CRYSTAL-STRUCTURE OF PB2SR2MCU3O8+DELTA SYSTEM (M = ND, SM, EU, GD, DY, Y1-XCAX, HO AND ER)

The crystal structure of Pb2Sr2MCu3O8 (M = Nd, Sm, Eu, Gd, Dy, Y1-xCax, Ho and Er) has been analyzed by the Rietveld method of X-ray powder diffraction data. In order to best fit the data several structural models with the different space groups have been tried. Among them the two models with the space groups of Cmmm and P22(1)2 in which the oxygen atoms on the PbO sheet are displaced along the b-axis from the midposition between Pb atoms within the PbO sheet gave the best results. It is shown that the lattice parameters and the interatomic distances vary with the ionic radius of lanthanoides. The systematic variation of the atomic arrangement in the unit cell which consists of stacking of sheets having a different role is argued in comparison with the prototype of the Ba2LnCu3O6+delta system. The bond lengths of the constituent atoms are also argued with special emphasis on the local environment of two copper sites. In the Pb2Sr2(Y1-xCax)Cu3O8 system, the interatomic distances change more largely by Ca-doping than by the effect of the size of the lanthanoide atoms.