Site Preference of Fe and V in PbFexV6-xO11 by Neutron Diffraction and M?ssbauer Spectroscopy: Low Temperature Investigation for x=1.75

The PbFexV6-xO11 series adopting the R block structural type were investigated by both powder eutron diffraction and M?ssbauer spectroscopy in order to quantify a Fe3+ cation occupancy in available crystallographic sites. Four samples with formal x ratio of 1, 1.21, 1.6, and 1.75 were investigated. The results obtained from the two techniques match rather well, providing reliable refinements. The preferential vanadium substitution by iron is clearly indicated in M(4) triangular bipyramidal sites, while M(2) and M(3) dimers are statistically occupied by Fe and V. Finally, the layers of octahedral M(1) sites remain solely occupied by vanadium. For the latter, M?ssbauer and neutron diffraction results slightly diverge because of the possible consideration of iron species within impurities present in the samples. The 2 K crystal structure refinement for PbFe1.75V4.25O11 is similar to that of the room temperature form, although there is a surprising c stacking axis increase. We have already described the magnetic ZFC/FC strong irreversibility in these series and assigned it to frustration arising in M(1)O6 layers. The magnetic structure refinement confirmed this model as possible in an identical magnetic/crystallographic unit cell.