Site preference of Fe and V in PbFexV6-xO11 by neutron diffraction and Mossbauer spectroscopy: Low temperature investigation for x=1.75

The PbFexV6-xO11 series adopting the R block structural type were investiga ted by both powder neutron diffraction and Mossbauer spectroscopy in order to quantify a Fe3+ cation occupancy in available crystallographic sites. Fo ur 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 cle arly indicated in M(4) triangular bipyramidal sites, while M(2) and M(3) di mers are statistically occupied by Fe and V. Finally, the layers of octahed ral M(1) sites remain solely occupied by vanadium. For the latter, Mossbaue r 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 o f the room temperature form, although there is a surprising c stacking axis increase. We have already described the magnetic ZFC/FC strong irreversibi lity in these series and assigned it to frustration arising in M(1)O-6 laye rs. The magnetic structure refinement confirmed this model as possible in a n identical magnetic/crystallographic unit cell. (C) 1999 Academic Press.