Calcium Aluminate Glasses
Calcium aluminate glasses have the potential for a variety of mechanical and optical applications; [20, 21, 38-46] however, they are difficult to form. Addition of SiO2 can be used to improve glass-forming ability, although this reduces the optical properties, particularly the transparency to infrared, so it is best avoided. Studies show that the best glass-forming composition in the CaO-Al2O3 binary is close to the composition 64CaO-36Al2O3 .
Calcium aluminate glasses form from “fragile” liquids , and these deviate from an Arrhenius viscosity-temperature relation. Because of these distinct rheological properties, calcium aluminate glasses have been extensively studied by diffraction and spectroscopic techniques. The composition-dependence of calcium aluminate structures was studied by McMillan for almost the entire range of CaO-Al2O3 liquids  using extremely rapid quench techniques. Extensive NMR and Raman data obtained from these rapidly quenched glasses show a range in Al-O coordination. For CaO/Al2O3 < 1, the glasses are dominated by [IV]Al. NMR and Raman data indicate that there are changes in mid-range order and also in relaxation time (i. e., viscosity), as expected for fragile liquids . The changes in Raman and NMR spectra are interpreted as different degrees of distortion of the Al-O coordination polyhedron as the identity of next-nearest neighbor changes. Raman data support this interpretation, in that there is no evidence for change in AlO4 polymerization. Similarly, X-ray absorption spectroscopy shows dramatic changes in spectra with quench rate, and changes in next-nearest neighbor. For calcium aluminates it is argued that the rearrangement of next-nearest neighbors reflects over – and under-bonding of the central ion in the Al-O coordination polyhedron, dependent on the degree of distortion.
Neutron and combined neutron and X-ray diffraction data for 64:36 and 50:50 calcium aluminate glasses [40, 48] have been used to determine Al-O and Ca-O coordination environments and mid-range order changes. These studies show that the Al-O correlation at 0.176 nm and the area below this peak yield a first-neighbor coordination number of 4.8. There is a peak in the pair-correlation function at 0.234 nm, which corresponds to Ca-O; the area beneath this peak yields a coordination number of 4.0, inconstant with the value obtained from the radial distance by bond-valence theory . Further examination of the diffraction data reveals a second Ca-O distance at 0.245 nm. Combined diffraction data suggest that the Ca-O polyhedron is quite distorted  and that the glass consists of a corner-shared Al-O framework with the Al-O units corner – and edge-shared with distorted Ca-O polyhedra.