engleski

Crystal Growth of Lead Fluoride Phases Using the Constant Composition Method. III. Effect of pH and Ionic Strength

The kinetics of crystal growth of the lead fluoride phases was studied at 25°C using the so-called constant composition method (pF-stat method). A linear growth law was obeyed for α ---PbF2 growth from equivalent lead nitrate and potassium fluoride solutions with low supersaturation and pH between 5.2 and 5.6 in pure water and in 0.1 mol dm− 3 KNO3 solution. The rate is about 5 times greater at high ionic strength. It decreases linearly with changing pH of the supersaturated solution 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 undergoes the recrystallization in water to pure α ---PbF2, and after annealing to 450°C decomposes into β -PbF2, and an unidentified phase (probably α ---(KNPbN− 1)F2− N phase with N varied) ; (b) with large excess of KF over Pb(NO3)2 and HNO3 to pH = 4.3, the β ---PbF2 phase was identified. The characterization of the grown phases was done on the basis of polarizing microscopy, Fourier transform infrared (FTIR) spectra and X-ray powder diffraction analysis.

lead fluoride ; crystal growth

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