Crystal nucleation in P2O5-doped lithium disilicate glasses

Abstract

The bulk (volume) crystallisation of Li₂O·2SiO₂ (LS₂) glasses with additions of 1, 2 and 5 mol % P₂O₅ (G1P, G2P and G5P) was investigated. In the G1P and G2P glasses heated at 454°C metastable α′- and β′-LS₂ crystal phases formed initially and only stable LS₂ at later stages. In G1P heated at 463°C transmission electron microscopy (TEM) revealed particles (≥2 μm) of stable LS₂ but also smaller (<0.1 μm) crystalline particles which were unstable under the electron beam and could not be identified. Crystalline lithium orthophosphate (Li₃PO₄) was detected by XRD and ³¹P magic angle spinning nuclear magnetic resonance (MAS-NMR) in both as-quenched and heat treated G5P but not in G1P and G2P. In G5P heated at 476°C, metastable α′-LS₂ phase initially formed and at later times stable LS₂. In as-quenched G5P, a trace of lithium metasilicate (LS) was observed. Li₃PO₄ crystals were not detected in G5P by TEM but a marked increase in nucleation rate due to P₂O₅ addition was observed. ²⁹Si MAS-NMR revealed amorphous Q² (LS) and Q⁴ (SiO₂) species in an average Q³ (LS₂) environment in the as-quenched glasses. Increased P₂O₅ concentration caused a greater degree of amorphous phase separation. ³¹P MAS-NMR showed amorphous lithium phosphate units in the as-quenched and nucleated G1P and G2P. The presence of crystalline Li₃PO₄ phase in glass G5P suggests that Li₃PO₄ crystals may act as sites for heterogeneous nucleation of stable lithium disilicate.