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Presentation Transcript

Slide 1: 

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?

Slide 2: 

ERTH 2001: X-Ray Diffraction What are X-rays? - a form of electromagnetic radiation (short λ, 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

Slide 3: 

ERTH 2001: X-Ray Diffraction What are X-rays? - a form of electromagnetic radiation (short λ, 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 (λ, E) is unique for each element ("atomic fingerprint")

Slide 4: 

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)

Slide 5: 

ERTH 2001: X-Ray Diffraction What is diffraction? - periodic atomic arrays in crystal lattice act like 3-D diffraction gratings atoms in lattice plane act as scattering centres diffracted X-rays act like "ripples" incident λ - where beams of scattered radiation emerge in phase, constructive interference produces “diffraction maxima” 1λ 2λ 3λ Blackburn & Dennen Ch. 13 1st ripple 2nd ripple

Slide 6: 

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 λ contributing to signal (nλ) polychromatic (white) light monochromatic light (e.g., laser)

Slide 7: 

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

Slide 8: 

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) given incident X-ray beams 1 and 2, in phase, with single λ:

Slide 9: 

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)

Slide 10: 

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) BRAGG'S LAW:  n  = 2d sin  single most important equation in X-ray crystallography - you must know this!

Slide 11: 

ERTH 2001: X-Ray Diffraction BRAGG'S LAW:  n  = 2d sin  single most important equation in X-ray crystallography - you must know this! How is Bragg's Law used to identify minerals? 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!!

Slide 12: 

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 λ are directed to unknown sample (= λ in Bragg's Law) best results when λ similar to expected d-spacing (for crystals, a few Ǻ)

Slide 13: 

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

Slide 14: 

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

Slide 15: 

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

Slide 16: 

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 sample moves  detector moves 2 geometry of X-ray diffractometer

Slide 17: 

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 sample moves  detector moves 2 chart recorder (paper or digital) geometry of X-ray diffractometer

Slide 18: 

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 intensity (I) position (2) detail sample moves  detector moves 2

Slide 19: 

ERTH 2001: X-Ray Diffraction JCPDS card output (diffractogram) 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

Slide 20: 

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 yields information on crystal structure only!!! Limitations:

Slide 22: 

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

Slide 23: 

ERTH 2001: X-Ray Diffraction