Applications to metal oxides and other inorganic solids

Phillips and Williams gave some general guidelines to determine whether good overlap would occur. A d band formation is likely to occur if:

(1) The formal charge of the cations is small.
Therefore TiO is metallic whereas TiO2 is an insulator.

(2) The cation occurs early in the transition series.
Therefore TiO and VO are metallic whereas NiO and CuO are poor semiconductors.

(3) The cation is in the 2nd or 3rd transition series.
Therefore Cr2O3 is a poor conductor whereas the lower oxides of Mo and W are good conductors.

(4) The anion is reasonably electropositive.
Therefore NiO is a poor conductor whereas NiS and NiSe are good conductors.


Effects (1) to (3) are involved in keeping the d orbitals spread out to reduce the amount of positive charge they feel from the parebt ion.

Effect (4) is associated with reducing ionicity and the band gap as previously discussed on page 18.


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