Abstract
A series of solid solutions of nominal composition Bi 8Sr 8− x Ca 4+ x Cu 9O 33− y (0 ⩽ x ⩽ 4) have been prepared and characterized by X-ray diffraction, copper valency and density measurements. The series is characteristic of the 2212 structure type with a domain of homogeneity which at 860 °C only slightly exceeds x Ca = 3. Within the calcium concentration interval studied, the lattice parameters (mainly the c parameter), average valency of copper and sample density all exhibit a smooth decrease vs. x Ca. The decrease in sample density is not as large as expected for the (Sr 1− x Ca x )-type substitution performed. The series can be described analytically via a structural model in which the oxygen sublattice (32 + δ atoms) remains practically intact throughout the range of homogeneity but the metallic sublattice exhibits an x Ca-dependent deficiency. The total number of vacancies present in the 28-atom metallic sublattice decreases vs. x Ca and at the terminal composition of the series ( x Ca = 3) it becomes zero. The observed effect of a steady densification of the Me sublattice explains satisfactorily why even in the case of isoelectronic cation substitution a substantial decrease in the average valency of copper can be observed. The paper also contains some important conclusions concerning local charge imbalances existing in Bi-based cuprates and their role as the main source of the overall power factor for oxidation higher than 2+ observed in these phases irrespective of whether they exhibit superconducting or non-superconducting character.
Published Version
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