The Role of Cation Point Defects and Oxygen Interstitials on Modulations in Undoped and Doped Bi-Sr-Ca-Cu-O Superconductors

Abstract

ABSTRACTVariations in superstructure modulations and transition tempeartures (Tc) in undoped and yttrium-doped Bi2Sr2CaCu2O8 (2212) superconductors annealed in different environments have been examined by microstructural analysis. The hole concentration is changed either by annealing the samples in reducing or oxidizing environments, or by partial substitution of Y3+ cations on the Ca-sites. Systematic investigations of the 2212 samples annealed in both the reducing (nitrogen) and oxidizing (oxygen) environments show commensurate and incommensurate modulations, respectively, and a decrease of Tc in oxygen. Commensurate modulations have also been found for samples prepared entirely in nitrogen. High spatial resolution microanalysis reveals that the modulations are insensitive to cation point defects. These results indicate that modulations are intrinsic to the 2212 materials and that they are caused by the lattice mismatch between the perovskite and Bi-O layers. Such periodic variations in interplanar spacings are expected to lead to satellite spots as observed in electron diffraction. The results are consistent with single crystal X-ray diffraction studies reported in the literature. In the Y- doped samples Tc values are similar in Bi2Sr2Ca1-xYxCu2O8 for x=0.2, in both the environments, but decrease with increasing value of x.