Glass-forming ability in the (BaO)${}_x$(Al${}_2$O${}_3$)${}_{1-x}$ system ($0\le x\le 1$) was investigated by using the containerless aerodynamic levitation and laser-heating method. The main glass-forming region was found to occur for 0.40(2) $\le x\le$ 0.48(2), where there is insufficient oxygen to form an ideal network of corner-sharing AlO${}_4$ tetrahedra in which the oxygen atoms are twofold coordinated, with another narrow glass-forming region at $x$ $=$ 0.62(2) around the eutectic composition. The glass corresponding to $x$ $=$ 0.4 was chosen for further investigation by using both neutron and x-ray diffraction, and a detailed atomistic model was built by applying a combination of molecular dynamics and reverse Monte Carlo methods. The results show a network structure based predominantly on corner-sharing tetrahedral AlO${}_4$ motifs in which triclusters (OAl${}_3$ units formed by three tetrahedral Al atoms sharing a common vertex) play an integral part, with as many as 21$\%$ of the oxygen atoms involved in these configurations. The barium ions bind to an average of 7.4 O atoms, most of which are twofold-coordinated bridging oxygen atoms. The larger size of barium compared to calcium narrows the range of glass-forming compositions in alkaline-earth aluminates such that the main glass-forming range corresponds to a regime in which an oxygen-deficient Al-O network is stabilized by the formation of triclusters.

• Received 10 November 2011

DOI:https://doi.org/10.1103/PhysRevB.85.064201