Electronic and magnetic structure of vivianite: cluster molecular orbital calculations

The electronic and magnetic structure of the octahydrophosphate vivianite, Fe-3(PO4)(2). 8H(2)O, has been investigated by cluster molecular orbital ca lculations in local spin density approximation. Optical and Mossbauer spect ra are well reproduced by the calculations, and the differences between the two iron sites can be correlated with differences in the geometrical struc ture of the first coordination sphere. The spin structure within the crysta llographic ac plane is derived and explained on the basis of different supe rexchange pathways via edges of the phosphate tetrahedra. The calculations demonstrate that quite large clusters (up to 118 atoms) are necessary to ar rive at reliable results.