One-step synthesis of single-phase, bulk Bi2Ti2O7 pyrochlores by deep undercooling rapid solidification and its formation mechanism

Preparation of dense Bi2Ti2O7 ceramics with single-phase pyrochlore structure is challenging using the conventional sintering method due to its thermal instability at ≥ 650 °C. Here we synthesized dense and phase-pure Bi2Ti2O7 ceramics from a mixture of Bi2O3 and TiO2 by a novel deep undercooling rapid solidification method using the aerodynamic levitation technique. We reported a surprising discovery that the pyrochlore phase can exist with Bi/Ti ratio varying between 0.9 and 1.1. By a comprehensive investigation on the phase purity, crystal structure, oxidation states and electrical properties, we revealed that the pyrochlore structure is primarily stabilized by the solidification process rather than presence of a large amount of Ti3+. With the advantages of being containerless, one-step and fast processing, deep undercooling rapid solidification is an effective and efficient approach to prepare dense Bi2Ti2O7 pyrochlores, and can be expanded to other thermally unstable compounds to obtain bulk materials.