Structural aspects of the glass-to-crystal transition in sodium-calcium silicate glasses

The structural aspects of the glass-to-crystal transition of Na2O-2CaO-3SiO2 (1-2-3) and 2Na2O-CaO-3SiO2 (2-1-3) glasses have been investigated using X-ray diffraction and multinuclear NMR spectroscopy. These materials represent a small group of glasses that undergo homogeneous nucleation rather than following the thermodynamically favored path of surface crystallization. To understand this unusual nucleation mechanism the structural resemblance of the glasses and their isochemical crystals was probed on different length scales. Definitive evidence of structural similarity at the level of intermediate-range order is obtained from 23Na spin echo decay experiments, which are sensitive to the spatial distribution of the sodium nuclei. In contrast the distributions of the silica network former units are quite different in the glassy and the crystalline states. These results suggest that a structural resemblance of the short- and intermediate-range order of the network modifiers rather than the network formers may be a key feature of homogeneously nucleating glasses.