. Routine crystallographic structure solution only reveals the average structure. The study of Bragg and diffuse scattering yields the local atomic arrangements holding the key to understanding increasingly complex materials. In this paper we review the pair distribution function technique used to unravel the local structure. We aim to give a practical overview and make this method easily accessible to the wider scientific community.

The combination of stroboscopic techniques and neutron diffraction can be used to study the kinetics of structural changes in condensed matter on a microscopic level. Transient states may be identified and characterized on time-scales down to the microsecond regime. Hence, valuable information about the underlying mechanisms can be obtained from time-resolved experiments. Particularly interesting subjects for this type of investigation are spatially modulated systems which undergo phase transitions or phase separation. The potential of stroboscopic neutron diffraction is demonstrated using three different examples in which samples are periodically perturbed by the variation of temperature, mechanical stress or electric field and their structural response is characterized by time-resolved diffraction. Spinodal decomposition in ionic crystals of the silver-alkali halide type is shown to be dominated by two different processes on different time-scales. The stress-induced phase transition into the incommensurate phase of quartz involves relaxation processes which are reflected by different kinetic behaviours of Bragg peaks and satellite reflections, respectively. Finally, metastable transient states are observed during the field induced lock-in transition in ferroelectric Rb 2 ZnCl 4 which are most probably due to strain fields and pinning effects.

The basic principles of small-angle neutron scattering and the two-phase model, frequently used for the interpretation of scattering data, are presented. Some studies on phase separation in alloy single crystals serve to illustrate the possibilities of the method. Small-angle neutron scattering of Ni-rich Ni—Ti studied during ageing at elevated temperatures indicates a complex sequence of phases during decomposition, involving two stages of metastable coherent precipitation preceding the formation of the stable hexagonal phase.

Complementary neutron and x-ray diffuse scattering may provide insight into structural super-ordering and disordering of decagonal quasicrystals (d-phases), and, in consequence, into the formation and stability of aperiodically ordered alloys. Neutron diffraction makes a contrasting almost isoelectronic atomic species possible, as well as a separation of elastic and inelastic diffuse intensity contributions. Experimental work at high temperatures is comparatively unproblematic. The method suffers, however, from the difficulty in obtaining sufficiently sized mono-grain samples and a lack of dedicated neutron diffraction instruments. Recent results, with a main focus on high-temperature (<1000°C) investigations of disordered decagonal Al—Ni—Co phases are reported and some tentative models are discussed.