Interaction of D2O with a Thick BaO Film: Formation of and Phase Transitions in Barium Hydroxides

The interaction of D2O with a thick BaO film ((ge)20 mono-layer equivalent (MLE)) on ultra-thin Al2O3/NiAl(110) was investigated with temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Upon D2O exposure of a thick BaO film amorphous barium hydroxide formed at room temperature that readily converted to crystalline Ba(OD)2 phases during annealing in ultra-high vacuum (UHV). The formation of crystalline hydroxide phases depends on the initial D2O exposure at 300 K. Following low D2O exposure at room temperature that results in the formation of amorphous barium hydroxide with no hydrating water, only the a-Ba(OD)2 phase was observed after 400 K annealing. The sample that was exposed to D2O extensively (i.e. hydrated amorphous barium hydroxide formed) showed a series of phase transformations as the sample was annealed to increasingly higher temperatures: amorphous-Ba(OD)2 · xD2O (x > 1) → β-Ba(OD)2 ·D2O → β-Ba(OD)2 → α-Ba(OD)2. The results of TPD experiments completely agreed with this phase transformation scheme: hydrating water molecules desorbed first at 425 K, allowing the formation of the β-Ba(OD)2 · D2O phase. Desorption of water from β-Ba(OD)2 · D2O at around 475 K leads to the formation of β-Ba(OD)2 and its subsequent conversion to α-Ba(OH)2. All the barium hydroxides thermally decomposed at T < 550 K. When the BaO film was exposed to D2O at 425 K crystalline b-Ba(OD)2 formed initially, which lead to the formation of a small amount of α-Ba(OD)2 as well at low D2O exposures. At high D2O exposures the dominant phase was β-Ba(OD)2 · D2O, and no α-phase was seen