Structural study of the oxide Pb2Sr2PrCeCu3O10, intergrowth of 0201- and 0222-type structures

The structure of the oxide Pb2Sr2LnCeCu3O10 was determined from powder X-ray and neutron diffraction data in the orthorhombic cell a = 5.4522 (2) Angstrom, b = 5.4807 (2) Angstrom and c = 37.012 (1) Angstrom, space group Fmmm. This study is completed by an H.R.E.M. study. The structure consists of an intergrowth of 0201 and 0222 structures, i.e., it derives from the Pb2Sr2YCu3O8 structure by replacing one [YO2]∞ simple fluorite layer by a [Ln2O4]∞ double fluorite layer. The main feature deals with the anionic disordering in the [PbO]∞ layers, leading to the tetrahedra PbO3L, in which L characterizes the 6s2 lone pair of Pb(II). The twofold coordination of Cu(1) is also shown but no conclusion about the linearity of the Cu1O2 group can be drawn owing to the existence of a mirror plane at the level of Cu(1). The CuO5 pyramids corresponding to Cu(2) exhibit a regular basal plane and the lanthanides are off center in the fluorite cages, having a 4 + 4 coordination. In spite of the existence of diffuse streaks and additional weak spots on the E.D. patterns, the H.R.E.M. images exhibit, for most of them, a very regular sequence in agreement with the N.D. and X.R.D. study; only some extended defects forming triple fluorite layers are observed in the bulk, whereas large fluorite domains appear on thin edges of the crystals. 21 refs., 11 figs., 3 tabs