Localized and Itinerant Electrons in Oxides

Recent work concerning the electrical properties of ReO
3
, PrO
x
, (1.5 ≤ x ≤ 2.0), TiO
2
, SrTiO
3
, perovskites, CrO
2
, NiO, Li
x
Zn
1−x
V
2
O
4
, V
2
O
3
, and Ti
2
O
3
has been selectively reviewed in attempts to elucidate the conduction properties of the charge carriers. A number of controversial issues are pointed out: Frequently it is not known whether mobilities are activated or not, nor whether a given material should be classified as a polaronic or mixed-carrier material; neither is there agreement on the nature of electrical transitions. Discrepancies in electrical properties can generally be traced to insufficient control over preparation and characterization of samples.