Structural Disorder in Intermetallic Boride Pr21M16Te6B30 (M = Mn, Fe): A Transition Metal Cluster and Its Evil Twin.

Reactions of boron, tellurium, and either iron or manganese in a praseodymium-nickel flux led to the production of Pr21M16Te6B30 (M = Fe or Mn) with a novel structure type that features M16B30 clusters surrounded by a Pr/Te framework. Due to disorder in the orientation of the transition metal boride clusters, these phases initially appear to form in the cubic space group Pm3̅m. However, analysis of site occupancy, bond lengths, and local structure in the M16B30 sublattice indicates the local symmetry is P4̅3m. This space group symmetry is supported by transmission electron microscopy studies including selected area electron diffraction (SAED) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), which indicate ordered regions. The M16B30 cluster twinning domain that could be as small as nanometer size inside a single crystal results in the misleading Pm3̅m symmetry. Electronic structure calculations indicate the Pr21M16Te6B30 phases are metals. Magnetic susceptibility measurements show that both the praseodymium and the transition metal have magnetic moments in these compounds. Pr21Mn16Te6B30 exhibits antiferromagnetic ordering at TN = 15 K, and Pr21Fe16Te6B30 undergoes a likely ferrimagnetic transition at TC = 23 K.