Structural phase transitions can be studied by a variety of techniques; macroscopic fields, X-ray, light and neutron diffraction, and resonance methods. The information which can be obtained by these different techniques about the order parameter and susceptibility at structural phase transitions is described and compared. Landau theory coupled with the ‘soft mode’ concept provides a simple picture of many structural phase transitions in terms of relatively few phenomenological constants. A review is given of the application of this type of theory to the wide variety of different phenomena which can occur at structural phase transitions. In particular, emphasis is placed on the nature of the fluctuations within Landau-Ginzburg theory and on the role of secondary order parameters. The nature of incommensurate phases and of lock-in phase transitions are discussed using both the conventional lattice dynamical approach and the soft soliton theories. The parameters of the Landau theory can, in princip...

Structural phase transitions I. Landau theory

We describe the design and current performance of the backscattering silicon spectrometer (BASIS), a time-of-flight backscattering spectrometer built at the spallation neutron source (SNS) of the Oak Ridge National Laboratory (ORNL). BASIS is the first silicon-based backscattering spectrometer installed at a spallation neutron source. In addition to high intensity, it offers a high-energy resolution of about 3.5 μeV and a large and variable energy transfer range. These ensure an excellent overlap with the dynamic ranges accessible at other inelastic spectrometers at the SNS.

A time-of-flight backscattering spectrometer at the Spallation Neutron Source, BASIS

Electrolyte solutions are of importance in a wide range of scientific contexts and as such have attracted considerable theoretical and experimental effort over the years. Nevertheless, there is still no generally agreed structural model of electrolyte solutions. In this review the authors show how new developments in both theory and experiment are beginning to make a major impact on the understanding of these systems.

https://iopscience.iop.org/article/10.1088/0034-4885/44/6/001/pdf

The structure of electrolyte solutions

We describe the design and current performance of the high-flux backscattering spectrometer located at the NIST Center for Neutron Research. The design incorporates several state-of-the-art neutron optical devices to achieve the highest flux on sample possible while maintaining an energy resolution of less than 1 μeV. Foremost among these is a novel phase-space transformation chopper that significantly reduces the mismatch between the beam divergences of the primary and secondary parts of the instrument. This resolves a long-standing problem of backscattering spectrometers, and produces a relative gain in neutron flux of 4.2. A high-speed Doppler-driven monochromator system has been built that is capable of achieving energy transfers of up to ±50 μeV, thereby extending the dynamic range of this type of spectrometer by more than a factor of 2 over that of other reactor-based backscattering instruments.

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The high-flux backscattering spectrometer at the NIST Center for Neutron Research

Quasielastic neutron scattering for the investigation of diffusive motions in solids and liquids.

Quasielastic Neutron Scattering for the Investigation of Diffusive Motions in Solids and Liquids

A neutron crystal spectrometer with extremely high energy resolution

A neutron crystal spectrometer with extremely high energy resolution.

Aus der inelastischen und quasielastischen Neutronenstreuung lassen sieh wesentliche Informationen fiber die Dynamik des festen und fltissigen Aggregatzustandes gewinnen. Die bisher verwendeten Neutronenspektrometer besitzen eine relative Energieaufl6sung AE/E bis etwa 0,5-10 -2. Die Aufl6sung des unten beschriebenen Spektrometers ist um fast zwei GrSgenordnungen besser. Die zugrundeliegende Idee des Spektrometers, das YOU I~{AIER-LEIBNITZ vorgeschlagen wurde, ist folgende: Erzeugt man monochromatische Neutronen durch Braggreflexion an Einkristallen unter einem Braggwinkel nahe 90 ~ (Riiekstreuung [1]), dann ist die Linienbreite der refiektierten Neutronen sehr

M. Birr

Papers

A neutron crystal spectrometer with extremely high energy resolution

A neutron crystal spectrometer with extremely high energy resolution.

Ein neues hochauflösendes Neutronenkristallspektrometer und seine Anwendung