Potentiometric and dynamic light scattering investigation of the precipitation of lanthanum fluoride

Abstract

Precipitation processes were characterised in an aqueous solution of La(NO3)3-KF at 25°C using a light scattering method and potentiometric pH and pF titrations with a fluoride ion selective electrode. Two processes are taking place according to the precipitation diagram: (I) a neutralisation process at high concentrations of precipitation components in the ratio 1:3 and with the attained pH at 4.0 ± 0.3, leading to the formation of pure LaF3(s); (II) ionic solubility at La(NO3)3 concentrations below (1.0 ± 0.05) × 10-3 mol/dm3 with large hydrated lanthanum ions in solution at a pH of 6.0 ± 0.3 (hydrolysis), leading to a fresh precipitate of lanthanum fluoride-hydroxide containing a large amount of water, which is hard to filter and to dry, and is not suitable for the preparation of pure LaF3. The steep increase in particle size (from submicron to micron size range, as determined by dynamic light scattering measurements) of systems at the solid/solution concentration limit is accompanied by the increase in pH of the solution from 4.6 to 5.9, dividing the concentration and pH regions I and II. In the former case I, the pF-stat crystal growth led to the formation of pure hexagonal LaF3(s), as determined by powder X-ray diffraction of the dried solid. The variety of morphologies found was a result of the growth and aggregation patterns of lanthanum fluoride particles from submicron to micron size and depended on the pH, supersaturation and time. The formation of dendrites (aggregates of monodisperse particles ∼300 nm), anisotropic fibers and ribbon-like multichains, reminiscent of liquid crystals or inorganic polymers, and finally large (several hundred microns) polycrystals, like hexagonal prisms and trigonal bipyramids, was characterised by dynamic light scattering and polarising microscopy. © Springer-Verlag 1999.