Current Status of Neutron-Scattering Research and Facilities in the United States (1984) / Chapter Skim
Currently Skimming:
Pages 34-89
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 34... ...
5. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE In this chapter we present selected summaries of important research areas and accomplishments during the past 6 years.
|
From page 35... ...
paragraphs we survey the contributions that neutron scattering has made to the various subfields. It is worth reflecting that most of the results discussed represent not only significant advances in our understanding of the physics of the materials but that neutron scattering provided unique information not available from other known techniques.
|
From page 36... ...
experimental tests of our understanding of simple random systems. Magnetic Systems The determination of magnetic structures was one of the first and most important applications of neutron diffraction.
|
From page 37... ...
rare-earth materials in which the rare-earth ion apparently has a fractional valence -- the intermediate- or mixed-valence materials. These materials do not show magnetic order, and the neutron-scattering spectra show a broad quasielastic line with an energy width in the range from 10 to 100 meV.
|
From page 38... ...
phase have revealed short-range magnetic order extending well above Tc. In another type of polarized-beam experiment, the appearance of "forbidden" magnons in Ni well below Tc indicates a deviation in the local and bulk magnetization directions.
|
From page 39... ...
case for the past 6 years. Phase transitions occur in a wide variety of systems so that the work discussed in this section will overlap with results presented in most of the other sections.
|
From page 40... ...
many cases major improvements in momentum or energy resolution, or both, currently available or under development in Europe, will be of increasing importance. In this brief report, it is impossible to survey all the beautiful experiments that have been performed in the last 6 years.
|
From page 41... ...
crystals of Cu-Mn have demonstrated strong short-range magnetic correlations associated with short-range nuclear order. Exotic effects are also observed in alloys with competing ferromagnetic and antiferromagnetic interactions.
|
From page 42... ...
complicating extraneous interactions. As in the past, neutrons will continue to provide an indispensible tool for such studies.
|
From page 43... ...
for the superconductivity by a barrier of inert atoms, preventing strong interaction between the two. In the case of materials such as DyMo6S8, heat-capacity measurements showed the presence of a new kind of ordering occurring below the temperature of the onset of superconductivity.
|
From page 44... ...
can be prepared by inserting, for example, alkali metal atoms between the graphite sheets. Among the fascinating and incompletely understood aspects of these resulting graphite intercalation compounds (GIC)
|
From page 45... ...
TMMC, by a group at ILL.
|
From page 46... ...
verified, in part, with neutron studies. The phenomenon of "locking in," whereby an initially incommensurate lattice distortion becomes pinned at a commensurate value was first observed by neutron studies, and recently an example of electricfield-induced lock-in transformation has been found in the ferroelectric thiourea.
|
From page 47... ...
precursors of charge-density wave instabilities such as the Peierls transformation discussed above. Of the various quasi one- and two-dimensional systems studied extensively with neutrons, the 1-D metal (KCP)
|
From page 48... ...
atom, but not every graphite atom, contributes to the interplanar stiffness. Although of great interest, from the viewpoint of 2-D physics, it has proven difficult to carry out similarly detailed studies of phonons propagating within the intercalant layers, because of sample size restrictions.
|
From page 49... ...
ion conductors. Here the time scale is somewhat shorter than is the case for hydrogen in metals so that triple-axis spectrometers can be employed.
|
From page 50... ...
that this field will continue to grow, and, indeed, certain of the problems (e.g., high-energy spectroscopy) are well matched to pulsed neutron sources, so that one can expect major activity at the new pulsed sources as they come online.
|
From page 51... ...
4He, 3He, Ar, and N2 on graphite. All of these exhibit triangular structures with a lattice constant either commensurate or incommensurate with respect to the graphite substrate.
|
From page 52... ...
have been carried out on surface systems. Interesting results have been obtained in both the United States and Europe for hydrogen chemisorbed on metallic surfaces such as Raney Ni, platinum, and palladium blacks.
|
From page 53... ...
of neutron scattering with the results of molecular-dynamics simulations, a technique that has so far been more successful for liquids than for glasses or amorphous solids. Nevertheless, real progress has been made in recent years in a fundamental understanding of the statistical physics of these systems, particularly in the use of kinetic theories.
|
From page 54... ...
in both normal and superfluid He. Recent neutron-diffraction results have led to very precise estimates of the condensate fraction in superfluid 4He, while high momentum transfer experiments on normal and superfluid 4He using epithermal neutrons show promise of giving very detailed measurements of the momentum density distribution, but again increased fluxes are needed to provide the necessary resolution and sensitivity.
|
From page 55... ...
