University of Surrey
Chemical Sciences
Level 3 Distance Learning
Crystal Structure Determination
As announced previously, this part assignment (the first, longer, part was Inorganic Spectroscopy) and the associated assessment are closely tied to tutorial teaching in Oxford Chemistry Primer 60, Crystal Structure Determination by Bill Clegg. Students may also find it beneficial to have access to a standard Physical Chemistry text such as that by Atkins.
The Primer is economically priced and students should note the availability of this text (e.g. from www.amazon.co.uk or from Oxford University Press http://ukcatalogue.oup.com)
ORDER IT NOW IF YOU HAVE NOT ALREADY DONE SO.
note that the Primer is printed to order and there could therefore be a brief delay in your receipt of it.
The assignment overleaf requires that you (i) read the primer and construct brief notes on key issues (for your own use, not to be submitted) and (ii) answer questions which exemplify the application of the underlying principles.
RCTS 12/11
The maximum mark that can be gained is 30/30.
Start each whole question on a new page.
Read each question carefully – the questions relate to coverage in the Clegg Primer but require you to apply more widely the knowledge and insights that you will be gaining.
1. What does the term crystalline mean? In that context, discuss what is meant by each of the terms:
(i) Structure (ii) Unit cell (iii) Lattice
[5]
2. Give an account of the physical chemistry underlying the link between:
(i) the existence of crystal planes (with {Miller} indices hkl) in ordered materials which have unit cells with known atomic contents and atomic positions (x,y,z) within them and
(ii) the observation of peaks in intensity I in diffracted monochromatic X-ray scattering at particular scattering angles J.
[5]
Your answer should include the Bragg law and also an equation linking atom positions with the structure factor F.
3. State the relationship between the observed intensities of peaks in scattering of monochromatic X-rays and the structure factor F applying to a unit cell. What is the phase problem in X-ray crystallography and how are steps taken to overcome it?
[5]
Include Patterson maps and direct methods in your answer.
4. Answer all parts.
(a) What is a 4-circle diffractometer (used historically) and how does an area detector improve data gathering?
[2]
(b) Give an account of twinning and of the effects of vibrational {thermal} parameters, noting that the amplitudes of vibrations are temperature dependent.
[1]
(c) In what ways is multi-variable least squares fitting important in crystallography? What are R factors and how are they used?
[2]
5. Answer all parts.
(a) In what ways are X-ray and neutron scattering similar and in what ways do they differ?
[2]
You need to consider the origin of the wavelength in both cases, differences in the angular variation of atomic/nuclear scattering factors, the sources of the radiation in each case, the size of sample needed for each technique.
(b) It can be very difficult to position hydrogen atoms using results from X-ray crystallography. Why might this be so?
[1]
(c) It can be similarly difficult to position oxygen atoms in oxides of heavy metals, such as in UO2+x. Suggest a technique that could be used to position the oxygen atoms in such an oxide.
[1]
(d) Lithium atoms often cannot be positioned accurately or precisely by crystallographic methods (neither by X-ray nor by neutron techniques). Suggest how this can be the case.
[1]
6. For 2 compounds of your own choosing, give flow charts showing the procedures enabling the determination of the crystal structure using X-ray crystallographic data.
[5]
An exemplar flow chart is given on p27 of the Primer. There are several word descriptions of crystal structure determinations in Chapter 3 of the same text; you need to make the charts specific to each compound.
/End of assignment
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