Structure and crystallization behavior of complex mold flux glasses in the system CaO‐Na2O‐Li2O‐CaF2‐B2O3‐SiO2: A multi‐nuclear NMR spectroscopic study

Abstract

AbstractThe structure of mold flux glasses in the system CaO‐(Na,Li)2O‐SiO2‐CaF2 with unusually high modifier contents, stabilized by the addition of ∼4 mol% B2O3, is studied using 7Li, 23Na, 19F, 11B, and 29Si magic‐angle‐spinning (MAS), and 7Li{19F} and 23Na{19F} rotational echo double‐resonance (REDOR) nuclear magnetic resonance (NMR) spectroscopy. When taken together, the spectroscopic results indicate that the structure of these glasses consists primarily of dimeric [Si2O7]−6 units that are linked to the (Ca,Na,Li)‐O coordination polyhedra, and are interspersed with chains of corner‐shared BO3 units. The F atoms in the structure are exclusively bonded to Ca atoms, forming Ca(O,F)n coordination polyhedra. This structural scenario is shown to be consistent with the crystallization of cuspidine (3CaO·2SiO2·CaF2) from the parent melts on slow supercooling. The progressive addition of Li to a Na‐containing base composition results in a corresponding increase in the undercooling required for the nucleation of cuspidine in the melt, which is attributed to the frustrated local structure caused by the mixing of alkali ions.