Pronounced impact of atmospheric conditions on Ba4Nb2O9 and Ba4Ta2O9

Abstract

Ba(4)Nb(2)O(9) and Ba(4)Ta(2)O(9) decompose upon heating in a CO(2)-rich atmosphere. Here, we report further in situ synchrotron X-ray diffraction studies under CO(2)-free atmospheres of variable humidity and under vacuum and show that this strongly influences the reconstructive phase transitions normally seen. Under normal atmospheric conditions, alpha-Ba(4)Nb(2)O(9) transforms to gamma-Ba(4)Nb(2)O(9) at 1493 K and alpha-Ba(4)Ta(2)O(9) transforms to 6H-Ba(4)Ta(2)O(9) at 1513 K. Under wet argon, these transitions shift to lower temperatures of 1453 K and 1443 K for Ba(4)Nb(2)O(9) and Ba(4)Ta(2)O(9) respectively. Upon heating both the alpha samples under vacuum, no phase transition is observed at all up to 1723 K. Heating 6H-Ba(4)Ta(2)O(9) under vacuum saw the monoclinic -> hexagonal symmetry-transition temperature increase from 1050 K under air to 1700K. The significance of these observations in terms of possible practical applications of their mixed oxide ionic, protonic and electronic conduction properties are discussed. © 2012, Elsevier Ltd.