The present invention is related to a method for preparing an amorphous metal fluoride of the formula M.sup.X+F.sub.X-.delta. comprising the steps of a) providing a precursor, whereby the precursor comprises a structure having a formula of M.sup.x+F.sub.(x-.delta.)-yB.sub.y; and b) reacting the precursor with a fluorinating agent generating the amorphous metal flouride having a formula of M.sup.x+F.sub.x-.delta., whereby M is selected from the group comprising metals of the second, third and fourth main group and any subgroup of the periodic table, B is a coordinately bound group; x is any integer of 2 or 3; y is any integer between 1 and 3; .delta. is 0 to 0.1; and x-.delta.>y.