The Bi-2212 sample chosen for this experiment was the Oxford crystal grown by the flux method in a platinum crucible and previously characterised in Chapter 3. The very high quality of this crystal was demonstrated by these measurements and makes it the most suitable for this study. The sample was mounted using silicon rubber on to the copper holder of the cryostat. The temperature was controlled by a closed cycle helium cryostat, maintained to within 0.5K. The x-ray alignment was as described in Chapter 3.
The experiment was first performed in low-resolution mode using graphite monochromator and analyser crystals. To ensure the reproducibility and highest possible accuracy of the results, four temperature cycles were completed over the range T=200-20K, measurements being made in stages, during both heating and cooling cycles, at temperatures of 197K, 138K, 118K, 98K, 69K and 20K. Stabilisation after each change in temperature was followed by a least-squares refinement of the lattice parameters, calculated from the positions of five fundamental reflections. Checks of the relative fundamental intensities showed the alignment of the crystal to be unchanged throughout the experiment. An attempt was also made to repeat the measurements using medium-resolution germanium monochromator and analyser crystals. The sensitivity of the medium-resolution measurements to very slight changes in the sample position with changes in temperature proved to be too much to maintain the alignment, however, and reliable intensity comparisons could not be made.