Neutrons play an increasingly important role in the characterization of advanced modern materials. They provide information that complements rather than competes with that provided by other scattering probes. Although neutrons require heroic efforts to produce, the techniques for using them are not particularly difficult, and with the advent of sufficient user friendly facilities, are becoming a routine tool in the arsenal of expanding numbers of materials scientists.

This article describes some of the neutron scattering techniques which can be used to probe structures of materials on length scales which range from less than 1 Å to 1 μm. The intent is to remove the veils of secrecy and mystery which have apparently prevented the wide application of these techniques to problems in materials science.

The Advanced Neutron Source (ANS) is a new user experimental facility planned to be operational at Oak Ridge in the late 1990's. The centerpiece of the ANS will be a steady-state research reactor of unprecedented thermal neutron flux (φth ≍ 9·1019 m−2·s−1) accompanied by extensive and comprehensive equipment and facilities for neutron-based research.

Neutron powder diffraction is a powerful technique that provides a detailed description of moderately complex crystal structures. This is nowhere more apparent than in the area of high temperature superconductors where neutron powder diffraction has provided precise structural and magnetic information, not only under ambient conditions but also at high and low temperatures and high pressures. Outside superconductor research, the variety of materials studied by neutron powder diffraction is equally impressive including zeolites, fast ionic conductors, permanent magnets and materials undergoing phase transitions. Recent advances that include high resolution studies and real-time crystallography will be presented. Future possibilities of neutron powder diffraction will be discussed.

Neutron topography has been carried out on organic single crystals of varying Xray sensitivity, in order to test the feasibility of the technique as an alternative to X-ray topography for the study of the influence of defects on the solid state reactivity of X-ray sensitive single crystals. Specimens studied include the diacetylene PTS, and Pyrene. A comparison of the strain sensitivity and spatial resolution of the neutron and X-ray based techniques is made. Preliminary results of dynamic neutron topographic studies of the UV induced polymerization in PTS are presented. These results are compared to those obtained from similar X-ray topographic studies.

Results indicate that the neutron technique can be a useful ally technique to the analogous X-ray techniques in studies of the influence of defects on reactivity in specimens of moderate X-ray sensitivity. In cases of extreme sensitivity, the neutron technique is the only one available for studies of this nature.

Multiple small angle neutron scattering (MSANS) was used to follow the evolution of the pore size distribution in α-Al2O3 through the intermediate and final stages of sintering. This technique makes it possible to determine microstructure morphology in the 0.08 to 10 μm size regime under conditions where the total scattering cross-section is dominated by elastic incoherent multiple scattering, as is often the case for ceramics. The MSANS results indicate an initial decrease in the effective pore radius from 0.19 μm at 57% of theoretical density (TD) to 0.17 μm at 79% TD. As the sample density further increased, there was a transition region after which the effective pore radius grew rapidly to ≤0.5 μm at 98% TD. Standard Porod analysis on scattering by the same samples also indicates a transition between the intermediate and final stages of processing. Both sets of results support a topological model of sintering in which the interconnected pore network in the intermediate stage of sintering decays in a stable manner. The pores become fewer, while retaining the same diameter, as densification proceeds. In the final stage, isolated pores remain.

Inelastic scattering from lattice phonons contributes a significant fraction of the observed Bragg intensity in time of flight neutron powder patterns of transition metal monoxides at elevated temperatures. Ignoring this thermal diffuse scattering (TDS) leads to errors in the site occupations of defects present in these materials. The intensity from one phonon TDS has been calculated as a function of time-of-flight and used to correct the measured intensities. The effect of this correction on the results of Rietveld profile refinement is discussed.

The use of high-energy neutrons from pulsed or hot sources allows the method of atomic pair distribution analysis to be applied to the structural determination of crystalline as well as amorphous solids. This method complements the standard crystallographic methods in studying non-periodic aspects of solids with or without long range order.

