Synthesis under high pressure of vacancy-ordered phases in the system CaFeO3 –y

Abstract

The perovskite-related oxide Ca₂Fe₂O₅ takes up additional oxygen under high pressure and elevated temperature. The Mössbauer spectra from a number of preparations establish the existence of new phases which are structurally related to the original lattice. The latter features an ordered arrangement of oxygen vacancies so as to form alternate layers of octahedral and tetrahedral Fe polyhedra. The introduction of oxygen to give CaFeO_2.60 initially increases the number of octahedral layers to give a sequence [ootot], and some of the iron in the double-octahedral layers undergoes oxidation to the 5+ oxidation state (by a nominal charge disproportionation of Fe⁴⁺ at low temperature). At least six distinct iron sites can be identified, and their coordination numbers established. It is believed that the charge disproportionation is reversed with increasing temperature. Further uptake of oxygen to give CaFeO_2.69 produces some layers with five-coordination. The Mössbauer spectra reveal the existence of unusually complicated and subtle magnetic interactions, which include spin-reorientation effects and may be triggered by a change in oxidation state of some cations. Oxidation can extend to at least CaFeO_2.80 before any substantial intergrowth of CaFeO₃ becomes apparent. It is clear that a wide range of non-stoichiometry can be achieved by the intergrowth of layers of Fe in different coordination, and the extra oxygen is counterbalanced by the generation of Fe⁵⁺ cations in octahedral coordination. The charge disproportionation appears to be established within two-dimensional layers of corner-sharing octahedra.