New M₃O₅-anatase intergrowth structures formed during low-temperature oxidation of anosovite

Oxidation of anosovite at temperatures ≤ 300°C produces new phases of general composition M(n)O(2n-1), based on unit cell intergrowth of M₃O₅ and anatase structural elements. The structural principles on which the intergrowth series are based were established by analysis of high-resolution transmission electron microscope images. The structure for the dominant intergrowth phase, with composition M₆O₁₁, was refined using the Rietveld method applied to powder X-ray data. This phase has monoclinic symmetry and space group C2/m, with a = 9.946(2), b = 3.744(2), c = 20.994(3) Å, and β = 93.30(1)°. Its structure comprises planar intergrowth of M₃O₅ compositional blocks with the pseudobrookite-type structure and MO₂ compositional blocks with the anatase-type structure, parallel to (001) ≡ (001)(M₃O₅) ≡ (101)(an). Alternatively, the structure can be considered as chemically twinned anatase with (101)(an) as the twin composition plane. The anatase blocks are alternately 2-octahedra and 3-octahedra wide along [001]. Local regions corresponding to compositions M₅O₉ (twinned anatase blocks are 2-octahedra wide) and M₇O₁₃ (3-octahedra wide anatase blocks) were also observed in TEM images. The results have relevance to the digestion of high-titania slags (anosovite-rich) in the sulfateroute titania pigment process because high sulfuric acid solubility is maintained after the low-temperature treatment, while the preoxidation of trivalent titanium avoids acid consumption by solution oxidation. (C) 2000 Academic Press.