X-Ray%20Diffraction

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X-Ray Diffraction
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The XRD Technique

• Takes a sample of the material and places a
  powdered sample which is then illuminated with
  x-rays of a fixed wave-length.
• The intensity of the reflected radiation is
  recorded using a goniometer.
• The data is analyzed for the reflection angle to
  calculate the inter-atomic spacing.
• The intensity is measured to discriminate the
  various D spacing and the results are compared to
  known data to identify possible matches.

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Powdering Samples

• The samples are powdered to give a random
  sampling of ALL atomic planes (crystal faces)
• Statistically accurate given samples are powdered
  finely AND randomly oriented on sample holder
• Intensities are a reflection of d-spacing
  abundance
• Problems arise with minerals that may
  preferentially orient on sample holder
• Micas and clays have special preparation
  techniques

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X-Rays
Wavelengths used for XRD
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What is X-Ray Diffraction??

• Crystalline substances (e.g. minerals) consist of
  parallel rows of atoms separated by a unique
  distance
• Simple Example
• Halite (Na and Cl)

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• Crystalline substances (e.g. minerals) consist of
  parallel rows of atoms separated by a unique
  distance
• Diffraction occurs when radiation enters a
  crystalline substance and is scattered
• Direction and intensity of diffraction depends on
  orientation of crystal lattice with radiation

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Schematic X-Ray Diffractometer
Detector
X-Ray Source
Powdered sample
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Sample XRD Pattern
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strong intensity prominent crystal plane
weak intensity subordinate crystal plane
background radiation
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Determine D-Spacing from XRD patterns

• Braggs Law
• n? 2dsin?
• n reflection order (1,2,3,4,etc)
• ? radiation wavelength (1.54 angstroms)
• d spacing between planes of atoms (angstroms)
• ? angle of incidence (degrees)

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strong intensity prominent crystal plane
n? 2dsin? (1)(1.54) 2dsin(15.5 degrees) 1.54
2d(0.267) d 2.88 angstroms
background radiation
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d-spacing Intensity
2.88 100
2.18 46
1.81 31
1.94 25
2.10 20
1.75 15
2.33 10
2.01 10
1.66 5
1.71 5
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Factors that affect XRD data

• Sample not powdered fine enough
• May not give all d-spacing data (not random
  enough)
• Analysis too fast (degrees/minute)
• May not give accurate peak data
• Mixture of minerals??
• Not crystalline glass!!

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Mixture of 2 Minerals
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Applications of XRD

• Unknown mineral ID
• Solid solution ID (e.g. feldspars, olivine)
• Mixtures of minerals
• Clay analyses
• Zeolites
• Crystallographic applications
• Material Science

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Created by Nicolas Barth2007Geology
114AUniversity of California, Santa
BarbaraSource material by Grant Yip