Phase evolution in lithium disilicate glass–ceramics based on non-stoichiometric compositions of a multi-component system: structural studies by 29Si single and double resonance solid state NMR

Abstract

The crystallization mechanism of a high-strength lithium disilicate glass–ceramic in the SiO₂–Li₂O–P₂O₅–Al₂O₃–K₂O–(ZrO₂) system, used as restorative dentistry material, has been examined on the basis of quantitative ²⁹Si magic angle spinning (MAS) and ²⁹Si{⁷Li} rotational echo double resonance (REDOR) NMR spectroscopy. Crystallization occurs in two stages: near 650 °C a significant fraction of the Q⁽³⁾ units disproportionates into crystalline Li₂SiO₃ and Q⁽⁴⁾ units. Upon further annealing of this glass–ceramic to 850 °C the crystalline Li₂SiO₃ phase reacts with the Q⁽⁴⁾ units of the softened residual glass matrix, resulting in the crystallization of Li₂Si₂O₅. The NMR experiments provide detailed insight into the spatial distribution of the lithium ions suggesting the absence of lithium ion clustering in the residual glassy component of the final glass–ceramic. ³¹P MAS-NMR spectra indicate that phosphate acts as a lithium ion scavenger, resulting in the predominant formation of orthophosphate (P⁽⁰⁾) and some pyrophosphate (P⁽¹⁾) groups. Crystallization of Li₂SiO₃ occurs concomitantly with the formation of a highly disordered Li₃PO₄ phase as evidenced from strong linebroadening effects in the ³¹P MAS-NMR spectra. Well-crystallized Li₃PO₄ is only formed at annealing conditions resulting in the formation of crystalline lithium disilicate. These results argue against an epitaxial nucleation process previously proposed in the literature and rather suggest that the nucleation of both lithium metasilicate and lithium disilicate starts at the phase boundary between the disordered lithium phosphate phase and the glass matrix.