ANALYSIS OF ROLE OF OXYGEN DEFICIENCY IN CRYSTAL-STRUCTURE AND CONDUCTION MECHANISM OF BABI0.25PB0.75O3-DELTA

The dependence of oxygen deficiency, delta, of BaBi0.25Pb0.75O3-delta on temperature, T, and oxygen partial pressure, po(2), was analyzed by thermogravimetry. A linear relationship with proportionality constant about -1/2 between log delta and log po(2) was observed, suggesting that the chemical potential of the electrons, i.e., effective concentration of electron carrier for conduction, is independent of the concentration of oxygen deficiency, delta. Based on the determined delta-T-po(2) relationships, the dependence of electric conductivity at high temperature on delta was calculated. Also, the samples with various oxygen deficiencies were systematically prepared, based on the data of thermogravimetry. Variations of crystal structure and the electric conduction behavior at low temperature with delta were investigated by the measurements of X-ray diffraction, resistivity and Seebeck coefficient. As delta increased, the crystal system of BaBi0.25Pb0.75O3-delta changed from orthorhombic through tetragonal to pseudo-cubic. Increase of resistivity at room temperature by an order of about four was observed as delta increased, suggesting that the mobility of electron carriers decreased drastically with oxygen deficiency. This decrease of electron mobility was consistent with the relationship between the oxygen content and electrical conductivity of the sample at high temperature. Also consistent with the decrease of electron mobility was the decrease of overlapping of wave function with introduction of oxygen deficiency, assuming variable range hopping conduction at low temperature. Decrease of Fermi level with delta, observed by Seebeck coefficient measurement, also agreed with the decrease of electron mobility.