Influence of microstructures on conductivity in Tysonite-type fluoride ion conductors

In this study, dense sintered polycrystalline compacts (relative density: >95%)of Tysonite-type fluoride ion conductor, La_0.93Ba_0.07F_2.93, with different grain sizes were prepared by applying the spark plasma sintering method, and their electrical conductivities were evaluated in order to understand the influence of microstructures on the ionic conductivity. The conductivity of the prepared compacts increased when increasing the sintering temperature up to 1273 K, and showed the highest value, e.g. 7.1 × 10⁻⁵ S cm⁻¹ at 300 K, which was much higher than the reported values for sintered compacts of the same material. These were because the application of the spark plasma sintering technique enabled us the high densification of the sintered compacts and the increase in the sintering temperature enhanced the grain growth resulting in the decrease in the grain boundary resistance. The inhibitory effect of grain boundary on the ionic conduction was not significant when the density of the polycrystalline compacts was sufficiently high. On the other hand, when the sintering temperature was higher than 1273 K, small pores were formed along the grain boundaries and the conductivity was degraded with increasing the sintering temperature. Such a decrease in the conductivity could be interpreted by the increase in the grain boundary resistance due to the pore formation by the high temperature sintering. This work demonstrated that, for sintering polycrystalline (La,Ba)F₃-based fluoride ion conductors, there is an optimum preparation condition to achieve the optimum microstructure i.e. high density, large grain size, and clean grain boundary, for high fluoride ion conduction.