Mossbauer characterisation of synthetic analogues of the helvite minerals Fe4M4[BeSiO4]6X2 ( M = Fe, Mn, Zn; X = S, Se)

We report on this paper on the Mossbauer characterisation of the family of synthetic helvite analogues, Fe4M4[BeSiO4](6)X-2 (M = Fe, Mn, Zn; X = S, Se). The data show iron to be present as high spin Fe(II) in tetrahedral coordination. The room temperature Mossbauer spectra are composed either by singlets or doublets with small quadrupole splitting values suggesting a small valence contribution at that temperature. From the dependence of the quadrupole splitting with temperature the separation Delta between the two e(g) orbitals has been estimated. The values of Delta range from 46.3 cm(-1) for the material Fe-8[BeSiO4](6)S-2 to 58.2 cm(1) for the material Fe4Zn4[BeSiO4](6)S-2. The lack of long-range magnetic order observed in the Mossbauer spectra was confirmed by neutron diffraction data which suggests that the M4X units are largely magnetically isolated within their cages leading to a frustrated magnet with no long range interaction for the sulfide species.