Glasses and Amorphous Solids The principal achievements in the fundamental understanding of this important class of materials has come from detailed measurements of the static structure factor by x-ray and neutron scattering, combined with the detailed information about local order derived from extended x-ray absorption fine structure (EXAFS)
|
From page 56... ...
concerning phonon density of states in amorphous materials based on the fractal nature of self-similar structures need to be explored by neutron-scattering experiments. As these and other efforts continue, neutron scattering will unquestionably provide the ultimate test of most theoretical models, since it provides data over a uniquely wide range of energy and momentum.
|
From page 57... ...
U. Bonse at a small reactor in Vienna, Austria, in 1974.
|
From page 58... ...
interaction, search for quaternions in quantum mechanics, observation of the neutron analog of the Fizeau effect, search for nonlinear terms in the Schrödinger equation, measurement of the longitudinal coherence length of the neutron, and a search for a neutron Aharonov-Bohm effect. Interference in spin space, using the nuclear magnetic resonance-Ramsey technique, has recently been used to observe optical rotation via parity-violating weak interactions.
|
From page 59... ...
that this field will take. However, there are various fundamental neutron-opticsrelated experiments currently in progress and others on the immediate horizon.
|
From page 60... ...
in which the entire diffraction pattern is fitted to a detailed crystallographic model. This technique, which has become known as the Rietveld method, has revolutionized structure studies on both sides of the Atlantic for a wide variety of materials that are not available as single crystals.
|
From page 61... ...
have been carried out, including such potentially technologically important materials as FeTiHx and LaNiH5. In studies of C-H/metal interactions, neutron diffraction has provided information on models for C-H bond activation processes that are of fundamental importance in catalysis.
|
From page 62... ...
in the partially oxidized tetracyanoplatinate complexes, the one-dimensional electronic conductivity has been studied as a function of metal-metal distance for a range of materials whose structures are known from neutron diffraction. More recent neutron-diffraction studies of organic, superconducting materials of the TMTSF family have explored the structure-property relationships that might exist between materials that exhibit superconductivity only at high pressure and materials that are superconducting on appropriate anion substitution.
|
From page 63... ...
the periodic table to compounds containing heavier elements. These studies are yielding important information on chemical bonding, including the distribution of d electrons in transition-metal systems.
|
From page 64... ...
High-Precision Structure Studies The high precision currently available from neutron-diffraction structure analysis has made possible a variety of new types of studies -- atomic positions may frequently be determined with a reproducibility of 0.001 Å or better. For example, investigations of a number of pyroelectric materials have shown that observed changes in polarization with temperature are consistent with the values calculated from the lattice constants and the nuclear positions.
|
From page 65... ...
such as Ni, Nd, and Cl. The method can be extended to nuclides with small differences in scattering length such as 12C and 13C or 14N and 15N.
|
From page 66... ...
Vibrational Spectroscopy Current U.S. research efforts in neutron molecular spectroscopy are concentrated on vibrational spectroscopy in the range 25-250 meV using recently developed specialized instrumentation at reactors and pulsed neutron sources.
|
From page 67... ...
on catalysts over a wide range of temperature and pressure. These studies have already provided direct information on the bonding states and interactions of H atoms on the surface of Ni and Pt particles and on adsorption and decomposition of organic molecular species on these catalysts.
|
From page 68... ...
sections of this report, in condensed-matter physics, polymer science, and biology. In chemical applications, low-energy neutron spectroscopy is an extremely sensitive tool for probing the intermolecular potentials and the details of rotation and diffusion of molecular species in condensed systems.
|
From page 69... ...
methane absorbed on graphite, it was possible to determine the relative importance of molecule-molecule and molecule-substrate forces by comparing tunneling levels for various types of rotations of the molecules on the surface. We should like to stress that this type of low-energy spectroscopy, when combined with IINS vibrational spectroscopy, provides a powerful and comprehensive probe for studying molecule-surface interactions.
|
From page 70... ...
and infrared light, electrons, and x rays have provided structural information ranging from the organization of the atoms in macromolecules to the arrangement of cells in organisms. With the development of high-intensity sources and improved instruments, it has recently become possible to use neutrons as an additional form of radiation with which to study the structure and dynamics of macromolecules and molecular assemblies.
|
From page 71... ...
