WO3 as a model solute in flux growth: The effect of lead ions

The nucleation and growth of WO3 from melts of different compositions under similar growth conditions was studied by observing the size and number of crystals obtained. The molar compositions were: (1) 100WO3-22Na2O, (2) 100WO3-44NAF, (3) 100WO3-22PbF2, as well as compositions based on (1) with PbO and on (2) with PbF2 substituted for part of the Na2O or NaF respectively. Amounts of Pb2+ as little as 0.25 mol% with respect to WO3 resulted in fewer, larger crystals instead of many small ones. The X-ray powder pattern data corresponded to those of WO3. With PbF2 as sole flux, a single sheet-like crystal was obtained. Electron probe microanalysis showed that the crystals contained no detectable Na+, Pb2+ or F-. It is suggested that lead ions may serve as a centre of condensation, as reported for alkali halides in aqueous solutions containing Pb2+, the resulting prenuclei of WO3 causing the exceptional growth rates required for few, large crystals.