Structural Study of the Oxide Pb 2Sr 2PrCeCu 3O 10, Intergrowth of 0201- and 0222-Type Structures

Abstract

The structure of the oxide Pb 2Sr 2LnCeCu 3O 10 was determined from powder X-ray and neutron diffraction data in the orthorhombic cell a = 5.4522(2) Å, b = 5.4807(2) Å and c = 37.012(1) Å, 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 Pb 2Sr 2YCu 3O 8 structure by replacing one [YO 2]∞ simple fluorite layer by a [ Ln 2O 4]∞ double fluorite layer. The main feature deals with the anionic disordering in the [PbO]∞ layers, leading to the tetrahedra PbO 3 L, in which L characterizes the 6 5 2 lone pair of Pb(II). The twofold coordination of CU (1) is also shown but no conclusion about the linearity of the Cu 1O 2 group can be drawn owing to the existence of a mirror plane at the level of CU (1). The CuO 5 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.