CONVERGENT-BEAM ELECTRON-DIFFRACTION AND HIGH-RESOLUTION ELECTRON-MICROSCOPY OF CAFETI2O6 PEROVSKITE

A new calcium iron(II) titanate ordered perovskite, CaFeTi2O6, synthes ized at 12-15 GPa and 1200-1400 degrees C has been studied using conve rgent beam electron diffraction (CBED) and high-resolution transmissio n electron microscopy (HREM) techniques, CBED and selected area diffra ction (SAD) patterns confirm that CaFeTi2O6 is tetragonal with a = 7.5 1 +/- 0.02 Angstrom, c = 7.54 +/- 0.02 Angstrom. The tetragonal struct ure is consistent with the symmetry of P4(2)/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 trans formation upon cooling, High resolution imaging and nanobeam analysis reveal three types of morphology, In addition to the major nearly perf ect perovskite phase, this material contains two sizes of iron precipi tates, One size class averages 4 nm in diameter and the other 15 nm in diameter, Crystalline perovskite-like domains with a high degree of d isorder were also observed, The formation of this type of domain may b e related to local variations of stoichiometry with considerable oxida tion, The coexistence of reduced and oxidized regions in the same spec imen is noteworthy, and probably indicates incomplete equilibration du ring high pressure synthesis. (C) 1996 Academic Press, Inc.