HIGH-ENERGY ELECTRON-DIFFRACTION AND X-RAY-DIFFRACTION STUDIES ON MAGNETIC-MATERIALS OF THE TYPE Y2FE17CX (0 LESS-THAN-OR-EQUAL-TO X LESS-THAN-OR-EQUAL-TO 1.6)

We have investigated the magnetic and crystallographic properties of compounds of the type Y2Fe17Cx. The Curie temperature increases strongly with x from Tc = 325 K for x = 0 to 550 K for x = 1.6. From the concentration dependence of the lattice constants it was derived that the carbon atoms occupy interstitial hole positions in Y2Fe17. With increasing carbon concentration the structure changes from the hexagonal Th2Ni17 type to the rhombohedral Th2Zn17 type. The change in crystal structure is accompanied by an expansion and contraction of the lattice in the a and c directions respectively. For concentrations near the phase transitions we observed a partial disordering of the lattice leading to an X-ray diagram of the CaCu5-type basis reflections without the superstructure lines of the Th2Ni17 or Th2Zn17 types. A detailed investigation was made by means of electron diffraction, revealing that the disorder cannot be described by a random stacking in the c-direction of hexagonal Th2Ni17-type layers and rhombohedral Th2Zn17-type layers. Actually, the disorder is a novel type of stacking in which homogeneous sheets of dumb-bell pairs of iron atoms and homogeneous sheets of rare earth atoms along particular ordering planes are stacked at random. © 1990.