Structural Disorder in Intermetallic Boride PrMTeB (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 PrMTeB (M = Fe or Mn) with a novel structure type that features MB 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 3̅. However, analysis of site occupancy, bond lengths, and local structure in the MB sublattice indicates the local symmetry is 4̅3. 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 MB cluster twinning domain that could be as small as nanometer size inside a single crystal results in the misleading 3̅ symmetry. Electronic structure calculations indicate the PrMTeB phases are metals. Magnetic susceptibility measurements show that both the praseodymium and the transition metal have magnetic moments in these compounds. PrMnTeB exhibits antiferromagnetic ordering at = 15 K, and PrFeTeB undergoes a likely ferrimagnetic transition at = 23 K.