Precipitation of β-quartz Solid Solutions from ZnO-Al₂O₃-SiO Glasses (ZnO/Al₂O₃>1)

Studies on the Crystallization of ZnO-Al₂O₃-SiO₂ Glasses, No. 4

Abstract

The precipitating region of β-quartz solid solutions from ZnO-Al₂O₃-SiO₂(ZnO/Al₂O₃>1) glasses, the types of substitution which may occur in their structures and its thermal decomposition products are investigated. Results obtained are as follows.
(1) As for ten glasses of different compositions studied, the glass compositional region in which oniy β-quartz solid solutions are precipitated is 3.45≥ZnO/Al₂O₃≥1 (molar ratio), SiO₂>46mol% and ZnO<38mol%. However, SiO₂ rich region where metastable phase separation occurs on quenching melts is excluded from the above compositioual region because single crystalline phase, β-quartz solid solution, is not obtained.
(2) While it is generally considered that 1/2 ZnO(VI)+Al(IV) can replace one Si(IV) of β-quartz structure with the molar ratio ZnO/Al₂O₃=1, it is presumed that at ZnO/Al₂O₃>1, there exist excess ZnO that can not enter the structure together with Al(IV) and part of them substitutes for the Si site owing to 4-fold coordination preference of Zn²⁺, the rest entering the holes of β-quartz structure, 6-fold coordination site, in order to maintain electrical neutrality.
(3) Change of lattice parameters of β-quartz solid solutions on their compositions (ZnO/Al₂O₃>1) can be well explained when their compositions are represented in the SiO₂-Zn (VI)⋅Zn(IV)O₂-Zn(VI)_1/2⋅Al(IV)O₂ pseudo-ternary system.
(4) β-quartz solid solution with ZnO/Al₂O₃>1 decomposes to β-quartz solid solution with ZnO/Al₂O₃=1 and β-Zn₂SiO₄ solid solution on heating to 950°C, while β-quartz solid solution with ZnO/Al₂O₃=1 decomposes to gahnite and SiO₂ rich β-quartz solid soiution at above 1000°C.