Neutron Diffraction from Novel Materials
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vironments for in situ studies. These strengths are widely accepted and have been exploited for many years. Previous reviews have focused on these topics.3 In this article, we discuss a few recent examples where neutron diffraction has had a major impact in rapidly developing areas of materials science and that illustrate advances in experimental technique and data analysis. In this recent work, there is a strong emphasis on "imaging" structural Information across a multidimensional parameter space—for example, chemical composition, temperature, atmosphere, pressure, and applied magnetic field—rather than obtaining structural Information from a Single diffraction pat tern. High-speed diffractometers with low background have enabled impressive structural studies on small samples previously thought to be beyond the capability of n e u t r o n diffraction. Precision measurements of structural parameters, including static and dynamic atomic displacements, have provided critical tests of theoretical modeis for the behavior of novel materials, and extreme sample en vironments have opened new experimen tal frontiers. The examples discussed here highlight the impact of neutron diffrac tion on condensed-matter science.
High-Temperature Superconductors There is not a more convincing ex ample of the use of neutron diffraction for the study of new materials than hightemperature superconductors (HTSs). (See the article by Aeppli and Hayden in this issue which includes an overview of the vortex structures of superconductors studied through neutron scattering.) The complexity of the structures is well matched to the Instrumentation, and the need to learn the locations and quantities of oxygen atoms among various heavier metal atoms exploited one of the natural
advantages of neutron diffraction. Each time a new HTS Compound was discov ered, a Rietveld refinement of the struc ture using neutron powder diffraction was either included in the initial paper or followed within weeks. 4 Diffraction studies quickly progressed beyond the initial determination of the crystal structure. It was realized that studies as a function of chemical compo sition, especially of oxygen content, were critical to understanding the behavior. For example, for the YBa2Cu306+.Y Com pound, such experiments validated the "charge transfer" model for copper oxide superconductors, in which changes in the chemistry of the charge reservoir layer control the carrier concentration of the C u 0 2 layers.5,6 Other important work included in situ studies at high tempera ture in controlled oxygen atmospheres, where the all-important oxygen defects could be studied in thermodynamic equilibrium. 7 This work provided the foun dation for the development of synthesis and processing schemes to achieve desired superconducting properties. As neutron-diffraction techniques were ap plied to these problems, it was shown that defects could be characterized in surprisingly small concentrations. 8 The study of HTS materials also motivated the development of methods for neu
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