Abstract
The fluorolytic sol-gel synthesis for binary metal fluorides (AlF3, CaF2, MgF2) has been extended to ternary and quaternary alkaline earth metal fluorides (CaAlF5, Ca2AlF7, LiMgAlF6). The formation and crystallization of nanoscopic ternary CaAlF5 and Ca2AlF7 sols in ethanol were studied by 19F liquid and solid state NMR (nuclear magnetic resonance) spectroscopy, as well as transmission electron microscopy (TEM). The crystalline phases of the annealed CaAlF5, Ca2AlF7, and LiMgAlF6 xerogels between 500 and 700 °C could be determined by X-ray powder diffraction (XRD) and 19F solid state NMR spectroscopy. The thermal behavior of un-annealed nanoscopic ternary and quaternary metal fluoride xerogels was ascertained by thermal analysis (TG/DTA). The obtained crystalline phases of CaAlF5 and Ca2AlF7 derived from non-aqueous sol-gel process were compared to crystalline phases from the literature. The corresponding nanoscopic complex metal fluoride could provide a new approach in ceramic and luminescence applications.
Highlights
The interest in complex metal fluorides (KMgF3, CaAlF5, SrAlF5, LiCaAlF6, LiSrAlF6, and LiMgAlF6) is mainly caused by their thermoluminescent and chemical properties
We succeeded in the synthesis of transparent and long-time stable CaF2 sols prepared from calcium ethoxide and calcium isopropoxide for the first time just by increasing the polarity of the solvent in the CaF2 system
We assume that the long-time sol stability of the CaF2 nanoparticles could be due to the higher viscosity and polarity of ethylene glycol compared to methanol and ethanol, respectively
Summary
The interest in complex metal fluorides (KMgF3, CaAlF5, SrAlF5, LiCaAlF6, LiSrAlF6, and LiMgAlF6) is mainly caused by their thermoluminescent and chemical properties. These complex metal fluorides are often used as host materials for a wide range of applications in the optical luminescence field. The fluorolytic sol-gel synthesis, discovered a few years ago, provides in general very convenient and easy access to nanoscopic fluorides [14]. This synthesis approach has not been deeply investigated for its applicability for the synthesis of complex fluorometallates. The chloride approach turned out to be a quite robust method compared to the alkoxide-based sol-gel route and enables transparent and long-time stable metal fluoride sols
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