Convergent beam electron diffraction and high resolution electron microscopy of CaFeTi₂O₆ perovskite

A new calcium iron(II) titanate ordered perovskite, CaFeTi₂O₆, synthesized at 12-15 GPa and 1200-1400°C has been studied using convergent beam electron diffraction (CBED) and high-resolution transmission electron microscopy (HREM) techniques. CBED and selected area diffraction (SAD) patterns confirm that CaFeTi₂O₆ is tetragonal with a = 7.51 ± 0.02 Å, c = 7.54 ± 0.02 Å. The tetragonal structure is consistent with the symmetry of P4₂/nmc proposed on the basis of X-ray diffraction studies. A CaFeTi₂O₆ crystal was studied at both ambient temperature and 84 K. In situ experiments show no phase transformation upon cooling. High resolution imaging and nanobeam analysis reveal three types of morphology. In addition to the major nearly perfect perovskite phase, this material contains two sizes of iron precipitates. One size class averages 4 nm in diameter and the other 15 nm in diameter. Crystalline perovskite-like domains with a high degree of disorder were also observed. The formation of this type of domain may be related to local variations of stoichiometry with considerable oxidation. The coexistence of reduced and oxidized regions in the same specimen is noteworthy, and probably indicates incomplete equilibration during high pressure Synthesis.