The Best Powder Patterns

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The Best Powder Patterns

• Suggestions on how to determine the best X-ray
  Powder Diffraction Pattern.

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The Powder DiffractometerReflection mode
?
R2
R1
Receiving Slit
?
?
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The Powder DiffractometerTransmission mode
Diffracted X-rays
bent crystal monochromator
source
Converging X-ray Beam
D
D R/sin?
R
2?
Sample
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The Best Powder Pattern

• Accurate positions of diffraction peaks
• Accurate intensities of diffraction peaks
• An accurate distribution of intensities as a
  function of the diffraction angle (resolution).

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The Best Powder Sample
Low X-ray Penetration
Smooth level surface
Small Uniform Crystallites
Few Voids
Random crystallite Orientations
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Factors that will Effect the Positions of
Diffraction Peaks

• Sample effects (reflection)
• Transparency
• ?2? sin2? / 2?R (radians)
• Displacement
• ?2? 2 s cos? / R (radians)
• ?2? ? 0.01o for s .015mm
• Instrument effects
• Flat sample
• Divergent x-ray beam

rs
f
rf
?
R
?
rf R/2sin?
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Factors that will Effect the Accurate
Determination of Peak Intensities

• Primary Extinction and Microabsorption
• Preferred Orientation (texture)
• Non-spherical crystallites pack, under stress,
  in a preferred direction according to their
  shapes.
• Needles gt plates gt cube
• Peak Overlap
• The larger the structure the denser the
  diffracted x-ray pattern, resulting in overlap of
  adjacent peaks.
• Peak broadening due to size, lattice strain and
  instrument effects results in overlap of adjacent
  peaks

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Preferred Orientation Aspirin
a 11.2853 b 6.5387 ?95.9302 c
11.2849 P21/c
Reflection
Transmission
Predicted
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Factors that effect the Intensity Distribution as
a Function of Diffraction Angle

• Instrument errors
• Divergence of the diffracted X-rays
  (perpendicular to the direction of the X-ray
  Beam)
• Crystallite Size
• As the crystallite size decreases the width of
  the diffraction peak increases.
• Bcryst k? / Lcos? L crystallite size
• Lattice Strain
• Symmetric strain will shift the diffraction peak
• Asymmetric stain will broaden the diffraction
  peak
• Bstrain ? tan? ? strain parameter

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Techniques to minimize peak shift errors

• Careful instrument alignment
• Standard addition
• Back loaded Samples
• Thin sample smears on zero background holders
  (ZBH) (specially cut single crystals of silicon
  or quartz)
• Samples are smeared as a thin film (lt0.001mm)
  on the surface of the holder.
• No transparency errors
• Small (lt0.001o) displacement error

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Techniques to minimize intensity errors

• Grease smears
• Vaseline, apeazon, mineral oil used to suspend
  powder on a ZBH.
• Side loading
• Sample holder that is filled from the side.
• Capillaries
• Thin walled glass or quartz tubes (.3 to 1.5 mm)
• Spray drying
• Samples that are mixed with solvent and are
  rapidly sprayed onto a surface
• Aerosols
• Particles are sucked onto glass filter paper
• Mechanical
• Sample spinners (not always effective)
• Data collection
• Reflection and/or transmission mode

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Sample Mount
Top
Back Side Pack Density
60 60 50
Capillaries
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Loading Techniques
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Nonstandard Mounting Techniques
Amyl Acetate (hair spray) Vaseline Non-silicon
grease
Mylar tape Frosted Glass Slide Single
Crystal Glass fiber
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Techniques to Optimize Intensity Distributions

• Instrument effects
• Crystallite sizes
• Sieves 200 (0.075mm) and 375 (0.045mm) mesh
• Lattice Stain
• Grinding
• Dry methods (mortar and pestle)
• Wet methods (solvents and greases)

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Typical Sample Preparation and Data Collection
for Good X-ray Powder Patterns

• Grinding and sieving
• Gently ground in a agate mortar
• A 200 mesh sieve is joined (top) to a 375 mesh
  sieve (bottom).
• The powder is placed in the top sieve (200 mesh)
  and both sieves are shaken.
• The sample is collected in the 375 mesh sieve.

Crystallites that remains in the 200 sieve are
too large
200 mesh 375 mesh
Crystallites that fall through the 375 sieve are
too small
Only crystallites in the 375 mesh sieve are used
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Data Collection

• Four samples are prepared
• smear on a ZBH (with internal standard)
• Accurate position of Intense peaks
• Top loaded sample (for Intensity)
• Side load sample (to check for preferred
  orientation)
• Capillary sample (transmission)