MAS-NMR study of lithium zinc silicate glasses and glass-ceramics with various ZnO content

Lithium zinc silicate glasses of composition (mol%): 17.5Li₂O-(72−x)SiO₂-xZnO−5.1Na₂O−1.3P₂O₅−4.1B₂O₃, 5.5?x?17.7, were prepared by conventional melt-quenched technique and converted to glass-ceramic by controlled crystallization process. ²⁹Si and ³¹P MAS-NMR was used to characterize the structure of both glass and glass-ceramic samples. Despite the complex glass composition, Q², Q³ and Q⁴ sites are identified from ²⁹Si MAS-NMR, which relative intensities are found to vary with the ZnO content, indicating a network depolymerization by ZnO. Moreover, well separated Q³ and Q⁴ resonances for low ZnO content indicates the occurrence of phase separation. From ³¹P MAS-NMR, it is seen that phosphorus is mainly present in the form of ortho-(Q⁰) and pyro-phosphate (Q¹) structural units and variation of ZnO content did not have much effect on these resonances, which provides an additional evidence for phase separation in the glass. On conversion to glass-ceramics, lithium disilicate (Li₂Si₂O₅), lithium zinc ortho-silicate (Li₃Zn_0.5SiO₄), tridymite (SiO₂) and cristobalite (SiO₂) were identified as major silicate crystalline phases. Using ²⁹Si MAS-NMR, quantification of these silicate crystalline phases is carried out and correlated with the ZnO content in the glass-ceramics samples. In addition, ³¹P spectra unambiguously revealed the presence of crystalline Li₃PO₄ and (Na,Li)₃PO₄ in the glass-ceramics.