In this paper some structural and chemical informations about the compound A1 are given. Transmission electron microscopy investigations reveal that it is composed of a mixture of two nanocrystalline compounds A1α and A1β, with atomic compositions close to Nd 2 0 Fe 8 0 and Nd 4 0 Fe 6 0 , respectively. Their formation is described in a metastable phase diagram including also NdFe 2 , and is confirmed by thermodynamic calculations on the basis of Miedema's model.