HIGH-PRESSURE PEROVSKITES ON THE JOIN CATIO3-FETIO3

An exploratory high-pressure study of the join CaTiO3-FeTiO3 has uncov ered two intermediate perovskites with the compositions CaFe3Ti4O12 an d CaFeTi2O6. These perovskites have ordering of Ca2+ and Fe2+ on the A sites. Both of these perovskites are unusual in that the A sites cont aining Fe2+ are either square planar or tetrahedral, due to the partic ular tilt geometries of the octahedral frameworks. For CaFe3Ti4O12, th e structure has been refined from a powder using the Rietveld techniqu e. This compound is a cubic double perovskite (SG Im ($) over bar 3, a =7.4672 Angstrom), isostructural with NaMn7O12. Fe2+ is in a square-pl anar A site (similar to Mn3+ in NaMn7O12) with Fe-O = 2.042(3) Angstro m, with distant second neighbors in a rectangle at Fe-O = 2.780(6) Ang strom. Calcium is in a distorted icosahedron with Ca-O = 2.635(5) Angs trom. CaFeTi2O6 crystallizes in a unique tetragonal double perovskite structure (SG P4(2)/nmc, a=7.5157(2), c=7.5548(2)), with A-site iron i n square-planar (Fe-O = 2.097(2) Angstrom) and tetrahedral (Fe-O = 2.0 84(2) Angstrom) coordination, again with distant second neighbor oxyge ns near 2.8 Angstrom. Rietveld refinement was also performed for the p reviously known perovskite-related form of FeTiO3 recovered from high pressure (lithium niobate type). This compound is trigonal R3c, with a =5.1233(1) and c=13.7602(2). The ordered perovskites appear to be stab le at 12-15 GPa and CaFe3Ti4O12 is found as low as 5 GPa. Thus these p erovskites may be important to upper mantle mineralogy, particularly i n kimberlites. These compounds are the first known quenchable perovski tes with large amounts of A-site ferrous iron, and add greatly to the known occurrences of ferrous iron in perovskites.