Structure and bonding in single- and two-phase alumina-based glasses

Pulsed neutron and high energy X-ray diffraction experiments on single- and two-phase Al2O3-Y2O3 and single-phase Al2O3-La2O3 glasses suggest that a polyamorphic phase transition occurs via changes in the Y-O coordination and connectivity. The transformation from a high temperature to a low temperature amorphous phase is mainly associated with the formation of YO8 polyhedra at the expense Of YO7 polyhedra. The transition produces a more ordered network structure with increased oxygen connectivity, while maintaining the overall metal:oxygen stoichiometry. It is estimated that for the two-phase sample studied similar to12% of the Y ions had transformed to the higher coordinate form in the supercooled liquid as the onset of the glass transition was reached.