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Powder X-ray diffraction – the uses

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1 Powder X-ray diffraction – the uses
Learning Outcomes By the end of this section you should: be able to describe the uses of powder X-ray diffraction and why these “work” be aware of diffraction/structure databases understand the limitations in each method

2 Powder XRD – the equipment

3 Uses: fingerprinting Single or multi-phase
Two different crystalline phases are present in this pattern – one in a very small amount NOT like spectroscopy. Whole patterns match.

4 Databases To match, we need a very large database of powder patterns
ICDD (International Centre for Diffraction Data) Powder Diffraction File contains (2007) 199,574 entries (172,360 inorganic & 30,728 organic) In ye olden days it was called JCPDS…(Joint Committee for Powder Diffraction Standards) and before that ASTM

5 ICDD Example Why d and not 2 ??

6 ICDD Good….

7 ICDD Bad….

8 Search/Match Search programs assist in identifying phase mixtures:

9 Inorganic Crystal Structure Database

10 Fingerprinting.. Advantages:
relatively quick and easy, can be non-destructive Problems: need reliable standards - new phases will not be in the PDF some things in the database are rubbish! often need other (chemical) information to narrow down searches not very sensitive - can “hide” up to 10% impurities (depending on relative “weights” – see later) problems from preferred orientation, etc. not much good for organics, organometallics.

11 Preferred Orientation
Remember: we rely on a random orientation of crystallites. When crystals are platey or needle-shaped (acicular) they will pack in a non-random fashion, preferentially exposing some planes to the incident radiation. Thus some diffraction peaks will be enhanced relative to others. This can also happen if a sample is packed down, or a thin film, etc. Brushite plates, SEM by Anna Fotheringham

12 Preferred Orientation
Intensity mismatch – due to using single crystal So e.g. all (n00) peaks may be enhanced…

13 Uses: different structures
NaCl KCl Even if two structures are the same (and they are chemically similar) differences can be observed: Peak positions (unit cell changes) and relative intensities (atoms) There is another major point here: K+ and Cl- are isoelectronic

14 Uses: different structures
BUT, sometimes you can’t really see any changes on visual inspection… Zeolite A This often happens in “open” structures where there is space for change of light atoms

15 Different polymorphs will have different powder patterns
Uses: polymorphs Different polymorphs will have different powder patterns e.g. Zn S

16 Uses: polymorphs K3SO4F: tetragonal & cubic forms

17 Peak Broadening In an X-ray diffraction pattern, peak width depends on
the instrument radiation not pure monochromatic Heisenberg uncertainty principle focussing geometry the sample… - a crystalline substance gives rise to sharp lines, whereas a truly amorphous material gives a broad “hump”. What happens between the two?

18 Peak Broadening If crystal size < 0.2 m, then peak broadening occurs At <50nm, becomes significant. Why? Bragg’s law gives the condition for constructive interference. At slightly higher  than the Bragg angle, each plane gives a “lag” in the diffracted beam. For many planes, these end up cancelling out and thus the net diffraction is zero. In small crystals, there are relatively fewer planes, so there is a “remanent” diffraction

19 Peak Broadening We can calculate the average size of the crystals from the broadening: Scherrer formula t is the thickness of the crystal,  the wavelength, B the Bragg angle. B is the line broadening, by reference to a standard, so that where BS is the halfwidth of the standard material in radians. (A normal halfwidth is around 0.1o)

20 Peak Broadening Halfwidth: “Full width at half-maximum” - FWHM
This can be different in different directions (anisotropic), so by noting which peaks are broadened we can also infer the shape of the crystals.

21 Uses: particle size determination
Here we see particle size increasing with temperature 30oC 1050oC

22 Particle size determination: Example
Peak at 28.2° 2 with FWHM of 0.36° 2 Standard material has FWHM of 0.16° 2  = CuK = Å 0.36 ° = 0.36 x /180 = rad 0.16 ° = 0.16 x /180 = rad B = rad t = 255 Å = m

23 Particle size determinaton
An estimate, rather than an absolute value - also will be dominated by smallest particles. Good for indication of trends. A useful complement to other measurements such as surface area, electron microscopy etc.

24 Amorphous / micro-crystalline?
It can be difficult to distinguish between an amorphous material and a crystalline sample with very small particle size. BUT the idea of such a small size “crystal” being crystalline doesn’t make sense! 5nm = 50Å = e.g. 10 unit cells Is this sufficient for long range order??

25 Unit cell refinement As the peak positions reflect the unit cell dimensions, it is an “easy” task to refine the unit cell. 2d sin =  and e.g. Thus if we can assign hkl values to each peak, we can gain accurate values for the unit cell We minimise the difference, e.g. This is known as “least squares” refinement. We will come back to this later.

26 Variable temperature/pressure
Need special apparatus Here (see previous) we could follow a phase transition as we heated the sample up – following the change in unit cell parameters. J. M .S. Skakle, J. G. Fletcher, A. R. West, Dalton

27 BaTiO3 T/P Variable pressure hard to do: neutron diffraction (later)
Much of these data actually from dielectric measurements. T. Ishidate, PRL (1997) S. A. Hayward, S. A. T. Redfern, H. J. Stone, M. G. Tucker, K. R. Whittle, W. G. Marshall, Z. Krist. (2005)

28 Uses: more advanced Structure refinement – the Rietveld method
A refinement technique, not determination Whole-pattern fitting - not just the Bragg reflections Needs a MODEL - pattern calculated from model, compared point-by-point with observed pattern. Originally developed (1967,1969) for use with neutron data - good reproducible peak shapes first report of application to X-ray data Hugo Rietveld, b1932

29 Uses: Rietveld Refinement
x y z Ca/Ce 0.3333 0.6667 (18) Ce 0.2337(4) 0.25 Si 0.403(3) 0.380(3) O1 0.316(4) 0.467(4) O2 0.597(5) O3 0.340(2) 0.252(3) 0.071(3) O4 Here there was a similarity between the powder pattern of this phase and an existing one – also chemical composition similar. J. M. S. Skakle, C. L. Dickson, F. P. Glasser, Powder Diffraction (2000) 15,

30 Uses: more advanced Quantitative phase analysis (how much of each)
Naïve approach - relative intensity of peak maxima? - Consider mixture of Ba,Si,O - Ba component would scatter more than Si component (e.g. Ba2SiO4 c.f. SiO2) Thus uses Rietveld method and takes into account relative scattering from each crystalline phase

31 Summary Many different uses for powder X-ray diffraction!
Fingerprinting: identifying phases, distinguishing similar materials, identifying polymorphs, (following chemical reactions) Indication of particle size from peak broadening Unit cell refinement Variable temperature/pressure measurements Crystal structure refinement Quantitative analysis

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