RARE-EARTH EFFECT ON THE SITE OCCUPATION OF AL AND C IN R2FE15AL2CY ALLOYS

Neutron diffraction has been used to investigate the rare-earth effect on the site occupations of Al and C atoms in the combined substitutio nal-interstitial compounds, R2Fe15Al2Cy (R=P-r, Tb, Ho). All three sam ples have the rhombohedral Th2Zn17-type structure. As the composition changes from light rare-earth to heavy rare-earth, the site occupation s of the substitutional Al atoms on the 6c and 18f increase while that on the 18h site decreases. However, the 18h site still has the highes t Al occupation for all samples. The site occupation of the C atoms on the octahedral interstitial site (9e) is much higher in the heavy rar e-earth (Ho) sample than in the light rare-earth (Pr) sample. However, the tetrahedral interstitial 18g site shows a reverse behavior. This 18g site occupation of C atoms has not been observed before except in gas phase interstitial compounds. The Al site occupation is determined by the crystal environment, while the C site occupation appears to be determined predominantly by site size. (C) 1997 American Institute of Physics.