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

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 non-conventional 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 glassy-like precursor, crystalline nanoparticles of high structural definition and monodispersity transit into hollow spheres with diameters of several hundred nm. These large vesicles contained smaller ones, which have shown the Maltese cross. After vesicle deformation and elongation into fibbers with extreme length, the nanocrystals are transferred into ribbon-like mesostructures. Assuming rod shape of nanoparticles with ellipsoidal cross-section, we calculate the semi-axes 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 r = b/a = 3. As was presented in Part I, rods of sub-micrometer size, determined by DLS, grown inside the ribbons, also have a semi-axes ellipsoidal ratio ρ = 3.0 +/- 0.1, and are monodisperse (polydispersity index defined as dw/dn = 1.01). The same axial ratio ρ = 3.0 +/- 0.5 was determined by POM on the micronsize 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 non-stoichiometric 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 favour of the formation of longer La-F distances instead of shorter La-O distances, which were firstly 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 their special applications.

Amorphous gel; Colloidal La(OH)3; Hierarchical structures of lanthanum hydroxide and fluoride; Nano-particles; Polarising optical microscopy (POM); SAXS; WAXS and/or XRD.

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