Nitrogen atom location in rhombohedral and hexagonal RE₂Fe ₁₇N_x compounds

High resolution neutron powder diffraction studies were performed on Nd₂Fe₁₇N_x and Y₂Fe ₁₇N_x. A strong preferential site occupation of the N atoms on the 9e position of the rhombohedral compound Nd₂Fe ₁₇N_x was observed. In the hexagonal compound Y ₂Fe₁₇N_x the N atoms occupy primarily the 6h sites (82%) and only a small fraction goes into the 12i site (12%). The refined nitrogen occupation numbers correspond to the formula compositions Nd ₂Fe₁₇N_2.6 and Y₂Fe ₁₇N_2.9.