Binary epsilon-Fe3N1098 and ternary epsilon-Fe2.322CO0.678N0.888 were synthesized as spherical bulk materials using novel high-pressure solid-state metathesis reactions (HPSSM). The structural and magnetic properties of the two nitrides were investigated using neutron powder diffraction (NPD), a vibrating sample magnetometer (VSM), and first-principle calculations. We found that at high pressure and high temperature (HPHT), nitrogen atoms enter the interstitial 2d sites of the nitrogen-rich epsilon-Fe3N1098 structure, space group P6(3)22, which were previously reported to be vacant. The stoichiometry of the nitride (N/Me ratio) and the level of disorder have a significant influence on the site occupancies and local magnetic moments in these iron-based nitrides. The substitution of Fe by Co in epsilon-Fe2.322CO0.678N0.888 did not reduce the mean magnetic moment (mu(m)) per metal atom at the Wyckoff 6 g site. NPD refinements showed that the magnetic moment per Fe atom in epsilon-Fe3N1098 (at 1.8 (4) mu(B)) was slightly higher than that of epsilon-Fe2.322CO0.678N0.888, where the moment per Fe/Co atom is 1.7(2) mu(B). (C) 2018 Elsevier B.V. All rights reserved.