Understanding magnetic interactions in the series A(2)FeX(5)center dot H2O (A = K, Rb; X = Cl, Br). I. Spin densities by polarized neutron diffraction and DFT calculations.

The compounds A(2)FeX(5)center dot H2O (A=alkali or NH4; X=Cl, Br) form a series of collinear antiferromagnets with transition temperatures in the range from 6 to 23 K. These ordering temperatures are much higher than in other hydrated salts of transition-metal ions with similar distances between magnetic ions. Spin-density distributions have been determined in Rb2FeBr5 center dot H2O and K2FeCl5 center dot H2O by means of the polarized-neutron-diffraction technique and ab initio calculations in order to elucidate the mechanism of such enhancement. The results show a large spin-density delocalization (similar to 20%) toward the ligand atoms, which explains the efficiency of the superexchange pathways in transmitting the magnetic interaction.