CRYSTAL-STRUCTURE AND DIELECTRIC-PROPERTIES OF THE MIXED AURIVILLIUS PHASE BI7TI4NBO21

The crystal structure of the ferroelectric mixed Aurivillius phase Bi7 Ti4NbO21 was determined for the first time by X-ray diffractometry usi ng a single crystal obtained by a flux method in Bi2O3 excess. It can be described as a regular stacking of double and triple perovskite-typ e slabs interleaved by Bi2O2 layers along the a axis. Nb is located in the double perovskite-type layers only with a random Nb/Ti distributi on whereas the octahedral sites within the triple perovskite-type laye rs are occupied by Ti only. The thermal evolution of the dielectric co nstant between room temperature and 900 degrees C show two phase trans itions at 650 and 830 degrees C. Combined analyses of experimental dat a support that the highest transition temperature corresponds to the C urie temperature, whereas the lowest one could be assigned to a phase transition within the orthorhombic symmetry by a change of the space g roup.