Structural features of intermetallic compounds Pr2M17 (M = Fe, Co) and implications on magnetic properties

Novel structural features have been shown by single-crystal x-ray diffraction (XRD) in Pr2Fe17 and Pr2Co17 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 Pr2M17 (M = Fe, Co) structure is R3m and not the usually considered centrosymmetric Rm 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 Pr2Fe17 compounds. 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. ©2003 The American Physical Society.