Powder X-ray Diffraction
Experimental Set-up:
Need ca. 20mg or more of microcrystalline powder
Individual crystallites ca. 50microns
Normally use monochromatic Cu-K radiation- Wavelength 1.54Å
Use 40kV, 50mA and 2kW sealed X-ray tube.
Measure to max. Bragg 2 of ca. 40o for organics, or 80-100o for inorganics
Results:
Get 1-D diffraction pattern or 'spectrum' of Bragg angle versus diffraction
intensity. This should be unique for each type of crystalline substance.
Results tell whether sample is
a)
Crystalline or not - peaks should be sharp and strong
b)
Simple pattern - high symmetry or preferred orientation
c)
Allows phase identification - compare with other samples
d)
Comparison with 'JCPDS' file - allows identification of compounds in
database
e)
For mixtures can use for semi-quantitative analysis
f)
Can refine the unit cell parameters by indexing the reflections.
g)
Can even solve or refine the structure if done carefully. (Rietveld
analysis) This is non-routine and a 'model' structure must normally be
used as a starting point.
Problems with Powder Diffraction:
a)
Preferred Orientation
Peak intensities can vary quite strongly from one sample to the next.
The database records patterns of substances prepared as microcrystalline
samples. If your crystallites have a different shape then peak intensities will
be affected by the probability that the crystallites are in diffracting orientation.
b)
Comparing Samples
The d-spacings of your unit cell and the diffraction angles that result should
not vary greatly. Sometimes there is a 'zero offset' error when you run the
pattern. Try subtracting or adding 0.1 or 0.15o and see whether two sets of
peaks now overlap !
Be careful in comparing mixtures with pure samples - i.e you may have two
sets of overlapping peaks, rather than a new material !
CHEM 534 Class Exercise:
Symmetry Operations, Equivalent Positions and Space Groups
The general positions for the monoclinic space group P2 are
1.
x, y, z
2.
-x, y, -z
What are the general positions for the following monoclinic space groups ?
a)
Pm
b)
P21 (Total of 2)
(Hint: Change 2-fold to 2-fold screw)
c)
C2 (total 4)
(Hint: add C-lattice centering to ab faces)
d)
P21/m (total 4)
(Hint: combine answers to P21 and Pm)
(Total of 2)
(Hint: Change 2-fold to a mirror)