THE FERRITES BIN+1SR2N+2BAN-1FEN+1O6N+4 - A SERIES OF INTERGROWTHS OF THE 2201 AND 0201 STRUCTURES

A new series of layered ferrites with the ideal formula Bi-n+1 ST2n+2Ban-1Fen+1O6n+4 has been synthesized. The ED and HREM studies of these phases show that they represent the three first members of a series of intergrowths of the 2201 (Bi2Sr2CuO6-type) and 0201 (K2NiF4-type) structures. The parameters of their tetragonal cells are related to the a(p) parameter of the perovskite and to the c(2201) and c(0201) parameters in the following way: a approximate to a(p) and c approximate to 1/2 (c(0201) + n.c(2201)) for the odd-n members or c approximate to c(0201) + n.c(2201) for the even-n members. The key point of this study deals with the stabilization of these structures by introducing barium ore the bismuth sites. Problems of nonstoichiometry on the cationic sites in the bismuth layers and of oxygen nonstoichiometry are discussed. The disappearance of the modulation of the structure by introducing barium, as is in the 2212 cuprate Bi1.5Ba2.5LaCu2O8.25, is also evidenced and interpreted in terms of space for the 6s(2) lone pair of Bi(III). (C) 1995 Academic Press, Inc.