X-ray structural studies, mainly by powder techniques, have been made of a number of ABO3-type mixed oxides. A cubic form of deformed perovskite structure, with eight formula units per cell, has been found in BaThO3, SrSnO3, and SrZrO 3. BaCeO.3 and BaSnO z have the ideal perovskite structure; orthorhombic distortions of this structure occur in SrCeOa and CaSnO3, and rhombohedral distortions in KIO 3 and TIIO 3. Doubt is cast on the identification of a number of 'perovskites' reported previously. Structure changes with composition in several series of perovskite solid solutions are described.

A detailed study has been made of products obtained by oxidizing magnetite FenFein04, prepared under a variety of controlled conditions, in oxygen at various temperatures. Specimens of magnetite which gave gamma ferric oxide on oxidation invariably contained appreciable percentages of water, while specimens prepared under dry conditions oxidized with great difficulty, never yielding the gamma oxide. It has also been found that the gamma oxide itself contains a small percentage of water, which apparently cannot be removed without destroying the characteristic spinel structure. It is suggested that current views on the defect structure of gamma ferric oxide are inadequate, and that the structure is stabilized by the presence of hydroxyl ions which replace some of the oxygen ions of the spinel lattice. Preliminary experiments on the oxidation of hercynite, FeA1204, a structural analogue of magnetite, are reported.Gamma ferric oxide (y-Fe2O3) is a relatively unstable, ferromagnetic, cubic form of the oxide, generally prepared by oxidation of magnetite (Fex1Fe2n*04) at a moderate temperature, or by dehydration of a corresponding gamma hydrate (7-FeOOH, lepidocrocite) prepared from certain ferrous compounds by oxidation in solution or in suspension.1 X-Ray examination shows the y-oxide to possess the well-known spinel structure, although the ratio of metal cations to oxygen ions in Fez03 is considerably smaller (0.67) than that characteristic of a spinel (0-75). The generally accepted structure of the y-oxide, proposed independently by Hagg 2 and by Verwey,3 is accordingly a defect structure ; the unit-cell content is formulated as Fe21.33032.00, and the 21.33 Fe3+ cations are considered to be distributed statistically over the 24 possible cation positions, 2.67 cation sites

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Accurate lattice constants for ferrous oxide (wustite) have been determined at 1oOO" over the composition range in which this non-stoichiometric phase is stable ; the limits of the range, at 10o0", have been established by extending the measurements into the two-phase regions on either side. The effects of varying conditions of heat-treatment have given information on the mode of decomposition of wustite into magnetite (Fe304) and iron.In preparation for later work, the curve of lattice constant against composition for solid solutions of wustite and manganous oxide has been established with increased accuracy. The effect of dissolved manganous oxide on the composition range for wustite has been determined at one Fe : Mn ratio.The measurements of lattice constant were carried out in conjunction with a thermodynamic study of the three-phase system : 0 2 (g) I " FeO+ MnO " solid solution I Fe+ Mn solid alloy.

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