Polyhedral Units and Network Connectivity in Calcium Aluminosilicate Glasses from High-Energy X-Ray Diffraction

Structure factors for $\mathrm{Ca}{}_{x/2\mathrm{}}\mathrm{Al}{}_x\mathrm{Si}{}_{1-x}\mathrm{O}{}_2$ glasses $(x=0,0.25,0.5,0.67\mathrm{})$ extended to a wave vector of magnitude $Q=40Å{}^{-1\mathrm{}}$ have been obtained by high-energy x-ray diffraction. For the first time, it is possible to resolve the contributions of Si-O, Al-O, and Ca-O coordination polyhedra to the experimental atomic pair distribution functions. It has been found that the connectivity of $\mathrm{Si}/\mathrm{Al}-\mathrm{O}$ tetrahedral network decreases with increasing $x$ due to the emerging of nonbridging oxygens located on Si-O tetrahedra. Calcium maintains a rather uniform coordination sphere for all values of $x$ and so it plays a certain role in determining the glass structure.