Phase formation in Cu3+1.5xR4-x(VO4)6 (R = Fe and Cr) systems:: Crystal structure of Cu2.5Fe4.333(VO4)6, Cu4Fe3.333(VO4)6, and Cu4.05Cr3.3(VO4)6

Phase formation in the Cu3+1.5xR4-x(VO4)(6) (R = Fe and Cr) systems was studied at 670 degreesC by powder X-ray diffraction. In the Cu3+1.5xFe4-x(VO4)(6) system there are two compounds having homogeneity ranges with formula Cu3+1.5xFe4-x(VO4)(6) (0.667 less than or equal to x < 0.778) and Cu3+1.5xFe4-x(VO4)(6) (-0.333 <less than or equal to> x less than or equal to -0.167) with structures of mineral lyonsite and mineral howardevansite (or Fe-7(PO4)(6)), respectively. The structures of Cu2.5Fe4.333(VO4)(6) and Cu4Fe3.333(VO4)(6) were refined by Rietveld method: space group P (1) over bar, Z = 1, a = 6.6139(1)Angstrom, b = 8.0468(2)Angstrom, c = 9.7566(2) Angstrom, alpha = 106.047(2), beta = 103.806(2), gamma = 102.157(2) for Cu2.5Fe4.333(VO4)(6) and space group Pnma, Z = 2, a = 4.91029(7) Angstrom, b = 10.2867(1) Angstrom, c = 17.2135(2) Angstrom for Cu4Fe3.333(VO4)(6). In the structure of Cu2.5Fe4.333(VO4)(6) vacancies are localized in the octahedral Cull) sites. Specimens Cu2.5Fe4.333(VO4)(6), Cu-2.75 Fe-4.167(VO4)(6), and Cu4Fe3.333(VO4)(6) were characterized by Mossbauer spectroscopy. There are three components in Mossbauer spectra for the first two compounds. Iron cations in them occupy two octahedral sites and partially occupy a trigonal-bipyramidal site. In the Cu3+1.5xCr4-x(VO4)(6) system only one homogeneity range was found (0.667 less than or equal to x less than or equal to 0.75). The compound Cu-4.05 Cr-3.3(VO4)(6) was isolated and determined to be isotypic to lyonsite and some molybdates and tungstates. The crystal structure of the compound Cu4.05Cr3.3(VO4)(6) was refined by Rietveld method (space group Pnma, Z = 2, a = 4.89650(8) Angstrom, b = 10.2035(2) Angstrom, c = 17.1407(3)Angstrom). The face-sharing octahedra M(3)O-6 form chains along the (100) direction and are partially occupied by Cu2+. Copper cations locate on the faces of the edge-sharing prisms M(1)O-6 which also form chains along the (100) direction. The edge- and corner-sharing octahedra M(2)O-6 are occupied by Cu2+ and Cr3+ and constitute incomplete layers in the (001) plane. (C) 2001 Academic Press.