Miller cubic phases

As already mentioned the l40Qd440 ratio may be assumed to be a measure of the distribution ratio between the occupancy of the tetrahedral and octahedral sites in the cubic cell of the spinel-type phase. According to Miller data of the octahedral site preference energies (31), it is assumed that the Cr ions are all located in octahedral sites, whereas the Zn and Cu ions are present in both tetrahedral and octahedral sites, according to results previously reported for Zn/Cr catalysts (14-16). From Table 3, it is possible to observe that the tendency of the M ions to be retained in the octahedral positions decreases with increasing calcination temperature and copper content, indicating that the CuO side phase detected may be mainly attributable to the segregation of Cu ions present in octahedral positions of the spinel- type phase. The behaviour of (2at B is noteworthy ... 

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