Structural similarity on multiple length scales and its relation to devitrification mechanism: a solid-state NMR study of alkali diborate glasses and crystals
The nature of the devitrification process is a fundamental problem in glass science and governs the ultimate stability of glass. It is hypothesized that the devitrification mechanism of a given glass composition is strongly correlated with the extent of structural similarity at the intermediate-range level between the glass and crystal phase to which it transforms. However, relatively little information is available until now to test this hypothesis because of the lack of long-range periodicity in glass and the absence of effective analysis methods. In this work Li2O-2B2O3 (L2B) and Na2O-2B2O3 (N2B) glasses, which respectively exhibit homogeneous (internal) and heterogeneous (surface) devitrification mechanisms, and plus corresponding crystals from same batches were prepared and comprehensively studied by using multiple solid state NMR techniques. Of those, 11B MAS and MQMAS experiments were applied to reveal short-range order information; 7Li and 23Na spin-echo decay and 11B{6Li} and 11B{23Na} rotational echo double resonance (REDOR) experiments were used to determine homonuclear and heteronuclear interactions that can reflect the intermediate-range order. It was found that the local bonding environments of the L2B crystal and glass are quite similar, while there are significant differences in the N2B system. Moreover, the experimental results indicated that the second moment (M2) values of both homo- and heterodipolar interactions measured for the L2B glass are very close to those of its isochemical crystal, in contrast, the M2 values for N2B glass differ significantly from those of its crystal phase. These observations indicate the existence of a strong correlation between structural similarity at both the short and intermediate length scale, and nucleation mechanis
