DEFECT STRUCTURE, ELECTRICAL-PROPERTIES AND TRANSPORT IN BARIUM-TITANATE .7. CHEMICAL DIFFUSION IN NB-DOPED BATIO3

The reequilibration kinetics of polycrystalline Nb-doped BaTiO3 were monitored by measuring electrical conductivity as a function of temperature and oxygen partial pressure over the ranges 1150 < T (K) < 1425 and 220 < p(O2) (Pa) < 10(5). Application of parabolic and logarithmic equations to the kinetic data gave the same values of the chemical diffusion coefficient for both oxidation and reduction runs as well as for both isothermal and isobaric experiments. The mobility of Ti vacancies (rather than that of Ba vacancies) is interpreted to be the major factor controlling gas/solid diffusion kinetics. The following relation between chemical diffusion coefficient and temperature was obtained: D(chem) = 8.83 x 10(-5) exp {-2.47[eV]/kT} (cm2s-1). This value is two orders of magnitude lower than that reported by Wernicke for La-doped BaTiO3. The difference is analysed using all the available data for each experiment. The p(O2) exponent of electrical conductivity was determined as 1/4. This value is in agreement with a compensation mechanism involving cation vacancies rather than electrons.