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 T_N = 15 K, and Pr₂₁Fe₁₆Te₆B₃₀ undergoes a likely ferrimagnetic transition at T_C = 23 K.