New M3O5-Anatase Intergrowth Structures Formed during Low-Temperature Oxidation of Anosovite

Abstract

Oxidation of anosovite at temperatures ≤300°C produces new phases of general composition MnO2n−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 sulfate-route 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.