Spinel long-range order

Fig. Z4. 1 1 long-range order in inverse spinels, resulting from the alternating occupancy of octahedral sites by two types of cation. Only octahedral cations are shown (Aburto et al 1982).

Fig. 2.9. Long-range order in spinels for 1 3 ratio of cations on octahedral sites (Aburto et al, 1982).

Less frequently, diffraction patterns such as those seen in Fig. 2.30b were obtained, typically from areas rich in promoter oxides. They arise from the spinel structure of magnetite/hercynite particles, with little long-range order. Such particles can only diffract X-rays very poorly and may well remain undetected in a phase analysis by XRD. The existence of a disordered phase has also been mentioned in Ref. 2. The poor crystallinity of the promoter oxides is in full agreement with their genesis as a product of exsolution from the oxide precurser, which has been described in the preceding sections. The dispersion of this poorly ordered phase was found to be much higher in catalysts 2, 3, and 5 than in catalyst 1, where this phase formed seams within the activated material. 

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