ERTH 2001: XRay Diffraction

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ERTH 2001 X-Ray Diffraction Nesse, Ch.8

• What is X-ray diffraction?
• What is Bragg's Law?
• How is XRD used to identify minerals?
• Advantages and limitations?

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ERTH 2001 X-Ray Diffraction
What are X-rays? - a form of electromagnetic
radiation (short ?, high energy)   -
generated when high velocity electrons
strike atoms of target material some e-
excited to higher energy shells X-rays
generated when e- return to normal    
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ERTH 2001 X-Ray Diffraction
What are X-rays? - a form of electromagnetic
radiation (short ?, high energy)   -
generated when high velocity electrons
strike atoms of target material some e-
excited to higher energy shells X-rays
generated when e- return to normal -
target atoms emit continuous and/or
characteristic radiation as electrons move
between shells spectrum of characteristic
radiation (?, E) is unique for each element
("atomic fingerprint")   
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ERTH 2001 X-Ray Diffraction
What is diffraction? - incident radiation (e.g.,
light, X-rays) scatters as it passes through a
finely spaced periodic array (e.g., grating,
crystal lattice)
polychromatic (white) light
monochromatic light (e.g., laser)
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ERTH 2001 X-Ray Diffraction
What is diffraction? - periodic atomic arrays
in crystal lattice act like 3-D diffraction
gratings
diffracted X-rays act like "ripples"
2nd ripple
1st ripple
atoms in lattice plane act as scattering centres
incident ?
- where beams of scattered radiation emerge
in phase, constructive interference produces
diffraction maxima 
1?
2?
3?
Blackburn Dennen Ch. 13
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ERTH 2001 X-Ray Diffraction
What is diffraction? - incident radiation (e.g.,
light, X-rays) scatters as it passes through a
finely spaced periodic array (e.g., grating,
crystal lattice) - where beams of scattered
radiation emerge from slit "in phase",
constructive interference produces
diffraction maxima  - position and intensity
of maxima depends on spacing of array and
integral number of ? contributing to
signal (n?)
polychromatic (white) light
monochromatic light (e.g., laser)
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ERTH 2001 X-Ray Diffraction
What is X-ray diffraction (XRD)
crystallography? - periodic atomic arrays in
crystal lattice act like 3-D diffraction
gratings - for practical purposes, diffraction
can be treated like reflection from multiple
equivalent lattice planes (hkl)
sharp peaks
broad peaks
diffuse, continuous spectrum
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ERTH 2001 X-Ray Diffraction
What is X-ray diffraction (XRD)
crystallography? - intensity and positions of
diffraction maxima depend on ? - wavelength of
incident radiation ? - angle of incident
radiation to given lattice plane (hkl) d -
distance between equivalent lattice planes (hkl)
given incident X-ray beams 1 and 2, in phase,
with single ?
for what combinations of ?, ?, d will beams 1 and
2 emerge in phase? (constructive interference
diffraction maximum or peak on spectrum)
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ERTH 2001 X-Ray Diffraction
What is X-ray diffraction (XRD)
crystallography? - intensity and positions of
diffraction maxima depend on ? - wavelength of
incident radiation ? - angle of incident
radiation to given lattice plane (hkl) d -
distance between equivalent lattice planes (hkl)
for what combinations of ?, ?, d will beams 1 and
2 emerge in phase? (constructive interference
diffraction maximum or peak on spectrum)
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ERTH 2001 X-Ray Diffraction
What is X-ray diffraction (XRD)
crystallography? - intensity and positions of
diffraction maxima depend on ? - wavelength of
incident radiation ? - angle of incident
radiation to given lattice plane (hkl) d -
distance between equivalent lattice planes (hkl)
BRAGG'S LAW  n ? 2d sin ?
single most important equation in X-ray
crystallography - you must know this!
for what combinations of ?, ?, d will beams 1 and
2 emerge in phase? (constructive interference
diffraction maximum or peak on spectrum)
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ERTH 2001 X-Ray Diffraction
How is Bragg's Law used to identify minerals?
BRAGG'S LAW  n ? 2d sin ?
single most important equation in X-ray
crystallography - you must know this!
beam X-rays of known ? at sample rotate sample
through range of known angles ? measure
positions and intensities of reulting maxima
(peaks) calculate d (lattice spacing) for each ?
where peak observed result is information on
