Phase diagram, chemical stability and physical properties of the solid-solution Ba4Nb2−Ta O9

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 ∼ 1273 K , and that this plays a critical role in the various reconstructive phase transitions as well as giving rise to proton conduction.