Crystallization, structure and thermal expansion properties of quartz solid solutions crystallized in copper aluminosilicate sol-gel glasses

• The presence of Cu 2+ ions is essential for Cu-Qz-ss formation, with the low processing temperatures employed in the sol-gel method preventing copper reduction. • Despite the complexity of the crystallization sequence, it was possible to crystallize between 81 and 99 wt% of Cu-Qz-ss in the spray-dried copper aluminosilicate compositions with Al/Si ratios ranging from 0.05 to 0.5. • Both low- and high-temperature modifications of Cu-Qz-ss exhibit positive thermal expansion coefficients, including SiO 2 -rich crystals. • Based on the electronic configuration of copper ions, the blue-green coloration observed in the glass powders indicates the predominant presence of copper in its divalent oxidation state. In order to expand the range of available quartz solid solutions (Qz-ss), copper aluminosilicate glasses were produced through different sol-gel methods. Favorable sample compositions were selected by analyzing dip-coated thin layers with different SiO 2 contents and CuO/Al 2 O 3 ratios. The phase assembly proved to be complex, as many different phases crystallized simultaneously, hampering stoichiometric precipitation of Qz-ss from the starting glasses. Despite these challenges, systematic heat treatments of four different spray-dried compositions with Al/Si ratios from 0.05 to 0.5 enabled the formation of Cu-stuffed Qz-ss with different structural stuffing degrees. The thermal expansion coefficients of the low and high temperature modifications of the different Qz-ss varied from 0.4 to 26 × 10 –6 K -1 , with no negative thermal expansion being detected. Copper ions are accommodated in both tetrahedral and octahedral sites in the Qz-ss channels, and the blue-green hue seen in the spray-dried powders may be assigned to Cu 2+ in 6-fold coordination.