Spin crossover transition in the cluster compounds ${\mathrm{Nb}}_6$${\mathrm{I}}_{11}$ and H${\mathrm{Nb}}_6$${\mathrm{I}}_{11}$

Our specific-heat measurements in ${\mathrm{Nb}}_6$ ${\mathrm{I}}_{11}$ indicate a phase transition (PT) at 274 K. Independent x-ray studies confirm that at $T_c$ a structural phase transition occurs. In H${\mathrm{Nb}}_6$ ${\mathrm{I}}_{11}$ 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 ${\mathrm{Nb}}_6$ ${\mathrm{I}}_{11}$, and from a high-temperature triplet to a singlet for H${\mathrm{Nb}}_6$ ${\mathrm{I}}_{11}$. 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.