Five Co-ordinated Titanium in K₂Ti₂O₅

Abstract

IT is widely believed that oxygen atoms may be extracted at random from the lattices of metallic oxides, and X-ray powder photographs have suggested that little or no modification of the structure of the original stoichiometric substance occurs. In particular, evidence has been adduced that the perovskite lattice can persist between the limits ABO ₃—ABO _2.5, provided that the six co-ordinated ion B is one of the metals, titanium, vanadium, cobalt or iron¹. In order to evaluate this proposal, we are examining the crystal structures of ordered, oxygen-deficient compounds in which the ionic radii comply with the rules enunciated by Goldschmidt² for the perovskite structure. K₂Ti₂O₅ was recently reported by Schmitz-Dumont and Reckhard³ to have a large unit cell of low symmetry, and this could, perhaps, be due to the regular omission of some of the oxygens from a perovskite lattice. We now wish to report briefly that a complete X-ray structure determination does not support this view.