Miran Čeh
“Jožef Stefan” Institute, Ceramics Department, Jamova 39, Ljubljana, Slovenia
 

ABSTRACT

Oxide-rich planar faults with a rock-salt-type structure in perovskite grains are the prevailing type of planar defects in polycrystalline AO-doped CaTiO₃ (A=Ca,Sr,Ba). These so-called Ruddlesden-Popper (RP) faults form random network structures or ordered polytypes and polytypoids, depending on the processing parameters. The ordering of RP faults was observed by various high-resolution STEM imaging techniques. The local chemical composition was investigated by energy-dispersive X-ray spectrometry (EDXS) and electron energy-loss spectrometry (EELS). The achieved point-to-point resolution in bright-field (BF) and high-angle annular dark-field (HAADF) STEM imaging was below 0.27 nm. In Z-contrast imaging, it was possible to resolve 15% of the difference in the average atomic number (Z) between two adjacent atomic columns. Chemical analysis showed that Sr and Ba preferentially substitute for Ca in the CaTiO₃ lattice, thus forming  a  solid  solution  of (Ca_1-xSr_x)TiO₃ and (Ca_1-xBa_x)TiO₃. The excess Ca ions form single or ordered CaO-rich planar faults in the host solid-solution phase. Sr and Ba do not substitute for Ca on the sites at the fault. The polytypoids are comprised of a sequence of single CaO faults followed by a different number of (Ca_1-xSr_x)TiO₃ or (Ca_1-xBa_x)TiO₃ perovskite layers