High-pressure synthesis and crystal structure of cafeti₂o₆, a new perovskite structure type

Single-crystal and powder samples of a new calcium iron(II) titanate, CaFeTi₂O₆, have been synthesized at 12-15 GPa and 1200-1400°C, and studied by X-ray diffraction. This compound is a variant of the perovskite structure and crystallizes in the tetragonal space group P 4₂/nmc, a = 7.5157(2) Å, c = 7.5548(2) Å, and Z = 4. Calcium and two iron atoms are ordered on three different A sites, with calcium in approximately 10-fold coordination, with one iron atom in tetrahedral coordination (Fe-O = 2.084(2)/Å), and the other iron atom in square-planar coordination (Fe-O = 2.097(2) Å). The system of octahedral tilts which this compound adopts has not been observed previously in a perovskite. The tilt system was described in a theoretical treatment of perovskites (a⁺a⁺c— in the notation of A. M. Glazer (Acta Crystallogr. Sect. B 28, 3384, 1972) and was (incorrectly) assigned to space group Pmmn.