ABSTRACT
Bismuth-based superconductors are often referred to as having serial compositions Bi2Sr2CaA!1Cum02m+4 with m = 1, 2, and 3 [1-3] and as being layered into ||BiO/SrO/ Cu(l)02/Ca(l)/ • • • /Ca(m - l)/Cu(m)02/SrO/BiO|| [4]. Both these compositions and structures are, however, considerably simplified in comparison with the real ones. The real compositions are nonstoichiometric with respect to both the cationic and anionic elements. In addition, the atomic positions in their real structures are considerably shifted from the "ideal" ones so that each phase is featured by an incommensurate, one-dimensional structural modulation.
