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