Chemistry and structural modulations in Bi2Sr2CuO6

Abstract

A series of samples along the composition lines Bi2+xSr2−xCuOy, and Bi2Sr2−xCuOy have been used to study the structural modulation, chemistry, and superconducting properties of pseudo-tetragonal Bi2Sr2CuO6 (2201). The 2201 phase can be formed from crystallization of thin glassy platelets. The sample displayed a strong crystallographic (00l) orientation which made it possible to determine incommensurate modulations near (00l) reflections using a conventional x-ray θ-2θ scan. From the crystallization of the 2201 phase, it was found that structural modulation was intrinsic to the phase, and ordering of the structure required a long time at high temperature. High temperature in situ x-ray diffraction of a 2201 Bi2Sr1.85CuOy platelet showed that the modulation existed at 875 °C in O2 (Tmelt ≍ 892 °C in O2). These suggest that the structural modulation cannot be caused solely by oxygen ordering and that metal-ion displacement must be involved. By removing 0.04 to 0.05 oxygen atom per formula unit from Bi2Sr2CuOy and Bi2Sr1.85CuOy, the c* components of the modulation changed from 0.31 to 0.26 and from 0.38 to 0.31, respectively, while the b* component of the modulation remained approximately 0.2. This demonstrates that oxygen, while not the sole cause, does play a role in the formation of the structural modulation. However, the invariance of bmod with respect to the change in oxygen content does not support the model that explained the modulation by inserting extra oxygen in the BiO plane. By varying metal-ion concentrations of Bi and Sr we found that both the lattice parameters and the modulation vectors depended more on the Bi/Sr ratio than on the Sr concentration alone. As the Bi/Sr ratio increased from 1.0 to 1.35, the modulation lines moved toward the (00l) reflections. The corresponding superstructural periodicities were calculated to vary from ∼1/5b* + 0.32 c* to ∼1/5 b* + 0.63 c*. Effects of oxygen content and metal-ion concentration on the 2201 phase formation and the superconducting properties will also be discussed.