CRYSTAL-GROWTH OF LEAD FLUORIDE PHASES USING THE CONSTANT COMPOSITIONMETHOD .3. EFFECT OF PH AND IONIC-STRENGTH

The kinetics of crystal growth of the lead fluoride phases was studied at 25-degrees-C using the so-called constant composition method (pF-s tat method). A linear growth law was obeyed for alpha-PbF2 growth from equivalent lead nitrate and potassium fluoride solutions with low sup ersaturation and pH between 5.2 and 5.6 in pure water and in 0.1 mol d m-3 KNO3 solution. The rate is about 5 times greater at high ionic str ength. It decreases linearly with changing pH of the supersaturated so lution both to higher and to lower values than mentioned (addition of KOH and HNO3, respectively), i.e. reaching the pH's at which two other lead fluoride phases grow: (a) with large excess of Pb(NO3)2 over KF and KOH to pH = 5.2 phase(s) of so far unresolved structure, which und ergoes the recrystallization in water to pure alpha-PbF2, and after an nealing to 450-degrees-C decomposes into beta-PbF2, and an unidentifie d phase (probably alpha-(KNPbN-1)F2-N phase with N varied); (b) with l arge excess of KF over Pb(NO3)2 and HNO3 to pH = 4.3, the beta-PbF2 ph ase was identified. The characterization of the grown phases was done on the basis of polarizing microscopy, Fourier transform infrared (FTI R) spectra and X-ray powder diffraction analysis.