The magnetic structures of double tungstates, NaM(WO4)2, M=Fe, Cr: Examples for superexchange couplings mediated by [NaO6]-octahedra

The crystal structures of the double tungstates NaM(WO4)2 with M=Fe, Cr and their solid solution are similar to the wolframite-type structure in the space group P2/c, but with doubled a lattice parameter. Magnetization and neutron-diffraction data reveal that NaFe(WO4)2 orders antiferromagnetically below 5 K with a commensurate propagation vector k=(12,12,12) and magnetic moments of Fe3+ ions oriented along the a-axis. NaCr(WO4)2 is antiferromagnetic below 10 K. Its magnetic structure is based on the propagation vector k=(12,12,0), and the magnetic moments of Cr3+ ions are aligned along the b-axis. The magnetic structure in the bc-plane is explained by a supersuperexchange mechanism. Long-range magnetic superexchange interactions along paths including [NaO6]-octahedra are necessary to explain the observed magnetic structures. Mixed NaFexCr1−x(WO4)2, with x=0.25, 0.5, 0.75, do not indicate magnetic order, neither in magnetization nor neutron-diffraction data.