Stabilization of the delta-phase in Bi2O3 thin films

• Normally Bi2O3 has a monoclinic structure (α) at room temperature, which transforms to cubic-fluorite (δ) structure at about 725 °C.
• However, the δ-phase can exist at room temperature when prepared as a nanometric thin film.
• The δ-Bi2O3 thin films can be stabilized from room temperature to 500 °C by Ta doping, while undoped films suffer phase changes at about 250 °C.

In this paper, we report the stabilization of the δ-phase of Bi2O3 thin films from room temperature (RT) to 500 °C by the addition of tantalum ions. The δ-Bi2O3 phase is the material presenting the highest ionic conductivity; as bulk it is stable in a reduced temperature range from 730 to 825 °C, while the α-phase is the RT stable phase. However, when produced by atomic aggregation methods as a nanometric thin film, the delta-phase can be maintained at atmospheric conditions and it actually reverts to the alpha-phase only after thermal annealing, such transformation usually occurs around 250–350 °Cby passing through the β-phase. In this work, we report that tantalum addition during the deposition of the Bi2O3 films by magnetron sputtering allows the maintenance of the delta phase up to 500 °C upon thermal annealing in air.