Oxygen vacancy ordering in CaTiO3-CaFeO2.5 perovskites:: From isolated defects to infinite sheets

The oxygen vacancy ordering process in the system CaTiO3-CaFeO2.5 has been studied as a function of composition and temperature. The samples (CaTi1-xFexO3-x/2 with 0 less than or equal to x less than or equal to 1) were annealed at temperatures between 900 degrees C and 1400 degrees C and characterized, after drop quench, by means of XRD, Mossbauer spectroscopy and TEM. Vacancy ordering starts from isolated oxygen vacancies randomly distributed in the lattice. They first cluster into chains of finite length which increases with increasing Fe content or decreasing temperature. Eventually, the chains become infinitely long and arrange in planes with tetrahedrally coordinated cations that alternate with layers of octahedrally coordinated ones. Long-range order of T (tetrahedral) and O (octahedral) planes develops in samples with x greater than or equal to 0.50. Mossbauer area ratio data indicate that at low Fe contents Ti must be present, partially, in pentacoordinated sites while at higher x this cation must be distributed over octahedral as well as tetrahedral sites.