have exploited enhanced contrast between protein and nucleic acid components of macromolecules. Specific deuteration of parts of macromolecules or macromolecular assemblies has been used to extend the range of information that can be obtained from solution scattering measurements.
|
From page 72... ...
of these proteins, while unexpected results concerning the details of hydrogen bonding between the enzyme and a substrate analog emerged from the analysis of ribonuclease. A second line of crystallographic studies has been to follow the exchange of hydrogen and deuterium, using the exchange patterns to derive ideas concerning the distribution of static and dynamic regions of the structure.
|
From page 73... ...
consisting of proteins and nucleic acids. Such studies are capable of delineating precise boundaries between the components and are useful if high-resolution data cannot be collected.
|
From page 74... ...
of such labeled particles could then be used to obtain a measurement of the distance separating the centers of the two proteins in question. By a process of triangulation, a three-dimensional map of the positions of proteins in a complex structure can be generated.
|
From page 75... ...
across the membrane of the organism, which it does by pumping protons from the cytoplasm to the outside in response to light. Understanding its structure may lead to improved ideas concerning the nature of membrane proteins and their relationship to lipid bilayers, as well as leading toward an understanding of the energy transduction process itself.
|
From page 76... ...
Inelastic Scattering The application of inelastic scattering to biological studies has only just begun. After a number of efforts in Europe, a start has been made in developing both the theoretical framework and the measurement techniques that will be needed for biological studies.
|
From page 77... ...
that such gains will continue to grow in the future, as improved instrumentation and new sources are developed. POLYMER AND COLLOID SCIENCE The availability of SANS facilities has led to revolutionary advances in polymer science over the past decade.
|
From page 78... ...
higher-energy frequency dynamics of polymers can also be quite valuable. A complete analysis requires large deuterated single-crystal samples, which are usually not available, but determination of large-amplitude modes (e.g., torsions)
|
From page 79... ...
ratios are considerable in systems of such low concentration, and the time for a single experimental run becomes unreasonably long. The dimensions of particles in colloids are comparable with those in polymers, contrast is established by substitution of deuterium for hydrogen, and therefore the SANS requirements are of the same kind.
|
From page 80... ...
absence of SANS, it was not possible to distinguish between these alternatives. SANS measurements on a number of systems strongly support assumption (a)
|
From page 81... ...
Block polymer molecules contain within a single polymer chain at least two different chemical subchains. In some interesting cases, these subchains separate into two microphases owing to mutual incompatibility of different parts of many molecules.
|
From page 82... ...
Instrumentation Needs Small-angle neutron scattering of high polymers and of colloids has become an essential research tool. More than 100 experimental studies have been published, the majority using neutrons from the D11 SANS facility on a coldneutron guide at the ILL.
|
From page 83... ...
be needed for optimal resolution and flexibility. Recent work in Japan has demonstrated that useful research on some problems can be done at modest-flux pulsed sources equipped with a cold source.
|
From page 84... ...
low that the testing procedure itself does not introduce any additional damage, as may occur with electron microscopy. The sensitivity of SANS to heterogeneities in the size range from a few nanometers to about a micrometer has made it possible to follow, often in detail, microstructural changes in metals and ceramics resulting from deformation, irradiation, or processing.
|
From page 85... ...
in a ferritic stainless steel developed for use in power-generation applications. Extended service causes microstructural alterations, which affect the mechanical properties of the steel such that it may no longer meet design requirements.
|
From page 86... ...
microcracks has been demonstrated in an elegant investigation of the ceramic YCrO3, which undergoes extensive microcracking when it passes through a phase transformation at about 1100°C. It was found that the small-angle scattering cross sections from the cracked YCrO3 could be fitted well by the form of the scattering expected from an ensemble of randomly oriented thin disks.
|
From page 87... ...
measurements show that, in contrast to the case of fatigue, no voids can be detected with sizes below the predicted value of the smallest stable void. As in the case of cyclic loading, no incubation time appears to be required for void nucleation.
|
From page 88... ...
The method now has been applied to the determination of stresses in a number of situations, e.g., in depleted uranium and in composites in cemented carbides. In the latter case, evidence was found of large hydrostatic components of stress between carbide and binder, which arise because of significant differences in their coefficients of thermal expansion.
|
From page 89... ...
place by nucleation and growth or by spinodal decomposition. In another recently reported in situ experiment at ILL, solute partitioning occurring during unmixing of a ternary alloy was studied.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.