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Variety of Aggregation and Growth Processes of Lanthanum Fluoride as a Function of La/F Activity Ratio. Part I. A potentiometric pF-stat and DLS Study of LaF3.

Constant composition method (CCM), in particular potentiometric pF-stat method, was proved to be applicable and convenient to study kinetics of sub-micrometer size lanthanum fluoride particles besides that of macroscopic crystals. The constancy of the state of ions in supersaturated solution (due to hydrolysis and the other chemical reasons), as it is: La-to- F ratio, the attained pH, ionic strength, temperature, the way and speed of mixing, etc., were kept constant during the whole growth process. Steady-state kinetics was achieved after about 150 % of growth as regards to the initial mass of seeds. Because of very reproducible results it is prominent for the preparation of new materials with respect to morphology and microstructure, even in nano-size scale (Part I and II), far from thermodynamic equilibrium and at non-stoichiometric condition. Depending on the activity ratio of La -to- F ions in supersaturated solution used for particle growth i.e. whether lanthanum nitrate or potassium fluoride is in a great excess, the kinetics and the mechanism is different. At the excess of lanthanum nitrate over potassium fluoride and at the attained pH about 4, the first order rate of growth was determined and the sub-micrometer scale particles were obtained and were characterized by the DLS method. Fast relaxation decay times measured at the scattering angles around 90o and slow relaxation decay times measured at smaller scattering angles (maximum at around 35o) with the assumption of spherical shape particles gave the estimation of the dH ~ 8 nm and dH ~ 24 nm for primary particles and larger vesicles, respectively. In this size region the rod shape may be assumed also and we got for the biaxial ratio of an oblate ellipsoidal cross-section, that aspect ratio is ρ =b/a=(3.0+/-0.2). Approaching to the stoichiometric condition in supersaturated solution and IEP at pH=(5.6+/-0.3), the power law kinetics prevailed and the order of growth was determined to be (4.0+/-0.2). All the data from pH 4 to 6 can be well fitted to a linear fit (ln vg vs ln s, supersaturation) with the overall order p=(3.3+/-0.4) and the growth rate constant kp about 7x10exp(-12) m s-1. This rate order supposes the polynuclear growth mechanism. Layer-by layer (sheet-by sheet) growth on the surface of several hundred microns large anisotropic LaF3 crystals was observed on polarizing optical microscopic (POM) pictures. In the POM images the systems prepared in the acidic pH region around 4 are shown as anisotropic dendrites (loose aggregates). Analysis of the SLS data proved that theoretically aggregates of sol particles are self-similar fractals with the universal fractal dimension determined to be Df=(2.10+/-0.05) for the reaction limited aggregation, as a vast number of cases in the scientific literature.

Crystal growth; kinetics and mechanism; Dynamic and static light scattering (DLS and SLS); Lanthanum fluoride; pF-stat kinetics and mechanism of crystal growth; Polarizing optical microscopy (POM); Rod shape particles.

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