Polymorphism and intergrowth structures in the nanostructured Pr-Co system: Pr2Co7 and Pr5Co19

A nanocrystalline powder was prepared in order to obtain the rhombohedral Pr5Co19-3R phase by arc melting and high-energy ball milling. Bright field images show a material characterized by nanoscale contrast bands with, in some cases, atomic plane stacking shifts at higher magnification. Calculations of HREM images demonstrate that these characteristics are partly intrinsic to this type of structure and can appear when projections are deviated from a zone axis. Previous XRD analyzes and magnetic M(H) measurements had shown characteristics of a powder containing one single hard magnetic phase identified as the Pr5Co19-3R structure. In the present study, a TEM analysis of FOLZ reflections in FFT power spectra unambiguously reveals the existence of nanograins exhibiting the hexagonal polytype structure Pr2Co7-2H. Some Pr2Co7-Pr5Co19 intergrowth domains also occur in the stacking of the subunit layers, which could be explained by a structural transition from the hightemperature stable Pr5Co19-3R phase. The presence of oxide precipitates was also revealed, identified as the cubic (Ia-3 space group) Pr2O3 crystal phase from atom probe tomography (APT) and high-resolution electron microscopy (HREM) studies.