Mechanochemical synthesis of MgF2 – MF2 composite systems (M = Ca, Sr, Ba)

• The capability of mechanochemical synthesis for the formation of MgF2–MF2 (M: Ca, Sr, Ba) composites was tested.
• A fluorination of fluorine-free metal sources with NH4F was performed directly at milling.
• An equimolar ratio of Mg2+ to the second cation allows access to tetrafluoromagnesates, MMgF4.

The capability of mechanochemical synthesis for the formation of MgF2–MF2 (M: Ca, Sr, Ba) composites, solid solutions or well-defined compounds was tested applying a fluorination of different fluorine-free metal sources with NH4F directly at milling. No evidence was found for a substitution of Mg2+ with Ca2+ (Sr2+, Ba2+) ions, or vice versa, in rutile or fluorite structure. However, an equimolar ratio of Mg2+ to the second cation allows the mechanochemical synthesis of tetrafluoromagnesates, MMgF4, which is more and more hampered the smaller the radius of the cation M2+ is. BaMgF4 is formed even phase pure from the acetates, SrMgF4 can only be observed in a mixture accompanied by the binary fluorides. In addition, 19F MAS NMR spectra along with calculations of 19F isotropic chemical shift values according to the superposition model point to the formation of a metastable phase of CaMgF4, which disappears at thermal treatment and decomposes into the binary fluorides CaF2 and MgF2.

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