All information on the site occupancies and atomic displacements which relate a pyrochlore superstructure to the parent fluorite-type subcell is contained in the normally-weak superstructure diffraction intensities. As Ti has a negative scattering length, the supercell maxima in the present phases are up to three times as intense as the fluorite-like reflections, and neutron diffraction provides an especially sensitive probe of the state of disorder. Y2Ti2O7 is found to have a fullyordered anion array and slight disorder, Y0.984Ti0.015, among the cations. In Y2(Zro.6TiO.4)2O7 the oxygen site normally vacant in pyrochlore is half filled and on the order of 15% exchange between cation sites has occurred.

Nanocrystalline compacts of TiO2 and Pd were prepared by first condensing the Ti or Pd vapors in an inert gas atmosphere. The Ti was oxidized in situ to TiO2. Samples were prepared by scraping off and compacting the nanophase materials into thin disks. The small angle neutron scattering was measured in the as-prepared condition and after isothermal anneals of up to 23 hrs at 550°C for the TiO2 and up to 3.3 hrs at 300°C for the Pd. Scattering data were obtained in absolute cross sections. Integrated small angle scattering and maximum entropy methods were used in estimating the structural parameters. The results are interpreted in terms of a model which consists of nanometer sized grains of the materials separated by boundary regions which are, on average, much less dense than the respective bulk materials; 21% for the TiO2 and about 56% for Pd. However, the boundary regions contain voids or pores, which contribute to these density decrements. Possible sources of error are discussed.

Several phenomena analogous to those observed in classical optics, such as reflection, refraction and interference, are also observed with slow neutrons. Information on surface properties, described by a refractive index profile, can be extracted from reflection experiments. This information is similar to that obtained by X-ray reflection. However, there are some instances where the new neutron method provides a distinct advantage. The refractive index is related to the scattering length density, a parameter which describes the neutron-matter interaction. Owing to the magnetic interaction, magnetic materials have a neutron spin dependent refractive index, and a critical reflection of polarized neutrons is a particularly sensitive probe of surface magnetism. On the other hand, in contrast to X-rays, neutron scattering length values vary randomly from element to element. Isotopic substitution can then produce a contrast in the scattering length density. Of particular importance is the large difference between hydrogen and deuterium. This is a distinct advantage for studiyng many problems in surface chemistry, particularly in the polymer field.

The interdiffusion of Cadmium Arachidate (CdA) in Langmuir-Blodgett films has been studied by neutron reflection at the Intense Pulsed Neutron Source of Argonne National Laboratory. One of the samples consisted of a few layers of perhydro H-CdA deposited on a silicon support, overlayered with a few layers of deuterated D-CdA, for a total thickness of ∼300 Angstroms. In a second sample the layers of perhydro-and deuterated- CdA were separated by two monolayers of Hn-octadecene/co-maleic acid copolymer. When heated for 15 minutes at 70°C, well below the disorder temperature [1], approximately 25% of the D-CdA molecules were replaced by H-CdA molecules, although the overall Langmuir-Blodgett film structure is known to remain unchanged [1]. The presence of copolymer layers limited the interdiffusion process to about 5%.

Using the technique of polarized-neutron reflectometry, we have studied the magnetic moment distribution in the interfacial region of an epitaxial film consisting of 97 Å Y(0001) atop 1000 Å Gd(0001). At T = 151 K, with an 80 gauss field applied parallel to the surface (basal plane), we observe two distinct ferromagnetic regions: the Gd bulk and a layer with reduced magnetic moment at the Y/Gd interface. Above the bulk Curie temperature, at T = 295 K , we observe no net magnetization in either the bulk or the interface.

Neutron diffraction patterns of MBE-grown single crystal zinc-blende epilayers of Cd1−xMnxSe with x=0.70 and 0.75 reveal the onset of long-range Type-III AF ordering at low T, in sharp contrast with earlier studies of bulk II-VI diluted magnetic semiconductors, where only short-range Type III correlations are observed. Results of first neutron diffraction studies of magnetic ordering in MBE-grown ZnSe/MnSe superlattices are also reported.

