maximum intensity, the upper scan having a step size of 0.10° and the lower 0.06°.
A comparison between the x-ray diffractograms of the silicon (111) peak made before and after the rebuild shown in Figure 3.9 and Table 3.2 demonstrates a considerable improvement in the peak profile and statistics; the same improvement is seen in the statistics for the (220) and (311) reflections, displayed in Table 3.2. Figure 3.10 plots the resolution function of the rebuilt diffractometer, showing a steady drop with angle; the minimum useful diffraction angle that could be measured with the rebuilt instrument was -4.0° 20.
Before Rebuild After Rebuild
Index FWHM S/N a FWHM S/N a
(111) 0.412 34.34 5.439 0.198 50.13 5.431
(220) 0.427 34.49 5.431 0.222 59.24 5.429
(311) 0.427 20.77 5.433 0.241 38.99 5.431
Table 3.2: the fu ll width at h a lf maximum (FWHM), signal to noise (S/N) and cubic cell dimension derived from dijfractograms o f the same silicon standard taken before and after the rebuild. At 298 K the standard value fo r a -5 .4 3 0 9 There is a great improvement in the fu ll width and signal to noise values.
0.008 - 0.007 - 0.006 - o CD < 0.005 - 0.004 - 0.003 -
Figure 3.10: Resolution o f the rebuilt diffractom eter fro m silicon standard, showing AO/6 (in degrees) versus the reflection angle o f the main silicon peaks.
3 .4 .7
XRD experimental procedure
The first step after loading a new sample was to determine its alignment with respect to the beam. The attachment between the cryostat and the centre stick allowed the stick, and thus the sample, to be rotated independently of the cryostat, this rotation being equivalent to the 0 rotation of the diffractometer. In order to find the sample's alignment the detector arm was driven to the 20 position of a strong reflection o f the sample; initially the graphite (1,0) peak was used, since this would be present with only a minor distortion in any graphite intercalation compound (see Chapter 1). The sample was rotated until a maximum in the x-ray flux at the detector was seen; since graphite (1,0) is an ab-plane peak, this corresponds to the sample being in correct alignment for transmission mode. Later the very strong (003) reflection was used instead after it had been determined for Rb- and K-GICs by early experiments; the flux maximum for this peak corresponds to
I I I I
30 40 50 60 70 80 90
the correct alignment for reflection mode. A 360° scale on the stick allowed the operator to rotate the orientation of the sample between reflection and transmission modes once the alignment of the sample is determined.
The experimental scans to get the ab-plane data on the samples were quite slow, usually taking 15 hours. This was a result of several factors: the samples were highly absorbing, with only ~8% transmission for the Rb-GICs, and were not easily made thinner after preparation; the polym er windows and beryllium can in the cryostat absorbed another 50% of the flux; the optical arrangement needed accommodate the cryostat made the beam flight path very long; and finally the a-b plane structures were often poorly ordered resulting in weak scattering intensities.
c-axis scans were much more rapid, with the most common scan time around 2 hours, as a result of the very strong c-axis peaks.
After data was collected it was uploaded using file transfer protocol (ftp) to one of the RSC's Hewlett-Packard Unix servers and from there transferred to other computers for reduction and analysis.
R eferen ces
1. A. Inaba, J. Skarbek, J.R. Lu, R.K. Thom as, C.J. Carlile, and D.S. Sivia, The librational ground state o f monodeuterated methane adsorbed on graphite. Journal of Chemical Physics, 1995. 103: p. 1627-1624.
2. C. Morphett, PhD Thesis, 1986, Oxford.
3. E.P. Gilbert, P.W. Reynolds, and J.W. W hite, Characterisation o f a Basal Plane Oriented Graphite. Journal of the Chemical Society, Faraday Transactions, 1998. 94(13): p. 1861-1868.
4. G. Lockhart, Personal communication, 1992.
5. J.P. Beaufils, T. Crow ley, T. Raym ent, R.K. Thom as, and J.W. W hite, Tunnelling o f hydrogen in alkali metal intercalation compounds I: C25Rb(H2)x and C24Cs(H2)x• Molecular Physics, 1981. 44: p. 1257-1269.
6. A. Herold, Recerches sur les composes d'insertion du graphite. Bull. Soc. Chim. Fr., 1955. 187: p. 999.
7. A. Herold, C rystalio-C hem istry o f Carbon Intercalation C om pounds, in In terca la ted Layered M a te ria ls, F. Levy, Editor. 1979, D. Riedel Publishing Company: Dordrecht, p. 321-422.