Semi controlled-valency

The incorporation on a cation site of an impurity cation of higher charge than the host cation can stabilize a lower valence state of the host cation. The addition of titanium (as Ti02> to the spinel, zinc ferrite, leads to an B-type semi-conductor of this type of formula, Zn (Ti + Fe t28 Fe ) O. The defect centre may be described as an impurity cation of higher relative charge on a cation site plus a quasi-free electron bound on a neighbouring host cation. A necessary condition for the application of the prindple of controlled valency is that the impurity cation should be of much the same size as the host cation. 

Baf Mna" " Mn t Ou -(2 - 8) H2O. It has not been found possible to differentiate Mn + and Mn + in these structures, presumably because they are not ordered. These two minerals and cryptomelane are natural controlled-valency semi-conductors. A more detailed study of the structural relationships of the manganese oxide noinerals should provide useful data on the geochemistry of manganese and on the defect solid state in general. 

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