STRUCTURAL TRANSITION IN BARIUM SUBOXIDE CLUSTERS

Ionization potentials of ${\mathrm{Ba}}_{n}{\mathrm{O}}_{m}$ clusters $(2\ensuremath{\le}n\ensuremath{\le}13,m\ensuremath{\le}n)$ have been measured by one-photon ionization. Beyond $n\ensuremath{\ge}8,$ a specific structural transition is observed when the oxygen/barium ratio increases. A tentative interpretation based on the interplay between Ba-Ba and Ba-O bindings and its consequences in terms of oxygen atom localization and stability of the bare barium skeleton are discussed. A comparison with mixed clusters involving other electronegative elements is approached. Combined with metallic atoms (alkali, alkaline-earth), oxygen seems to have a specific behavior which excludes segregation.