Structural features and thermal expansion of zinc aluminosilicate quartz solid solutions

Zinc aluminosilicate (ZAS) glasses were synthesized along the peraluminous join (SiO2 contents between 95 and 50 mol%) by melt-quenching or sol-gel spray-drying and subjected to controlled heat treatments to obtain ZAS quartz solid solutions (Zn-Qz-ss). The precipitation of Zn-Qz-ss is non-stoichiometric with respect to their parent glass and the crystals undergo a compositional evolution during annealing, even in absence of secondary crystalline phases. Near-hexagonal high-quartz-like crystals with negative thermal expansion (down to -3 × 10-6 K-1) can be obtained in the range 50-80 mol% SiO2, while SiO2-richer crystals exhibit positive thermal expansion at room temperature (up to 29 × 10-6 K-1). Zn2+ ions occupy two distinct positions within the structural channels of quartz, with a strong preference for 4-fold coordination. In analogy to lithium aluminosilicate quartz solid solutions (Li-Qz-ss), the low-to-high phase transition exhibits a linear correlation to the composition of the crystals.