A new cuprate, Bi6Ba4Cu2O15, with a monoclinic symmetry (a 12.191 Å, b = 5.555 Å, c = 27.00 Å β = 93.31°) has been isolated. The high-resolution electron microscopy study of this phase has allowed a structural model to be established. The structure is deduced from that of the 2201-modulated superconductor by a double “collapsing operation,” and is called for this reason “double collapsed 2201-type” structure. Indeed, a first translation along c2201 of ap(1 + 2/2), every five octahedral block leads to the hypothetical n = 5-member of the collapsed family (ap is the parameter of the ideal cubic cell of the perovskite); a second translation performed in the latter structure along b2201 of ap 2 leads to the present structure. In fact, this structure consists of “2201”-type slices, parallel to (012)2201, whereas the n = 5 collapsed structure would consist of identical slices but parallel to (010)2201. The analogy of these crystallographic collapsed structures with the crystallographic shear structures suggests the existence of two families of structures characterized by two kinds of crystallographic collapsing planes (CCP), {010}2201 and {012}2201, respectively. The oxides Bi17Sr16Cu7O49-δ and Bi15(Sr, Ba)14Cu6O42 represent the members n = 8 and 7 of the first family called collapsed, whereas the cuprate Bi6Ba4Cu2O15 represents the member n = 5 of the second family called 2201-type double collapsed. © 1993 Academic Press, Inc.
