Environmental Engineering Reference
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Figure 5.10 (a) Schematic illustration of an oxidant-deficient compound MX 2 δ in
which doubly charged oxygen vacancies predominate. The electrons are assumed to be
localized at M atoms on regular lattice sites, M′ M . (b) Schematic illustration of an oxidant-
deficient compound (excess metal), M 1 δ X 2 , in which doubly charged interstitial cations
predominate. The electrons are assumed to be localized at M
M .
oxides the electronic conductivity involves transport of positive holes for which
they are called p-type semiconductors (positive charge carrier).
In metal-deficient oxides where the disorder is limited to the cation sublattice,
particularly in those cases when the concentration of defects is high, the point
defects may be more complex than that presented in Figs. 5.10 and 5.11. Neutron
diffraction studies by Roth [12] on w ¨ stite (Fe 1 δ O) have indicated that the va-
cancy concentration is not in accord with the ideal defect model concept. The
resultant complex defect for this system may thus be considered as an interstitial
ion adjoined by two vacancies, (V
Fe ), as illustrated in Fig. 5.12. They
are referred to as Roth clusters. Such complexes can further increase in size with
association of additional vacancies, leading to clusters containing 4 trivalent inter-
Fe Fe •• i V
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