Nucleation and crystallization of WO₃ from pure and Pb²⁺ doped NaF fluxes

DTA runs and flux growth experiments have shown that the crystallization temperatures of WO₃ from NaF flux are in the range of 1020-1090°C. Addition of Pb²⁺ influences the crystal growth and the crystallization temperatures are altered (within this range). A thermal effect giving rise to an exothermal DTA peak was observed above the crystallization temperature. This peak occurs at a higher temperature when lead ions are present. We propose an explanation based on pre-crystallization clustering and on the number of nucleation sites on the surface of the platinum crucible.