Pronounced impact of atmospheric conditions on Ba4Nb2O9 and Ba4Ta2O9

► Wet argon lowers the temperature of the phase transitions in Ba4Nb2O9 and Ba4Ta2O9. ► Heating under vacuum allows the low-temperature alpha phase to persist up to 1723 K. ► Vacuum retards the monoclinic ➔ hexagonal symmetry transition in 6H-Ba4Ta2O9..