Rare-earth effect on the site occupation of Al and C in R2Fe15Al2Cy alloys

Abstract

Neutron diffraction has been used to investigate the rare-earth effect on the site occupations of Al and C atoms in the combined substitutional–interstitial compounds, R2Fe15Al2Cy (R=Pr, Tb, Ho). All three samples have the rhombohedral Th2Zn17-type structure. As the composition changes from light rare-earth to heavy rare-earth, the site occupations 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 highest Al occupation for all samples. The site occupation of the C atoms on the octahedral interstitial site (9e) is much higher in the heavy rare-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.