Distorted spinels

Both compounds Mn304 (i.e., hausman-nite) and /-Mn203 crystallize with a tetragonally distorted spinel-type structure (see Fig. 17). 

Acceptor doping, as in lithium oxide doping of nickel oxide, produces p-type thermistors. The situation in nickel-oxide-doped Mn304 is similar but slightly more complex. This oxide has a distorted spinel structure (Supplementary Material SI), with Mn2+ occupying tetrahedral sites and Mn3+ occupying octahedral sites in the crystal, to give a formula Mn2+[Mn3+]204, where the square parentheses enclose the ions in octahedral sites. The dopant Ni2+ ions preferentially occupy... 

When compiling the information gathered by different techniques of various investigators, the view can be expressed that the promoters, among them especially CaO and AI2O3, are uniformly distributed in one or more spinel or distorted spinel phases, the latter partly existing as separate phases. This view is confirmed by the available x-ray data. 

Mn304 occurs as hausmannite, with a distorted spinel structure. It is formed when any other oxide is heated in air to 1000°. 

It is only in structures in which small proportions of the two types of hole are occupied that the nearest neighbours of an atom in a tetrahedral (octahedral) hole are only the nearest 4 (6) c.p. atoms. This is true, for example, in the spinel and olivine structures (Table 4.8). The CogSg structure is closely related to the spinel structure, which has 32 c.p. 0 atoms in the cubic unit cell. In CogSg there are 32 c.p. S atoms with 4 Co in octahedral and 32 Co in tetrahedral holes compare C03S4 with a slightly distorted spinel structure, also with 32 S in the cubic unit cell, but 16 Co in octahedral and 8 Co in tetrahedral holes. The spinel structure is discussed in detail on p.490 for the olivine structure see p. 811. An expanded version of part of Table 4.8 will be found on p. 619, where the structures of a number... 

Among the wide variety of the phases formed by the thermal decomposition of aluminum hydroxides we shall consider only alumina forms which derive from pseudoboehmite, boehmite and bayerite. Information about these structures is rather poor. With the exception of the spinel form (t -, y-), the species are mainly characterized by the dimensions of the unit cell and by some indications on their crystallographic system [20]. The structure of y-AbOs was considered as a tetragonal distorted spinel lattice (a superstructure with one unit cell parameter tripled in the case of 8-AI2O3) and is depicted by the formula A18/3 [ ] 1/3 O4, where [ ] is a cation vacancy. This formula is consistant with the constraints of stoichiometry and electrical neutrality. It is difficult to know the exact distribution of the cations in the octahedral or tetrahedral sites despite an abundance of work on this subject [21-23]. The 0-phase, isomorphous with... 

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