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In x-ray and neutron diffraction, polycrystalline solid or powdered specimens
are placed in a monoenergetic, collimated beam. Appropriately oriented grains
diffract the x-rays or neutrons into a detector. By sweeping the angle of
incidence and detection, a spectrum of diffraction peaks corresponding to
the crystal lattice spacings is produced. The measured lattice spacings are
compared with spacings of known compounds
to identify the crystalline phases and are used to refine the unit cell
parameters. Analysis of unit cell parameters as a function of temperature
yields the lattice thermal expansion. Reactions, phase transformation, and
crystallization are all studied by high temperature diffraction methods.
Crystal structure analysis (e.g. atomic location, thermal motion, site occupancy)
utilizes both the line position and intensity.
The Diffraction User Center includes both room temperature
and furnace-equipped
x-ray, synchrotron,
and neutron diffractometers.
The high temperature x-ray diffractometer is a theta-theta diffractometer
(specimen remains horizontal at all times) with a specimen-heating furnace
capable of operation at 3000 K and a position sensitive detector for high
speed measurements. The X-ray furnace is used for studies of materials properties
at temperatures up to 2700oC in vacuum and up to 1600oC
in air. Temperatures up to 1700oC in nitrogen have been achieved.
A similar x-ray furnace is available for high resolution, fast data collection
studies utilizing synchrotron radiation. Neutron powder diffraction at elevated
temperatures (1600oC, vacuum) is conducted at the High Flux Isotope
Reactor in collaboration with the ORNL Solid State Division Neutron Scattering Group.
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