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

Abstract

Phase formation in the Cu3+1.5xR4−x(VO4)6 (R=Fe and Cr) systems was studied at 670°C 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≤x<0.778) and Cu3+1.5xFe4−x(VO4)6 (−0.333≤x≤−0.167) with structures of mineral lyonsite and mineral howardevansite (or Fe7(PO4)6), respectively. The structures of Cu2.5Fe4.333(VO4)6 and Cu4Fe3.333(VO4)6 were refined by Rietveld method: space group P1, Z=1, a=6.6139(1) Å, b=8.0468(2) Å, c=9.7566(2) Å, α=106.047(2), β=103.806(2), γ=102.157(2) for Cu2.5Fe4.333(VO4)6 and space group Pnma, Z=2, a=4.91029(7) Å, b=10.2867(1) Å, c=17.2135(2) Å for Cu4Fe3.333(VO4)6. In the structure of Cu2.5Fe4.333(VO4)6 vacancies are localized in the octahedral Cu(1) sites. Specimens Cu2.5Fe4.333(VO4)6, Cu2.75Fe4.167(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≤x≤0.75). The compound Cu4.05Cr3.3(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) Å, b=10.2035(2) Å, c=17.1407(3) Å). The face-sharing octahedra M(3)O6 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)O6 which also form chains along the (100) direction. The edge- and corner-sharing octahedra M(2)O6 are occupied by Cu2+ and Cr3+ and constitute incomplete layers in the (001) plane.