Structure, redox and transport properties of acceptor doped cerium niobates

In the search for solid oxide fuel cell (SOFC) materials, which exhibit improved oxide ion conductivity at lower temperatures, attention has recently been focussed towards materials with atypical structural chemistry and diffusion pathways. For example high oxygen diffusivity is reported in fergusonite structured CeNbO4+δ, which can incorporate a range of oxygen excess stoichiometries by oxidation of Ce3+ to Ce4+ and as a result, four commensurate or incommensurately modulated superstructures of the monoclinic parent cell are possible. An alternative strategy to reduce migration energy barriers is to change the charge carrier from oxide ions to protons, and through the introduction of oxide ion vacancies, develop materials that can conduct both species through a O2(g)/H+(g) partial pressure dependence. Alkaline earth doped rare earth niobates RE1-xAxNbO4-δ (RE=La, Nd, Gd, Tb, Er A=Ca, Sr, Ba) have been shown to exhibit proton conductivities that can be maintained to much higher temperatures compared to traditional perovskite materials, with reported stability to both protonic and acidic media. However they are hindered by low dopant solubility, and an intermediate temperature monoclinic to tetragonal phase transition that is problematic for device applications. Owing to the unique structural and redox behaviours of the cerium analogue of the RENbO4 series, this worked aimed to investigate the transport properties of acceptor doped Ca1-xAxNbO4±δ. This work has shown that Ce1-xAxNbO4±δ can cycle between oxygen hyper- and hypostoichiometry, and in the process alternate between mixed electronic interstitial oxide ion conduction, and (mixed electronic) protonic conduction respectively. Under both oxidising and reducing conditions the conductivity increases by over one order of magnitude relative to both CeNbO4+δ and the most protonically conductive of the series La1-xAxNbO4-δ respectively. This is a result of the greater solid solubility of strontium and calcium in CeNbO4+δ, relative to LaNbO4 (1-2%), and is speculated to arise from the charge compensation mechanism of electron hole formation via Ce3+-Ce4+ oxidation, where the change in both cerium valance and coordination may act to stabilise the dopant. The likelihood of electronic charge carriers in the hypostoichiometric phases of Ca1-xAxNbO4±δ, is of interest due to limited number of mixed proton-electron conductors if the properties can be optimised further. Furthermore the transition temperature raises from ~520 °C to ~650 °C on exchange of lanthanum with cerium. Unusually the hyperstoichiometric phases are also suggested to show proton conductivity, indicating Ca1-xAxNbO4±δ may exhibit more unusual defect reactions.Open Acces