Defektverbindung und fremdionen-dotierung bei der ausfällung von eisen(II)- und eisen(III)- phosphaten

The products obtained by precipitation of iron(III)-phosphates in the pH-range from 2 to 13.35 differ in their composition from the ones reported in the literature, e.g. FePO₄2H₂O or FeH₂(OH)₂PO₄, except at low pH values.The molar ratio of 1 : 1 for P and Fe is disturbed to a significant extent and foreign ions are also incorporated as integral parts of the precipitation products.The change of the P/Fe ratio within the pH-range from 2 to 11 is the result of continuous substitution of OH^? for PO₄^? ions. This exchange may be explained as a transition between phosphate- and hydroxide-“Defektserie” (reference is to sets of compounds which form partial rather than complete isomorphous series), whereby the critical composition and the critical precipitation product are defined by the inflection point of the exchange curve.During the hydrothermal recrystallization of amorphous precipitation products, crystalline iron phosphate (strengite) is formed at low pH values, and crystalline iron hydroxide is formed at high pH values. The precipitation products formed at intermediate pH values have a heterogeneous composition and fail to crystallize under these conditions.