Thermal and XRD analysis of synthesis of fluoro-topaz

The present work deals with thermal and XRD analysis of fluoro-topaz synthesis by sintering of aluminium and silicon oxides (in the form of corundum and quartz) with ammonium fluoride and carried out in open Pt crucibles (for XRD studies) and a closed furnace of simultaneous DTA/TG thermoanalytical apparatus called MOM Derivatograph. The furnaces are connected to a gas suction device to remove gas decomposition products.The study shows the endothermic peak at 125–155 °C, representing the formation of ammonium aluminium hexafluoride and cryptohalite. The endothermic peak at 220–240 °C represents the beginning of dissociation of ammonium aluminium hexafluoride to the more stable ammonium aluminium tetrafluoride. The endothermic peak at 340–350 °C represents the dissociation of cryptohalite and ammonium aluminium tetrafluoride with the formation of aluminium fluoride.The formation of fluoro-topaz is represented by the wide endothermic peak at 770–800 °C. The medium wide or sharp endothermic peaks at 950 °C and 1000 °C represent the subsequent dissociation of fluoro-topaz to corundum or mullite, respectively.

► The synthesis of topaz by the studied method is a complicated one. ► Different products of sintering are obtained. ► Intensive formation of topaz takes place at 770–800 °C. ► The produced topaz crystallizes in orthorhombic system and is optically positive.