We have developed an ab-initio method for calculating the static response functions in substitutional alloys. For magnetic alloys, in addition to the nuclear diffuse scattering, a contribution to the alloy diffuse scattering intensities results from the response of the local moments to changes in the ‘local’ chemical environment (i.e. ∂μi/∂cj). We present results of firstprinciples calculations of these ‘local’ response functions in magnetic alloys. These response functions, which may be directly compared to neutron-scattering and Mößbauer experiments, are derived via a mean-field statistical mechanical description of compositional fluctuations in alloys. The statistical averages are performed via the Korringa-Kohn-Rostoker coherent potential approximation, which incorporates the electronic structure of the high-temperature, chemically disordered state. As a first application of the theory, we have investigated the environmental dependence of the moments in NiFe alloys and FeV alloys and multilayers. We compare our results with experiments on bulk alloys and multilayers. Also, a comparison is made to a set of first-principle ‘supercell’ calculations. Although preliminary, the results demonstrate the utility of these response functions for investigating the effects of changes in the chemical environment on the alloy moments and for aiding experimental interpretation in other multilayer systems that are less experimentally amenable than FeV.

The density of hydrogen in thin films or multilayers of metal hydrides can be inferred from an expansion of the host lattice as measured by conventional x-ray diffraction techniques. However, because hydrogen and deuterium have scattering lengths for neutrons that are comparable to those of most metal nuclei, unlike the corresponding case for x-ray atomic scattering amplitudes, the hydrogen density profile normal to the surface of a flat, thin film can be determined directly from neutron reflectivity measurements. The hydrogen (deuterium) density modulation in an artificial superlattice along the growth direction can also be determined in this manner. The thin film or multilayer host metal material need not even be crystalline. Furthermore, because relatively large, flat single crystal substrates such as Si or quartz are nearly transparent to neutrons, the substrate can serve as the incident medium and the reflectivities of films or multilayers in contact with liquid solutions can be obtained. Thus, in situ studies employing active electrochemical cells, for example, can be performed. In addition to discussing the sensitivity of the method, the results of some neutron reflectivity experiments on metal multilayers and films, electrolytically loaded in situ with deuterium, are reported in this paper.

The magnetic structure has been measured with neutron scattering and SQUID magnetometry for a number of epitaxial multilayers and films of the magnetic rare-earths dysprosium and erbium grown with non-magnetic yttrium. In almost all cases the first order ferromagnetic phase transitions are suppressed, although the Néel temperatures are within 5% of the bulk values. The observed critical fields are explained by including epitaxial constraints with the bulk theory of magnetoelasticity.

The specular reflectivity of neutrons has been used to characterize quantitatively the microphase separated morphology of symmetric, diblock copolymers of polystyrene, PS and polymethylmethacrylate, PMMA, as a function of the total molecular weight of the copolymer where either block is perdeuterated. It is shown that the hyperbolic tangent function, as opposed to a linear or cosine squared function, most closely describes the concentration gradient at the interface between the lamellar copolymer microdomains. The effective width of the interface is found to be independent of the molecular weight of the copolymer blocks and has a value of 50 ± 3Å. This interface is also found to be identical to that of PS and PMMA, homopolymers. However, using measured values of the Flory-Huggins interaction parameter for PS and PMMA current theoretical treatments cannot describe the observed widths of the interface.

The order-disorder transition in thin films of symmetric diblock copolymers of polystyrene and polymethylmethacrylate has been investigated by neutron reflectivity. At temperatures above the order-disorder temperature, TMST, a surface induced oscillatory segment density profile with an exponential decay length, χ, is observed. The inverse of decay length 1/≤ TMST a lamellar morphology oriented parallel to the surface propagates through the entire specimen.

The study of surfaces and interfaces using the specular reflection of neutrons on the reflectometer CRISP at the ISIS pulsed neutron source is discussed. The instrumentation, is briefly described, and the performance of the spectrometer being illustrated by some standard examples. Recent experimental results on the adsorption of surfactants at the air-water interface will be presented.