Sols of nanosized magnesium fluoride: formation and stabilisation of nanoparticles

Abstract

The formation of magnesium fluoride sols and xerogels according to the fluorolytic sol–gel synthesis based on the reaction of Mg(OMe)₂ with non-aqueous HF has been thoroughly investigated by X-ray scattering (WAXS/XRD), TEM, SAXS, DLS and ¹⁹F MAS NMR spectroscopy. Mechanistic insights were gained by following the reaction progress and formation of intermediate phases of the fluorination of magnesium alkoxides. For F : Mg ratios of 0.3 and 0.4 the formation of two crystalline phases was observed containing the recently obtained compound [Mg₆F₂(OCH₃)₁₀(CH₃OH)₁₄] hexanuclear dicubane units. The stoichiometric reaction yields magnesium fluoride nanoparticles with crystallite sizes below 5 nm, which show broad reflections in the X-ray diffraction pattern. Metal fluoride sols prepared by this way undergo tremendous changes over the first several weeks after synthesis. Immediately after the fluorination, particles of about 120 nm—probably agglomerates—are formed, which break apart in the course of about one month of ageing and low-viscous, transparent sols with particles of about 12 nm are obtained. At the same time structural re-organisation processes within the magnesium fluoride particles are observed by an increase of the (110) reflection in WAXS.