  From density functional DFT investigations helped with crystal chemistry
rationale, diiron (pairs of Fe), mostly known in molecular diiron nona-carbonyl
Fe2(CO)9 and diiron-mono-carbide Fe2C carbide, are embedded in hexagonal C6
substructures. Generated Fe2C6 and Fe2C12 are shown to be more cohesive than
the mono-carbide on one hand, and increasingly cohesive from hexa- C6 to
dodeca- C12 on the other hand. From energy differences, the ground state is
spin-polarized SP, versus a non-spin-polarized NSP configuration, and
identified as ferromagnetic versus a higher energy anti-ferromagnetic
hypothesis. The projection of the magnetic charge density on Fe and C, shows
that only Fe carries the magnetic moment, while carbon receives charges from Fe
as illustrated by the electron localization function ELF 3D and 2D mapping. SP
configuration induces an enlarged c/a hexagonal ratio, versus NSP, while
a(hex.) remains constant thanks to the rigid C6 carbon substructure network,
resulting in an anisotropic magneto-volume response. This feature, essentially
due to in-plane diiron, is discussed from the energy-volume (E, V) NSP and SP
equations of state EOS and derived quantities like volume- and d(Fe-Fe)-
changes of the magnetization.
