MODELING THE ATOMIC DISPLACEMENTS IN BI2SR2CAN-1CUNOX SUPERCONDUCTORS

The incommensurate displacive modulation waves in Bi2Sr2Can-1CunOx are investigated using high resolution transmission electron microscopy. The image contrast is compared with simulated images calculated with d ifferent models of the modulation waves. The static atomic displacemen ts are calculated in one step on all equivalent sites of a periodicall y continued average structure. In contrast to other methods symmetry o perations are not required for the evaluation of the correct phase par ameters to create the whole modulated structure. This facilitates the creation of the simulated structure, and incommensurate modulation wav e vectors can be used, too. The superstructures of the 2212, 2223 and especially the 2201 phase can be simulated by this method. For the 220 1 phase both types of the incommensurate modulation known from the lit erature are simulated. A connection between the type of the 2201 modul ation and the modulation wave vector is found. The Pb modulation in (B i,Pb)2Sr2Can-1CunOx is also described by a similar formalism using dat a from the literature. Different models of the oxygen arrangement in t he Bi layers of the 2212 phase are compared using HREM images and cont rast simulations. The correlation between atomic shifts of the Bi atom s in [100] direction and the occupations of different oxygen sites in the BiO layers is shown.