Abstract
Two types of materials in which the highest filled and lowest empty electron bands are mainly cationic have been investigated, viz. Bi 4Ti 3O 12 and Bi 2WO 6 with a filled Bi 6s band and empty Ti(W) 3d(5d) bands and NiNb 2O 6 and CoNb 2O 6 with filled narrow transition metal bands and empty Nb 4d bands. Bi 4Ti 3O 12 shows weak photocurrents in a photoelectrochemical water decomposition cell, whereas Bi 2WO 6 does not. Also NiNb 2O 6 and CoNb 2O 6 behave differently. Whereas NiNb 2O 6 shows photocurrents, CoNb 2O 6 does not. These results are discussed in terms of bulk properties. The absence of a photocurrent is related to the formation of small polarons with high activation energy for migration. In agreement with this the compound Bi 2WO 6 shows luminescence.
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