Presentation on theme: "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."— Presentation transcript:
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
Powder XRD – the equipment
Uses: fingerprinting Single or multi-phase NOT like spectroscopy. Whole patterns match. Two different crystalline phases are present in this pattern – one in a very small amount
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
ICDD Example Why d and not 2 ??
Search/Match Search programs assist in identifying phase mixtures:
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.
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. This can also happen if a sample is packed down, or a thin film, etc. Brushite plates, SEM by Anna Fotheringham Thus some diffraction peaks will be enhanced relative to others.
Preferred Orientation Intensity mismatch – due to using single crystal So e.g. all (n00) peaks may be enhanced…
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
Uses: different structures BUT, sometimes you cant really see any changes on visual inspection… This often happens in open structures where there is space for change of light atoms Zeolite A
Uses: polymorphs Different polymorphs will have different powder patterns e.g. Zn S
Uses: polymorphs K 3 SO 4 F: tetragonal & cubic forms
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?
Peak Broadening If crystal size < 0.2 m, then peak broadening occurs At <50nm, becomes significant. Why? Braggs 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
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 B S is the halfwidth of the standard material in radians. (A normal halfwidth is around 0.1 o )
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.
Uses: particle size determination Here we see particle size increasing with temperature 30 o C 1050 o C
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
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.
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 doesnt make sense! 5nm = 50Å = e.g. 10 unit cells Is this sufficient for long range order??
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.
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
BaTiO 3 T/P S. A. Hayward, S. A. T. Redfern, H. J. Stone, M. G. Tucker, K. R. Whittle, W. G. Marshall, Z. Krist. (2005) T. Ishidate, PRL (1997) Variable pressure hard to do: neutron diffraction (later) Much of these data actually from dielectric measurements.
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
Uses: Rietveld Refinement xyz Ca/Ce (18) Ce0.2337(4) Si0.403(3) 0.380(3) 0.25 O10.316(4) 0.467(4) 0.25 O20.597(5) 0.467(4) 0.25 O30.340(2) 0.252(3) 0.071(3) O 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,
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. Ba 2 SiO 4 c.f. SiO 2 ) Thus uses Rietveld method and takes into account relative scattering from each crystalline phase
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