Pronounced impact of atmospheric conditions on Ba4Nb2O9 and Ba4Ta2O9

Ba4Nb2O9 and Ba4Ta2O9 decompose upon heating in a CO2-rich atmosphere. Here, we report further in situ synchrotron X-ray diffraction studies under CO2-free atmospheres of variable humidity and under vacuum and show that this strongly influences the reconstructive phase transitions normally seen. Under normal atmospheric conditions, α-Ba4Nb2O9 transforms to γ-Ba4Nb2O9 at 1493 K and α-Ba4Ta2O9 transforms to 6H-Ba4Ta2O9 at 1513 K. Under wet argon, these transitions shift to lower temperatures of 1453 K and 1443 K for Ba4Nb2O9 and Ba4Ta2O9 respectively. Upon heating both the α samples under vacuum, no phase transition is observed at all up to 1723 K. Heating 6H-Ba4Ta2O9 under vacuum saw the monoclinic → hexagonal symmetry-transition temperature increase from 1050 K under air to 1700 K. The significance of these observations in terms of possible practical applications of their mixed oxide ionic, protonic and electronic conduction properties are discussed.