Conclusion

In conclusion, no changes in the incommensurate modulation of $\rm {Bi_2Sr_2CaCu_2O_{8+\delta}}$ are observed over the temperature range investigated (200K - 18K). The results confine the existence of any changes in the modulation, such as those proposed by EXAFS studies to be associated with superconductivity, to short range distortions with a correlation less than the 100${\rm\AA}$ coherence of the x-rays, or to distortions with long range correlation but which induce less than a 5% change in the intensity. A small deviation from expected behaviour is observed, as a weakening in intensity over the temperature range T$\simeq$140-100K. The anomaly is not restricted to the modulation alone as it is also observed in a fundamental reflection. The effect could be associated with an anomalous disordering, but it is at the limits of these measurements and cannot be observed with any certainty. It is noted that the temperature range for this anomaly is coincident with that cited for the anomalies observed in studies of elastic constants and internal friction peaks. In particular, the acoustic damping changes of Anderson [175] are convincing evidence for a structural change below 160K, a change suggested to be associated with oxygen arrangements [183]. An attempt to further investigate this possibility using medium-resolution methods was unsuccessful due to poor thermal stability of the sample holder combined with insufficient intensity. A repeat of the experiment at a high intensity synchrotron source would be necessary to make further progress.