Temperature-dependent changes in the structure of high-quality polycrystalline Bi2Sr2CaCu2O8+x material

Refinement of neutron powder diffraction patterns from high-quality polycrystalline Bi2Sr2CaCu2O8+x (Bi-2212) material has revealed a number of temperature-dependent structural changes coinciding with changes in the measured physical properties. Changes in the b-axis lattice parameter and in the period of the incommensurate structural modulation at 160 K coincide with a sharp internal friction peak measured by an acoustic composite bar technique. Further, these changes should have an effect on angle-resolved photoemission studies and may provide an alternative explanation for recent results which cite a pseudogap explanation. Below 40 K, the changes correlate with single-crystal neutron-depolarization measurements which show that flux pinning appears. These results also seem to indicate a differential expansion between the BiSrO3 rocksalt sublattice and the CaCu2O3 perovskite-like sublattice over a broad temperature range. We suggest that the cause of the observed modulation in the Bi-2212 structure is not due to extra oxygen atoms binding onto the Bi atoms but is more likely simply the mismatch of the two sublattices.