Structural, magnetic, and optical properties of A3V4(PO4)6 (A = Mg, Mn, Fe, Co, Ni)
Combined synchrotron and neutron powder diffraction indicates that A3V4(PO4)6 (A = Mg, Mn, Fe, Co, Ni) compounds crystallize with triclinic P1 symmetry. Lattice parameters expand as expected with successive increases in the ionic radius of the A2+ ion. Cation disorder on the octahedral sites increases as the ionic radii of A2+ ion decreases. Direct-current magnetic susceptibility measurements indicate that all compounds with magnetic A2+ ions order anti-ferromagnetically with transition temperatures ranging from 12 to 15 K. Effective magnetic moments for A3V4(PO4)6 (A = Mg, Mn, Fe, Co, Ni) are 5.16, 11.04, 10.08, 9.76, and 7.96 μB per formula unit, respectively, in line with calculated values for high-spin transition metal ions. With the exception of Co3V4(PO4)6 the ultraviolet-visible spectra are dominated by d-d transitions of the V3+ ions. The striking emerald green color of Co3V4(PO4)6 arises from the combined effects of d-d transitions involving both V3+ and Co2+.