crystal structure, NOT chemical composition!!
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ERTH 2001 X-Ray Diffraction
Generation of X-rays for diffraction experiments
X-ray tube
- electrons strike target (typically Cu) - target
emits X-rays (continuous and
characteristic) - characteristic X-rays of
desired ? are directed to unknown sample
( ? in Bragg's Law)
best results when ? similar to expected d-spacing
(for crystals, a few ?)
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ERTH 2001 X-Ray Diffraction
X-ray diffraction methods (some of
them!!) photography - intensity of diffraction
maxima detected on film diffractometry -
intensity measured by electronic detector  1.
Single crystal diffraction Laue method -
fixed crystal, fixed (flat) film - sample
mounted in specified crystallographic orientation
with respect to X-ray beam
Precession method - - both crystal and (flat)
film rotate
Single crystal methods are best for determining
structure and symmetry of unknown minerals
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ERTH 2001 X-Ray Diffraction
X-ray diffraction methods (some of
them!!) photography - intensity of diffraction
maxima detected on film diffractometry -
intensity measured by electronic detector 2.
Powder diffraction photography - fine,
randomly oriented crystals (powder) mounted in
X-ray beam - many planes in correct
orientations to yield diffraction maxima -
yields series of nested diffraction cones  
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ERTH 2001 X-Ray Diffraction
X-ray diffraction methods (some of
them!!) photography - intensity of diffraction
maxima detected on film diffractometry -
intensity measured by electronic detector 2.
Powder diffraction photography - powder
diffraction camera - film surrounds fixed
mount records 2? up to 180 - diffraction cones
generate circular lines on film (or pairs of
semi-circular lines) - compare line spacing and
intensity with JCPDS database  
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ERTH 2001 X-Ray Diffraction
X-ray diffraction methods (some of
them!!) photography - intensity of diffraction
maxima detected on film diffractometry -
intensity measured by electronic detector 3.
X-Ray Powder Diffractometry (XRD) (what you will
do!) - powder mounted in path of
monochromatic X-ray beam - both powder mount
and detector rotate so that detector picks up
diffraction maxima separately
geometry of X-ray diffractometer
sample moves ? detector moves 2?
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ERTH 2001 X-Ray Diffraction
X-ray diffraction methods (some of
them!!) photography - intensity of diffraction
maxima detected on film diffractometry -
intensity measured by electronic detector 3.
X-Ray Powder Diffractometry (XRD) (what you will
do!) - powder mounted in path of
monochromatic X-ray beam - both powder mount
and detector rotate so that detector picks up
diffraction maxima separately
geometry of X-ray diffractometer
chart recorder (paper or digital)
sample moves ? detector moves 2?
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ERTH 2001 X-Ray Diffraction
X-ray diffraction methods (some of
them!!) photography - intensity of diffraction
maxima detected on film diffractometry -
intensity measured by electronic detector 3.
X-Ray Powder Diffractometry (XRD) (what you will
do!) - position (2?) and intensity (I) of
maxima recorded as peaks on chart
recorder (or digital equivalent) measurement
easier and more accurate than powder
diffraction film
sample moves ? detector moves 2?
detail
intensity (I)
position (2?)
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ERTH 2001 X-Ray Diffraction
X-ray diffraction methods (some of
them!!) photography - intensity of diffraction
maxima detected on film diffractometry -
intensity measured by electronic detector 3.
X-Ray Powder Diffractometry (XRD) (what you will
do!) - compare with JCPDS database (done by
computer) - generally find 1-5
minerals compatible with diffraction data
use other information to work out correct
choice
JCPDS card
output (diffractogram)
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ERTH 2001 X-Ray Diffraction
Advantages
- fast and easy to use - theory and practice very
well established - thousands of substances in
database - can be applied to any crystalline
material (minerals, synthetic materials,
proteins, etc.....) - more than one material
may be compatible with data - multi-mineralic
samples can be difficult to interpret - for best
results, need pure mineral separate - no
information on chemical composition
Limitations
yields information on crystal structure only!!!

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ERTH 2001 X-Ray Diffraction Nesse, Ch.8
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ERTH 2001 X-Ray Diffraction