Sebastian Wachowski

Influence of Sb-substitution on ionic transport in lanthanum orthoniobates

The results of ionic transport measurements for the lanthanum orthoniobate substituted with 10 and 30 mol% of antimony (LaNb0.9Sb0.1O4 and LaNb0.7Sb0.3O4) are presented and discussed. The influence of calcium co-doping on these properties has also been analysed. It has been shown that for the investigated material protonic conductivity predominates at temperatures up to 800 °C in oxidizing atmospheres under wet conditions. The maximum observed protonic conductivity reaches ∼10−4 S cm−1 at 800 °C (in humidified air); under dry conditions, the increasing influence of oxygen vacancies and holes is detected. Oxygen self-diffusivity has also been analysed by isotopic exchange to investigate the possible diffusion paths.

Tailoring structural properties of lanthanum orthoniobates through an isovalent substitution on the Nb-site

A tetragonal polymorph of lanthanum orthoniobate can be stabilized to room temperature by the substitution of Nb with an isovalent element. LaNb1−xAsxO4 (0 < x ≤ 0.3), where As is an element stabilizing the tetragonal structure, were successfully synthesized using a combined co-precipitation and solid-state reaction method. The phase transition temperature, above which the material has a tetragonal structure, decreases linearly with increasing As content, and LaNb0.7As0.3O4 is tetragonal at room temperature. The analysis of the influence of different isovalent substituents, namely V, Sb and Ta, has shown that there is a relation between the properties of the chemical element and its effect on the structure. It was proposed that the electronegativity of the substituent determines the type of stabilization – the tetragonal/monoclinic structure is stabilized by chemical elements with electronegativity higher/lower than that of niobium. The slope of the phase transition temperature dependence on the substituent content has been proposed as a parameter determining the “quality” of the stabilization, since a steeper function leads to a larger decrease of transition temperature for the same content of different substituents. The analysis has shown that apart from the electronegativity, the stabilization of the tetragonal structure depends also on the ionic radius of a substituent. Arsenic has been found to be a better stabilizer of the tetragonal polymorph of lanthanum orthoniobate than Sb, but worse than V.