Determining Structures: A Case Study in Experiment Design

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Determining StructuresA Case Study in
Experiment Design
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Typical Experiment Design

• It Starts with a question.
• Literature Search
• Equipment / Facilities / People
• Work Plan
• Tasks
• Schedule
• Funding?
• Agency / Program / Program Manager
• Proposal

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Perovskite Structures
orthorhombic
tetragonal
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Bragg Law
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KCl
KBr
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Neutron Diffraction

• H? ions are emitted from an ion source.
• The ions are accelerated in two steps to 50 MeV.
• The two electrons are stripped off and the
  protons are accelerated further to 450 MeV.
• This process is repeated 30 times per second
  resulting in 80 ns pulses.
• The protons strike an enriched uranium target.
• Neutrons are emitted from the target and are
  slowed to useful energies as they pass through a
  Gd-poisoned, liquid methane moderator.
• Pulses of neutrons head toward our 9 to 12 g
  sample in the diffractometer.

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Neutron Diffraction
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Rietveld Refinement
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I 2/m Monoclinic Structure
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Bi and Pb have nearly identical covalent radii
(because both are nominally 4 valence), but Pb
has a larger atomic radius.
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Mixed Phases Observed by Neutron Diffraction
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Mixed Phases Observed by Neutron Diffraction
tetragonal
orthorhombic
T O
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Process Makes a difference
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High Temperature X-ray Diffraction
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High Temperature Neutron Diffraction
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Phase Diagram
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