Experimental Details

The ${\rm Bi_2Sr_2CaCu_2O_{8+\delta}}$ sample chosen for this study was the large Beijing crystal which had been previously well characterised at room temperature, the results having been presented in Chapter 3. Although of an intermediate quality, it was chosen as it was not wished to inflict the irreversible degradation, which could result from high temperature annealing, upon a crystal of higher quality. The Beijing sample also has the advantage of allowing the diffuse scattering around the odd (0 0 $l$) positions, something not present in the high quality crystals, to be observed.

The alignment for the x-ray measurements was as described in Chapter 3. Triple crystal geometry was utilised with graphite monochromator and analyser crystals chosen to make available the maximum possible x-ray intensity. The sample was mounted on a ceramic disc using Ge ceramic cement as the adhesive, and the disc was then screwed into the copper block support of the high-temperature furnace. The furnace is specially constructed for the x-ray diffractometer to allow in situ measurements of this kind. A Lakeshore temperature controller was used with a chromel-alumel thermocouple attached to the ceramic mount to monitor temperature. The temperature was found to be stable to at least $\pm$1$^o$C over the entire scale. The experiment was carried out with the furnace under a permanent vacuum, which was maintained in the region 4.0x10$^{-8}$ to 1.0x10$^{-8}$ bar throughout.

The experiment was performed in two essentially distinct stages, and the presentation of the results will follow this accordingly. The first stage involved observations at intervals from room temperature up to 400$^o$C, at which temperature the sample was allowed to anneal for a period of 110 hours whilst more detailed measurements were made. At the end of each stage, the sample was returned to room temperature for comparison with the previous set of measurements to allow the extent of any irreversible changes to be established. The second stage involved the heating of the sample to 450$^o$C, at which temperature it was then maintained for an extended period, to allow time evolving processes induced solely by the vacuum annealing to be observed. The lattice parameters were determined at frequent intervals during each of the heating cycles and always before a set of measurements. The results for this are presented in their entirety, for the complete experiment, as a final section.