ИСТИНА

The Fe7(PO4)6 [1] represents a rare case of parent compound of widespread structure type with general formula (where Me ̶ first row transition metal, Mg; X ̶ P, V, As, Mo or In) with poorly investigated physical properties. Introduction of transition metals, i.e. manganese, cobalt, nickel and copper into Fe7(PO4)6 structure results in preferable occupation of iron positions by Me2+ ions so that total formula transforms into , , and . Available information about their magnetic properties is limited to antiferromagnetic order formation at TN = 47 K in nickel–doped compound. Trivalent positions can be occupied by titanium (Me ̶ Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn), vanadium (Me ̶ Mg, Cr, Mn, Fe, Co, Ni, Zn), chromium (Me ̶ Mg, Cr, Mn, Fe, Co, Cu, Zn) or indium (Me ̶ Mg, Co, Ni, Zn). The magnesium comprising compounds are mostly paramagnets. The combinations of transition metals with vanadium experience antiferromagnetic ordering at low temperatures of about 12 – 15 K. Another route for chemical modifications of Fe7(PO4)6 structure relates with introduction of alkali metals ions, i.e. Na+, into the crystal structure. In our work we focused on parent compound Fe7(PO4)6, which is isotypic with mineral vanadate howardevansite NaCuFe2(VO4)3, and its counterpart compound doped with sodium metals ions, NaxFe7(PO4)6. Here we present basic thermodynamic and magnetic properties of these compounds and their primary characterization.