Electrical property and defect structure of lanthanum-doped polycrystalline barium titanate

Electrical conductivity of two types of lanthanum-doped barium titanote ceramics with different dopant levels was measured at temperatures between 900 and 1250° C andP _{O 2}from 10^–5 to 1 atm. The activation energies of the conduction for the two are interpreted in terms of the formation energy of ionized oxygen vacancies even in such a highP _{O 2}region. This fact is in contrast with a well -known controlled -valency model proposed for rare- earth -doped semi-conducting perovskites. In a lightly lanthanum-doped specimen, semiconduction achieved at elevated temperatures is retained on cooling the specimen to room temperature, whereas in a heavily doped specimen, the resultant high-temperature semiconduction changed to insulation on cooling. The former behaviour on cooling is successfully explained by a metastabilization of oxygen vacancies accompanied by electrons formed at elevated temperatures.