STRUCTURE DEFECTS IN RARE-EARTH IRON CARBIDES WITH THE HEXAGONAL TH2NI17 STRUCTURE TYPE - A STUDY BY MEANS OF HIGH-RESOLUTION ELECTRON-MICROSCOPY

Rare-earth iron carbides (RE)2Fe17Cx have the hexagonal (hP38) Th2Ni17 structure from terbium to lutetium and for yttrium, provided that the carbon concentration is below the value of the hexagonal-to-rhombohedral transformation point. High resolution electron microscopy reveals structure defects which change the site symmetry and therefore the magnetocrystalline anisotropy of the rare-earth sublattice on a local scale. Under suitably chosen imaging conditions, the rare-earth atom columns can be identified as white dots in the high resolution images. Less bright or even absent dots at some particular positions of the rare-earth columns are interpreted in terms of partially disordered atom columns with a mixed occupation of rare-earth atoms and iron dumbbells. Image simulations confirm this hypothesis of local disorder. Conservative antiphase boundaries are observed. Defect areas containing successive nonconservative antiphase boundaries are identified as local slabs of material with the ThMn12 structure type. © 1991.