A neutron and x-ray diffraction study of calcium aluminate glasses

Spallation neutron diffraction and high-energy x-ray diffraction methods have been used to study CaO:Al2O3 glasses at the 64:36 mol% eutectic and 50:50 mol% compositions. The samples were produced by the containerless cooling of liquid droplets heated by a laser beam and suspended in an aerodynamic levitator. The results show aluminium on average to be surrounded by 4.0(1) oxygen atoms at a distance of 1.76(1) Angstrom in CaAl2O4, which increases to 4.8(1) at the eutectic composition. The two techniques have also been combined to reveal the local structure of the calcium atoms in the glass. In CaAl2O4 the calcium is found to be surrounded, on average, by 5.6(2) oxygen atoms at a distance of 2.38 Angstrom. The Ca coordination decreases to the unusually low value of 3.9(2) oxygen atoms at a distance of 2.40 Angstrom at the eutectic composition. No additional Ca-O correlations are observed up to 2.7 Angstrom, but longer bonds cannot be ruled out. The higher-r correlations are shown to be similar in the two glasses, suggesting that both Al and Ca may act as network formers. The results are compared to previous studies on splat-quenched glasses with compositions near the eutectic.