Phase diagram, chemical stability and physical properties of the solid-solution Ba4Nb2-xTaxO9

Through the construction of the Ba4Nb2-xTaxO9 phase diagram, it was discovered that the unique high-temperature γ phase is a thermodynamic intermediate between the low-temperature α phase (Sr4Ru2O9-type) and a 6H-perovskite. Refined site occupancies for the γ phase across the Ba4Nb2-xTaxO9 solid-solution indicate that Nb preferentially occupies the tetrahedral sites over the octahedral sites in the structure. When annealed in a CO2-rich atmosphere, all of the phases studied absorb large amounts of CO2 at high temperatures between ∼700 and 1300 K. In situ controlled-atmosphere diffraction studies show that this behaviour is linked to the formation of BaCO3 on the surface of the material, accompanied by a Ba5(Nb,Ta)4O15 impurity phase. In situ diffraction in humid atmospheres also confirms that these materials hydrate below ∼1273K, and that this plays a critical role in the various reconstructive phase transitions as well as giving rise to proton conduction. - Graphical abstract: Thermodynamic phase diagram of Ba4Nb2-xTaxO9. Highlights: → γ-Ba4Nb2O9 phase is a structural intermediate between the α and 6H-perovskite phases. → Ba4Nb2O9 and Ba4Ta2O9 decompose at high temperatures in the presence of CO2. → These materials all absorb between 5% and 6% of CO2 by mass between ∼800 and 1200 K.