New mixed-valent iron (II/III) phosphates, Cu3-xFe4+x(PO4)(6)

The phase formation in the system Cu3-xFe4+x(PO4)(6) (0 less than or equal to x less than or equal to 1) was investigated at 920 degrees C under air. These phosphates were studied by X-ray powder diffraction and Mossbauer spe ctroscopy. At 0.3 less than or equal to x less than or equal to 0.95 new ph osphates, CU3-xFe4+x (PO4)(6), are formed which are isotypic with Fe-7(PO4) (6). Mossbauer spectroscopy confirms that the structure stabilizes Fe2+ cat ions rip to 19% under air. At 0 less than or equal to x < 0.3 the synthesiz ed specimens contain Cu2P2O7, while at 0.95 < x less than or equal to 1 the y contain FePO4 as the second phases. The crystal structures of Cu2.4Fe4.6( PO4)(6) and Cu2.05Fe4.95(PO4)(6) specimens were studied by the Rietveld met hod. They crystallize in the triclinic space group P-1 (No. 2), Z = 1 with a = 7.9231(2) Angstrom, b = 9.3099(2) Angstrom, c = 6.2582(2) Angstrom, alp ha = 107.395(2), beta = 100.980(2), gamma = 105.675(2) for Cu2.4Fe4.6(PO4)( 6) and a = 7.9434(2) Angstrom, b = 9.3089(2) Angstrom, c = 6.2654(2) Angstr om, alpha = 107.564(2), beta = 101.040(2), gamma = 105.643(2) for Cu2.05Fe4 .95(PO4)(6). The Fe3+ cations are localized in M(2)O-6 and M(4)O-6 sites. T he trigonal bipyramidal M(3)O-5 sites are fully occupied by Cu2+ cations in both structures. With increasing iron content in Cu3-xFe4+x(PO4)(6), the s ubstitution of Fe2+ for Cu2+ is realized in M(1)O-6 sites. (C) 2000 Academi c Press.