Defect structure and electrical conduction behavior of Bi-based pyrochlores

Pyrochlore-type quaternary systems with the generic formula (Bi_1.5Zn_0.5)(Nb_0.5M_1.5)O₇ (BZNM, M=Ti, Sn, Zr, and Ce) have been synthesized by the standard solid-state reaction route. The study of positron annihilation lifetime spectra provides direct evidence for defect characterization for BZNM ceramics with different tetravalent element. The variation of the annihilation lifetime suggests that the defect structure undergoes significant changes. The defect structure is further described by means of the complex-defect model. Electrical conduction in the samples is expected to result mainly from the defects present in the lattice. Conduction measurement shows there are different conduction mechanisms. For temperatures less than 350 °C, electronic conduction dominates, while for temperatures greater than 350 °C, ionic conduction is dominant in the present system. The conduction mechanism includes the formation of charged defect dipoles, causing internal bias fields, creating oxygen vacancies that promote ionic conductivity.