Co-Doping Effect of Mn and Y on Charge and Mass Transport Properties of BaTiO3

BaTiO^sub 3^-based multilayer-ceramic capacitors (MLCC) using base metal (Ni) electrodes normally contains Mn and Y each approximately on the order of 0.5 mol%. It is only empirically known that the co-doping of Y and Mn facilitates sintering with the base-metal electrodes as well as improves the device performance and life time. In order to understand the effect of the co-doping, we have measured the electrical conductivity and chemical diffusivity on polycrystalline BaTiO^sub 3^ that is co-doped with Y and Mn each by 0.5 mol% against oxygen partial pressure at elevated temperatures. It is found that while the n-type conductivity in reducing atmospheres (e.g., Po^sub 2^ < 10^sup - 6^ atm at 1000^sup ^C) remains similar to that of undoped or acceptor-doped BaTiO^sub 3^, its p-type conductivity in oxidizing atmospheres (e.g., Po^sub 2^ > 10^sup - 6^at 1000^sup ^C) is remarkably suppressed compared to the latter. The chemical diffusivity is also similar to that of the latter in magnitude (e.g., 10^sup - 2^-10^sup - 5^ cm^sup 2^/s at 1000^sup ^C), but its trend of variation with oxygen partial pressure is rather opposite. These variations of the conductivity and chemical diffusivity are mainly attributed to Mn ions changing their valence from +2 to +3 to +4 with increasing oxygen partial pressure. It is explained from a defect-chemical view why the codoping of fixed-valent donor (Y) and variable-valent acceptor (Mn) has been practiced in MLCC processing.[PUBLICATION ABSTRACT]