High temperature protonic conduction in aragonite-type NdBO₃

Electrical conduction in undoped and Ca-doped NdBO₃ with the orthorhombic aragonite-type structure was investigated by using conductivity measurements under 0.4-5 kPa of p(H₂O) and 0.01-100 kPa of p(O ₂) at 500-925°C. From humidity dependence and H/D isotope effect of the conductivity, it was found that NdBO₃ became a protonic conductor under wet conditions by partially substituting Ca for Nd. The conductivities of l mol% Ca-doped NdBO₃ were 10^-5.4- 10^-4.1 S ·cm^-1 under wet condition at 500-925°C. As temperature increased, electron holes became to contribute to the conduction, although protons were estimated as dominant charge carriers under the conditions employed. The p(H₂O) and p(O₂) dependence of the conductivity were discussed in terms of defect equilibrium. Conductivity behaviors as well as conductivity values of Ca-doped NdBO ₃ were rather similar to those of the Sr-doped LaBO₃ with the same crystal structure.