A series of NdDyFe(17-y-x)CoxSiy solid solutions with = 2 and 3 and = 0.5 1.0 and 1.5 were prepared by induction melting stoichiometric amounts of high-purity elements. The postannealed samples consist of two phases belonging to the space groups R3 m and P63 mmc . The lattice parameters and the unit cell volumes were calculated from the refinements of the magnetic and structural unit cells using the FULLPROF version of the Rietveld program. For a fixed content of Co, the maximum Curie temperatures (305 C to 405 C) were observed in samples with = 1 and having two phases, a disordered rhombohedral (DR) structure and a disordered hexagonal (DH) structure. An increase in the Curie temperature of 70 C per atom of cobalt is observed in NdDyFe(17-y-x)CoxSiy with y = 1 and x\u3c 3, suggesting that with a suitable choice of rare earths this DR phase may be a promising candidate for high-energy product permanent magnets. The magnetization versus temperature (M versus T) plots of the solid solutions, which consist of two phases, exhibit only a single magnetic ordering transition temperature
