ON THE CRYSTALLOCHEMISTRY AND SUPERCONDUCTIVITY OF THE (BI1+ -XCU1+/-X)8SR6CA6CU9O24+Z/

A series of solid solutions of the (Bi1-xCux)8Sr6Ca6Cu9O24+z type with 0.08 greater-than-or-equal-to x greater-than-or-equal-to -0.225 has b een prepared in air at 850-degrees-C and identified through X-ray. den sity, average valency of copper and low temperature resistivity measur ements. The Cu-rich side of the series starts from x congruent-to 0.06 , whereas that of Bi-rich one, comparatively wider, reaches compositio n very close to x = -0.202. Within the single phase region, lattice pa rameter a of the 2212-structure type unit cell, remains practically co nstant (around 0.54 nm). At the same time, the parameter c exhibits a regular decrease vs. growing concentration of bismuth and finally it r eaches the value 3.0625 nm at x = 0.202. The average valency of copper within the series shows a minimum value 2.17 at x = 0.00. Starting fr om this value it grows up linearly in both directions, slightly along the Cu-rich side and more efficiently along the Bi-rich one. The atomi c content of the elementary unit cell (Z), expressed as number of mole cules of MeO1+y-type (Me = sum of the metallic structure components in their atomic fractions) and calculated from sample densities, indicat es that the structure deficiency is characteristic of all the series. However, within its Bi-rich part the deficiency occurs to be much deep er than in the Cu-rich equivalent. Thus, the intermediate composition with x = 0 i.e. Bi8Sr6Ca6Cu24+9V(Cu)/2, characteristic of minimum defi ciency, minimum valency of copper and minimum T(c), divides the series into two sub-regions, each of extremely different type of structure d eficiency and different critical temperature behaviour.