It was noted that the X-ray diffraction patterns of catalysts that were based upon this material exhibited peak broadening effects which could not be explained by strain and crystallite size effects. It was shown that the extensive broadening of peaks for which the h or l indices were odd was due to extended defects. The latter could be modelled as stacking faults which were perpendicular to the a- and c-axes. The proposed defects also explained streaking effects which were seen in single-crystal X-ray diffraction and electron diffraction patterns. One of the proposed defects could explain the presence of V5+ in this phase. The amount of V5+ could be closely correlated with the defect concentration. Nuclear magnetic resonance data also confirmed that the V5+ was associated with the present material, rather than with a second phase. The defects were most prominent in samples which were designed to be optimum catalysts for the oxidation of n-butane to maleic anhydride.

P.T.Nguyen, A.W.Sleight, N.Roberts, W.W.Warren: Journal of Solid State Chemistry, 1996, 122[2], 259-65