5 - From fluorinated clusters to metal fluoride composites: small particles with marvelous properties

Abstract

The fluorolytic sol–gel synthesis is a new synthesis approach that gives access to a wide variety of homodisperse nanoscopic metal fluorides. During the reaction of metal alkoxides with anhydrous hydrogen fluoride, new cluster-like metal alkoxide fluoride molecules containing between 3 and 10 metal centers are formed. Further fluorination results finally in pure metal fluorides; however incomplete fluorination allows functionalization of these clusters via their remaining OR–groups giving finally organically functionalized metal fluoride nanoparticles (NPs). Particles functionalized this way can be introduced into organic polymers forming metal-fluoride-based composite materials with improved thermal, mechanical, and optical properties. However this synthesis approach is not only restricted on binary metal fluorides, but also ternary, quaternary, or even more complex nanoscopic metal fluorides are accessible this way. Thus for example, nano-CaF2, SrF2, and BaF2 doped with rare-earth metal cations (RE3+) up to 40% RE3+ content are easily available. Monodisperse quasispherical NPs with a size of 5–15nm are accessible. The synthesized clear sols exhibit intense luminescence under UV excitation at room temperature and there is no restriction regarding composition and combination of different RE metals in different metal fluoride matrices. Moreover, based on unreacted, remaining alkoxide groups inside the metal fluoride particles, even such luminescent NPs can be introduced into organic polymers, and consequently, the obtained composites show excellent luminescent properties. This chapter will give insights into some aspects of the synthesis mechanism, the characteristic properties of such NPs and the wide application of metal fluoride NPs in composite, especially luminescent materials.