On the soft magnetic properties of the compounds of the series NaxMn4.5-x/2(VO4)3 and the magnetic structure of h.t.-Mn3(VO4)2 (x=1)

The high temperature phase of manganese vanadate h.t.-Mn-3(VO4)(2) and the solid solution with NaMn4(VO4)(3) (NaxMn4.5-x/2(VO4)(3)) were shown to order ferrimagnetically below 55 K for x < 1, whereas NaMn4(VO4)(3) is an antiferromagnet. The materials show very soft magnetic properties with low coercitive fields required for demagnetisation (H-c < 0.001 T). The magnetic structure of h.t.-Mn-3(VO4)(2) was determined by Rietveld analysis of low temperature powder neutron diffraction data, and shows antiferromagnetic alignment of the magnetic moments of the Mn2+ ions on the 8c and 8d sites. The ferrimagnetic moments were shown to result from the magnetic moments of the Mn2+ cations located on the 4b site in unusual dodecahedral coordination (Hoard dodecahedron). This coordination can be understood as two penetrating oxygen coordination tetrahedra, one showing shorter and one showing longer Mn-O distances. The magnetic moments of the Mn2+ ions on the 4b site are aligned parallel to the ones on 8d and antiparallel to the ones on 8c, being in good agreement with the GKA rules. The local exchange interactions between the Mn2+ ions on the 4b to those on the 8c/8d sites are likely to be similar in strength and competitive and therefore probably contribute to the soft magnetic properties.