X-ray crystallography

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X-ray crystallography
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What tools do we have to ID minerals so far?
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How do we know atomic structure of minerals?

• Atoms are 1-5Å apart so how do we see this?

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X-rays

• Between 0.1 and 10Å (1Å 0.1 nm)

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X-rays

• Amplitude
• Wavelength

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X-ray interference
Constructive is in phase
Destructive if out of phase
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Wave behavior
Interference and diffraction If a wave encounters
a barrier with several holes, the propagating
wave will have sharper peaks of intensity at
certain angles.
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X-rays interact with atoms

• Atoms cause scattering
• Diffraction coherent scattering of waves off a
  periodic arrangement of matter

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Generation of X-rays

• Need
• Vacuum environment
• Source of electrons
• Large accelerating voltage
• Target metal

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Generation of X-rays

• Tungsten filament as source of e-
• E- sent to target metal (usually Cu)
• X-rays generated off Cu
• Higher voltage, faster e-, more energy

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X-ray Crystallography

• X-ray generation

W

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A
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electrons
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Two types of X-rays generated

• Continuous spectra (white radiation)
• Electrons hit target surface, loose energy, stop
• NO change of target electron configuration
• Removed by filtering

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Two types of X-rays generated

• Characteristic radiation
• Electrons interact with target electron
  configuration
• Fingerprint of target metal

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Two types of X-rays generated

• Bombarding electrons dislodge electron from
  target
• K-shell or L-shell
• If vacancy in K-shell filled from L-shell,
  K??radiation
• If vacancy if K-shell filled from M-shell, K?
  radiation

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Two types of X-rays generated

• K? radiation has more energy than K? radiation

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Two types of X-rays generated

• K? radiation has more energy than K? radiation
• Quantized energy and characteristic of metal
  target

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Table 2-1 Wavelengths in Nanometer Units of
Characteristic Emission Lines for Selected
Elements. Element Kalpha Kalpha1 Kalpha2
Kbeta1 Lalpha1 Cr 0.229092 0.228962
0.229351 0.228480 Cu 0.154178 0.154051
0.1544330 0.139217 1.3357 Mo 0.071069
0.070926 0.071354 0.063225 0.54062 W
0.021060 0.020899 0.021381 0.018436 0.14763
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• Characteristic radiation more intense than
  continuous
• Want Ka radiation to go to sample so need to
  filter everything else

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Sample sees monochromatic radiation
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X-ray detection

• Photographically
• Electronic detectors
• Result see planes of atoms and what orientation
  planes are in
• Dont get image of individual atoms

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X-ray diffraction

• Works thanks to Bragg Equation
• X-rays diffract at specific angles based on
  spacing of atomic planes

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X-ray Crystallography

• Destructive and constructive interference of
  waves
• Bragg Equation

in phase
in phase
Y
x
q
q
q
d
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X-ray Crystallography

• nl2dsinq n is the order
• As soon as the crystal is rotated, the beam
  ceases (This is diffraction, not reflection)
• Only get diffraction at certain angles!
• Relation between l and d and q

Y
x
q
q
d