SITE PREFERENCE OF Fe AND V IN PbFexV6-xO11 BY NEUTRON DIFFRACTION AND MOSSBAUER SPECTROSCOPY: LOW TEMPERATURE INVESTIGATION FOR x=1.75

摘要

The PbFexV6-xO11 series adopting the R block structural type were investigated by both powder neutron diffraction and Mossbauer 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 V substitution by Fe 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 V. For the latter, Mossbauer and neutron diffraction results slightly diverge because of the possible consideration of Fe species within impurities present in the samples. The 2 K crystal structure refinement for PbFe1.75V4.25O11 is similar to that of the RT form, although there is a surprising c stacking axis increase. Authors 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. 27 refs.