Variety of Aggregation and Growth Processes of Lanthanum Fluoride as a Function of La/F Activity Ratio. 2. Excess of F over La Region. Transformation of Amorphous to Crystalline Phase, POM, SAXS, WAXS, and XRD Study

Abstract

Preparation of lanthanum hydroxy-fluorides using steady-state pF-stat experiments, which ensures constant, very low concentrations of the precipitation components and constant pH during the whole growth process, results in a structural evolution of objects with defined physical parameters and a variety of nonconventional morphologies. This structural evolution was characterized by a combination of techniques, namely, static and dynamic light scattering (SLS and DLS), small-angle and wide-angle X-ray scattering (SAXS and WAXS), X-ray diffraction (XRD), and polarized optical microscopy (POM). It is shown that after the immediate formation of an amorphous glassylike precursor, crystalline nanoparticles of high structural definition and monodispersity transit into hollow spheres with diameters of several hundred nanometers. These large vesicles contained smaller ones, which have shown the Maltese cross. After vesicle deformation and elongation into fibers with extreme length, the nanocrystals are transferred into ribbonlike mesostructures. Assuming rod-shaped nanoparticles with ellipsoidal cross-section, we calculate the semiaxes a = 3.2 ± 0.2 nm and b = 9.25 ± 0.3 nm from the cross-section radius of gyration Rc = 4.9 ± 0.1 nm and thickness radius of gyration Rt = 1.6 ± 0.1 nm as determined by SAXS measurements. These data yield an axial ratio ρ = b/a = 3. As was presented in part 1, rods of submicrometer size, determined by DLS, grown inside the ribbons, also have a semiaxis ellipsoidal ratio ρ = 3.0 ± 0.1 and are monodisperse (the polydispersity index is defined as dw/dn = 1.01). The same axial ratio ρ = 3.0 ± 0.5 was determined by POM on the micron-size scale range. These are three independent methods, which confirm the consistency of morphology. The XRD data (providing an average structure, not a local one) suggest that mainly nonstoichiometric materials of tetragonal structure transform to the stoichiometric equilibrium hexagonal LaF3 structure. The shift of the two strongest peaks to longer distances occurs. This confirms the change in favor of the formation of longer La−F distances instead of shorter La−O distances, which were first formed during the transformation of amorphous gel to crystalline phase, shown on the WAXS and XRD diffractograms. These independent methods convincingly confirm the self-similarity of the particle shape and axial ratio and of the structure evolution in different scale lengths, as in a vast number of cases in the scientific literature. This conclusion is crucial in possible preparation of materials for special applications.