Knut W. Urban (1,2), Chun-Lin Jia (1,2) and Hong Wang (2)

(1) PGI-5 and Ernst Ruska Center, Research Center Juelich, Juelich/Germany.
(2) School of Electronic and Information Engineering and State Key Laboratory for Mechanical Behaviour of Materials, Xi'an Jiaotong University, Xi'an/China.

Pyrochlores, chemical formula A2B2O7, form an extended class of materials with interesting properties. The compound Bi1.5ZnNb1.5O7 (BZN) is prototypical. Its excellent dielectric properties make it attractive, e.g. for capacitors, tunable microwave devices and electric-energy storage equipment. Investigating BZN we report on the first study on a pyrochlore by atomic-resolution transmission electron microscopy. In this study – at the forefront of what modern electron microscopy can do – we are able to realize picometer-precision in our images of the close atom pairs with a separation of only 20 pm. We find that the position of the atomic intensity maxima do not coincide with the projected Wyckoff positions of the basic pyrochlore lattice. This supplies atomic-scale evidence for displacive disorder on split (cage-like) A-type sites. From our studies on atom mobility during microscopic observation we can conclude that it is the Zn-atom jumps and not those of Bi or Nb that are responsible for dielectric relaxation.