Spin crossover transition in the cluster compounds Nb6I11 and HNb6I11

Our specific-heat measurements in Nb6I11 indicate a phase transition (PT) at 274 K. Independent x-ray studies confirm that at Tc a structural phase transition occurs. In HNb6I11 the equivalent PT occurs at 324 K. The magnetic-susceptibility measurements indicate that the electronic ground-state degeneracy of the cluster is reduced by the PT, going from a high-temperature quartet to a doublet for Nb6I11, and from a high-temperature triplet to a singlet for HNb6I11. Independent band-structure calculations suggest that, in distinction to the cooperative Jahn-Teller effect, the ground-level degeneracy is removed here by the crossing of the electronic levels which start to move with the onset of the structural deformation. The mean-field theory for the susceptibility of a two-level-model system is presented here and fitted to the experiment.