Geo-Inspired Phosphors Based on Rare-Earth Metal(III) Fluorides with Complex Oxoanions: I. Fluoride Oxocarbonates and Oxosilicates

摘要

The quest for luminescent materials with high energy efficiency in illumination devices has become a serious issue for saving our planet earth from global warming. On the search for new and effective solids as host lattices for luminescence (Dieke 1968; Blasse and Grabmaier 1994; Jüstel et al. 1998; Ronda 2008; H?ppe 2009), synthetic oxosilicates containing rare-earth metal(III) cations and fluoride anions got into the focus of our work, because hard anions (Pearson 1963) such as oxide and fluoride seem to be most suitable to preserve the applied energy and prevent it from being transformed into heat by lattice-vibration processes. During the everlasting fight photons versus phonons, the transfer of energy originating from external UV radiation to luminescent cations needs to prevail over any kind of non-radiative fading. The most simple combination of both anionic species, oxide O~(2-) and fluoride F~-, is realized in the rare-earth metal oxide fluorides with the composition MOF (M~(3+) = Y, La; Ce - Nd, Sm - Lu), which are not easy to prepare. A straightforward method to obtain them phase-pure and on the gram-scale results from the thermal decomposition of bastnaesite-type fluoride oxocarbonates MF[CO_3] at reasonable low temperatures. Moreover, they can serve as active educts for the synthesis of fluoride oxosilicates, where all oxide anions are bonded to silicon central atoms in isolated or vertex-shared [SiO_4]~(4-)tetrahedra and thus get in close proximity to each another. Therefore high coordination numbers most suitable for M~(3+) cations can be achieved in a few prototypic mineral-related compounds such as La_3F_3[Si_3O_9], Y_3F[Si_3O_(10)], Er_4F_2[Si_2O_7] [SiO_4] and Eu_5F[SiO_4]_3 (≡ (Eu~(II))_2(Eu~(III)_3F[SiO_4]_3).