ELECTRON-DIFFRACTION AND SUBSTITUTIONAL DISORDER PHENOMENA IN Y2FE17CX (0 LESS-THAN-OR-EQUAL-TO X LESS-THAN-OR-EQUAL-TO 1.6)

Magnetic materials of the type Y2Fe17C(x) (0 less-than-or-equal-to X less-than-or-equal-to 1.6) show for increasing C-concentration a structural transformation from the hexagonal to the rhombohedral structure. For concentrations near the phase transition, X-ray diagrams reveal only basic diffraction lines, and are interpreted in terms of the CaCu5-structure. Electron diffraction disproves this interpretation, since diffuse streaks in between basal hexagonal planes in reciprocal space are observed. These streaks indicate a substitutionally disordered state based upon the rhombohedral structure: homogeneously filled sheets either of Fedumbbells or of Y-atoms are stacked randomly along the six [101]-ordering planes.