Synthesis,structure and spin-crossover transition of The Cluster Compound Nb6I11−xBrx (0 ⩽ x ⩽ 2.7)

The homogeneous phase Nb₆I_11?xBr_x (0 ? x ? 2.7) is synthesized from Nb₃Br₈, Nb₃I₈ and Nb in sealed Nb capsules at 1 130 K. A second-order phase transition is found as for the composition Nb₆I₁₁ itself, changing the space group from P2₁cn (low temperatures) to Pccn, accompanied by a spin-crossover from a doublet to a quartet state. With increasing Br content the lattice constants decrease and the transition temperature shifts from 274 to 170 K while the transition interval is broadened simultaneously. Single crystal investigations for x = 0.5 and 2.3, each at 110 and 298 K, indicate a preferred substitution of one of the bridging I positions (I6) by Br atoms.