Polarised neutron diffraction study of spin localisation in Nb6I11

Abstract

The spin density distribution in single crystals of Nb₆I₁₁ in an applied field of 4.6 T has been studied using polarised neutron diffraction at 285 and 40 K. At 285 K each Nb₆I₈ cluster in Nb₆I₁₁ carries three unpaired spins whereas at 40 K, below the spin-crossover transition, the cluster carries only one unpaired spin. It has been shown that at 40 K the spin density is not delocalised in the cluster, but can be associated with the bonds formed with just one of the Nb atoms. In the higher temperature phase the spin density is found to be more evenly distributed over the cluster, although the moment on one of the three pairs of equivalent Nb atoms is significantly higher than that on the other two. These results can be understood in terms of earlier molecular orbital calculations.