BI-2201 PHASES - SYNTHESIS, STRUCTURES AND SUPERCONDUCTING PROPERTIES

The Bi2+xSr2-xCuO6+delta (Bi-2201) phases and La substituted Bi-2201 c ompounds with general formulas Bi2Sr2-xLaxCuO6+delta and Bi23-xSr1.7La xCuO6+delta were prepared and characterized by X-ray powder diffractio n, electron diffraction, iodometric titration and resistance measureme nts. The monophasic samples of Bi2+xSr2-xCuO6+delta were obtained in a ir at 850 degrees C for 0.15 less than or equal to x less than or equa l to 0.4; while synthesis at 740 degrees C in oxygen flow extended thi s range to the stoichiometric cation composition (0 less than or equal to x less than or equal to 0.4). The superconducting transitions were not detected for these monophasic samples in contrast to the La subst ituted phases which exhibited superconductivity, and the highest T-c,T -onset = 33 K was found for the Bi2Sr1.6La0.4CuO(6.33) compound. Super conductivity in the Bi2Sr2-xLaxCuO6+delta series exists in overdoped a nd underdoped regions. Incommensurately modulated structures of nonsup erconducting Bi2.3Sr1.7CuO6.23 phase and superconducting Bi2Sr1.7La0.3 CuO6.28 phase (T-c,T-onset = 26 K) were refined from X-ray powder data by a Rietveld technique using the four-dimensional space group P:A2/a :-11. The maximal Cu displacements from the average position in the fi rst structure was found to be sufficiently larger than in the latter o ne. The distorted structural arrangement of the (CuO2) layers in the B i2.3Sr1.7CuO6.23 structure can be a reason for the suppression of supe rconductivity in this phase, while their less corrugated configuration in the La containing Bi-2201 structure leads to the existence of supe rconductivity.