Phase stability and electrical conductivity of Ca-doped LaNb1−xTaxO4−δ high temperature proton conductors

The electrical conductivity, crystal structure and phase stability of La0.99Ca0.01Nb1−xTaxO4−δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, δ = 0.005), a potential candidate for proton conductor for solid oxide fuel cells (SOFCs), have been investigated using AC impedance technique and in situ X-ray powder diffraction. Partially substituting Nb with Ta elevates the phase transition temperature (from a monoclinic to a tetragonal structure) from ∼520 °C for x = 0 to above 800 °C for x = 0.4. AC conductivity of the La0.99Ca0.01Nb1−xTaxO4−δ both in dry and wet air decreased slightly with increasing Ta content above 750 °C, while below 500 °C, it decreased by nearly one order of magnitude for x = 0.4. It was also determined that the activation energy for the total conductivity increases with increasing Ta content from 0.50 eV (x = 0) to 0.58 eV (x = 0.3) for the tetragonal phase, while it decreases with increasing Ta content from 1.18 eV (x = 0) to 1.08 eV (x = 0.4) for the monoclinic phase. By removing the detrimental structural phase transition from the intermediate-temperature range, consequently avoiding the severe thermal expansion problem up to 800 °C, partial substitution of Nb with Ta brings this class of material closer to its application in electrode-supported thin-film intermediate-temperature SOFCs.

► We measured the electrical conductivity, crystal structure and phase stability of La0.99Ca0.01Nb1−xTaxO4−δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, δ = 0.005).
► Substituting Nb with Ta increased the monoclinic-to-tetragonal phase transition temperatures from ∼520 °C for x = 0 to near 800 °C for x = 0.4.
► The conductivity decreased slightly from 3.5 × 10−4 S cm−1 for x = 0 to 2.3 × 10−4 S cm−1 for x = 0.4 at 850 °C in humidified air.
► Substitution of Ta onto Nb slightly increases the activation energy of the tetragonal phase, and it decreases the activation energy for the monoclinic phase.