Naslov
Study of Anisotropic Structures from Amorphous to Crystalline Phases of Lanthanum Hydroxy-Fluorides using Light Imaging and Scattering

Autori
Stubičar, Nada

Vrsta, podvrsta i kategorija rada
Radovi u zbornicima skupova, cjeloviti rad (in extenso), znanstveni

Izvornik
Proceedings of the 6^th Multinational Congress on Microscopy - European Extension / Milat, Ognjen ; Ježek, Davor - Zagreb : Hrvatsko mikroskopijsko društvo, 2003, 215-217

Skup
6^th Multynational Congress on Microscopy -European Extension

Mjesto i datum
Pula, Hrvatska, 01.06.2003. - 05.06.2003

Vrsta sudjelovanja
Pozvano predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
anisotropic particles; dynamic light scattering; lanthanum hydroxy-fluorides; polarizing light microscopy; small-angle X-ray scattering; morphology of particles; nanoparticles

Sažetak
The detailed understanding of the earliest stages of crystal growth or even the nucleation from the aqueous solutions of potassium fluoride-lanthanum nitrate is a great challenge, because a wide variety of different dynamic structures and interfaces i.e. differently ordered heterogeneous meso-phases and solid phases have been formed. This might lead to the rational design of new materials and their wide application, because of their structural, optical, electrical (super-ionic), properties, etc. Solid-state chemistry and physics of LaF_3 crystals has been incomparably more extensively investigated than that in solution and none from the amorphous gel phase. Studying these process deals with very short time scales (sub-millisecond range), as well as with very low concentrations in solution, which should be kept constant during the growth process investigated, to avoid the interplay with secondary nucleation. Hence, the steady-state pF-stat method, known as the constant composition method, was employed for the preparation of phases from the solution supersaturated with regard to lanthanum hydroxide and only saturated with regard to the more soluble lanthanum fluoride phase. The activity of La^3+ ions was kept very low: 10^-6 mol L^-1 and with the excess of F^- ions, so that their activity ratio was 1/8 during the growth and the attained pH was 7.2. The pF-stat method enables us to study processes far from thermodynamic equilibrium and at non-stoichiometric condition. The characterization of particles was done using polarizing optical microscopy (POM) in the micron-size scale range, dynamic light scattering (DLS) in the sub micron-size scale range, and small-angle X-ray scattering (SAXS) in the nano-size scale range. The formation of crystalline from the amorphous gel phase was evidenced by wide-angle X-ray scattering (WAXS) or diffraction (XRD). POM reveals different shapes. Some of them are shown in Figure 1. Large vesicles entrapped with the monodisperse primary particles and ribbons with rods grown inside them. The ratios of the major and minor semi-axes of rods in microscopic images are ρ ~3 (+/- 0.5). DLS experiments give us two relaxation modes of particle motion: slow mode at smaller angles (at 35o) and fast one at 90o, corresponding to the size of vesicles, expressed as median, weight average hydrodynamic diameter dH ~23.7 +/- 3 nm and of primary particles of about 4.2 +/- 0 nm, assuming the spherical shape. Assuming the rod shape of sub-micron size particles the DLS data give the ratio of ellipsoidal semi-axes ρ = b/a = 6 and oblate ellipsoid is distinguished. We do not need to have depolarized DLS experiments, because in this case qL = 4 (q is scattering wave vector and L is the length of rod), and the rotation of rods may be neglected (so we calculated the size from the translational diffusion coefficient, using Stokes-Einstein equation). The analysis of the desmeared X-ray scattering curve yielded a cross-section radius of gyration Rc = 4.9 +/- 0.1 nm and a thickness radius of gyration Rt = 1.6 +/- 0.1 nm. Assuming the rod shape with ellipsoidal cross-section the following semi-axes of the cross-section were estimated to be: a =3.2 +/- 0.2 nm and b = 9.25 +/- 0.3 nm. Again the ratio of the semi-axes is ρ ~3. These three independent techniques give a hard confirmation of the self-similarity of particle shape and structure in different scale length, as a vast number of cases in the scientific literature. Figure 1. Optical microscopic pictures of the heterogeneous systems prepared from lanthanum nitrate and potassium fluoride aqueous solutions (concentrations and pH are above in the text) using the potenciometric pF-stat method. Pictures were taken with polarized light and λ plate. Acknowledgements: The author is thankful to Prof. Dr. Peter Zipper, Graz, Austria, and Prof. Dr. Mirko Stubicar, who were involved in this study. Nada Stubicar: E-mail: stubicar@chem.pmf.hr

Izvorni jezik
Engleski

Znanstvena područja
Kemija

Napomena
ISBN 953-99339-0-0

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