On the nitrogen occupation in the Y 2Fe 17 lattice

Abstract

Insertion of nitrogen of carbon into the interstitial sites of the R{sub 2}Fe{sub 17} compounds (R represents a rare-earth element) produces a significant improvement in their magnetic properties. The N and C atoms are accommodated mostly at the octahedral interstitial (6h) sites. Since the improvement originates from both the lattice expansion and the increase of electric field gradient (EFG) of the R site upon atomic insertion, and complete filling of the 6h sites, which yields a formula unit of R{sub 2}Fe{sub 17}N{sub 3}, is expected to be ideal for achieving both the largest lattice expansion and the largest, and most homogeneous, increase in the EFG. To date, efforts have been made to increase the amount of N and/or C inserted into the 2:17 phase by: (1) nitrogenation under high gas pressure, (2) by using relatively high temperature, (3) by combined nitrogenation and carbonation processing, and (4) by element substitution. However, the general results show that, while the reported N content varies over a wide range from about 2 to 3 or more, very similar lattice expansion values were obtained in the various studies. These results raise the fundamental questions: (1) ``what is the actual N content in the nitride R{submore » 2}Fe{sub 17} phase?`` and (2) ``why is there such a large variation in the reported N content values?`` Here, it should be emphasized that it is the N content which is actually in the 2:17 phase that is important. In this regard, the most frequently used macroscopic measurement methods such as gravimetry or volumetry are not accurate since they account for the total absorption of N by the sample and not the N absorbed solely into the 2:17 phase. In this work, the authors present the results of a {sup 89}Y and {sup 57}Fe spin-echo NMR study which enables a determination of the N coordination in the Y{sub 2}Fe{sub 17}N{sub x} system.« less