Structure of spinel and related compounds

The mixed-metal oxide spinel, MgA fTt, is one of the most important inorganic materials. The structure of spinel can be regarded as a ccp structure of O2-anions with Mg2+ ions orderly occupying /8 of the tetrahedral interstices, and Al3+ ions orderly occupying half of the octahedral interstices the remainder 7/x tetrahedral interstices and half octahedral interstices are unoccupied. The sites of the three kinds of ions in the face-centered cubic unit cell are displayed in Fig. 9.6.29. 

According to the cation distribution over the 8(a) and 16(d) sites, the spinels of formula AB2O4 can be divided into normal and inverse classes. Examples of normal spinels are MgA CU and MgTi204, in which the oxidation states of A1 and Ti are both +3 their respective structural formulas are Mg2+ t A]2+ 004 and [Mg2+]t[Ti2+]o04, in which the subscripts t and 0 indicate tetrahedral and octahedral sites, respectively. 

In inverse spinels, half of the B ions are in tetrahedral 8(a) sites, leaving the remaining B ions and the A ions to fill the 16(d) sites. Usually, the occupancy of these 16(d) sites is disordered. Examples of inverse spinels are MgFe2C 4 and Fe304 their structural formulas are [Fe3+]t[Mg2+Fe3+]o04 and [Fe3+]t [Fe2+Fe3+]004, respectively. 

In intermediate spinels any combination of cation arrangement between the extremes of normal and inverse spinels is possible. Table 10.1.7 lists some normal spinels and inverse spinels and the values of their unit-cell parameter a. 

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