It is shown that two vanadium oxides of unknown composition, i.e. VOx [Tilley & Hyde, J. Phys. Chem. Solids, (1970), 31, 1613-1619] and phase Q [Colpaert, PhD Thesis, Rijksuniv. Gent (1972)] are identical with V4O9 [Théobald, Cabala & Bernard, C. R. Acad. Sci. Paris, SOr. C, (1969), 269, 1209-1212]. The unit cell of V4O9 is orthorhombic, with a = 8.235, b = 10.32, c = 16.47 Å. A model for the structure of V4O9 is proposed. It is based on an ordered superlattice of O vacancies in V2O5, and is in agreement with X-ray oscillation photographs. The model accounts for the low density of this material, its low temperature of formation, the typical appearance of twins and the topotaxic growth of V4O9 on V2O5.
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Grymonprez et al. (1977) studied this question.