Novel structural features have been shown by single-crystal x-ray diffraction ~XRD! in Pr 2 Fe 17 and Pr 2 Co 17 intermetallic compounds. Some of these features are common characteristics of the two compounds, while others appear to be specific to the cobalt- or iron-based phase. It has been found that in both cases the suitable space group for the description of the rhombohedral Pr 2 M 17 (M5Fe, Co) structure is R3m and not the usually considered centrosymmetric R3 ¯ m group. This results in a puckered arrangement of the M atoms in the 3d-metal plane and in the nonsymmetric position of the two crystallographically independent Pr atoms with respect to the 3d metal atoms in the mixed Pr-M layer. The Pr-Fe compounds are always stoichiometric even in the presence of excess Pr in the starting nominal composition. The real structure is characterized by a very frequent occurrence of obverse-reverse twinning domains stacked along the c axis. The presence of multiple Pr sites and, as a consequence, of different contributions to the magnetocrystalline anisotropy is the key to explaining the occurrence of a double field-induced transition in the magnetization curve of Pr 2 Fe 17 com- pounds. In contrast some excess Pr was found in the cobalt compounds. Two groups of samples having, respectively, low (.1%) and high (.5%) excess Pr and consequently different anisotropy field values were found, depending on the starting nominal composition. The origin of the differences observed in Fe and Co compounds is discussed.
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