In accordance with well-established knowledge, no stable structures of the rhombohedral modification can be found at or below the strongly stabilized half-sodiated Na Analyzing the theoretical convex hull based solely on the rhombohedral data points, represented by the orange dashed line in Figure , reveals stable structures at Na 1 FeHCF and Na 1.66 FeHCF. It appears that even in the rhombohedral system -where the tetragonal sodium arrangement is replaced by layers of sodium perpendicular to the c-axis -the alternating arrangement of Fe 2+ -C and Fe 3+ -N in Na 1 FeHCF remains energetically favorable.