Highly ordered carbon penetration into the Mn₅Ge₃C_x lattice: A superstructure in Mn₅Ge₃C_0.5 inferred from a 55 Mn NMR study

Local magnetic properties of the Mn5Ge3Cx (001) epitaxial films grown on Ge(111) with a nominal carbon concentration 0 <= x <= 0.85 have been studied by means of Mn-55 nuclear magnetic resonance (NMR). The NMR spectra were recorded from the demagnetized films and from the films fully magnetized along the c direction as well as in the hexagonal c plane. The data unambiguously show a synchronized carbon penetration of the crystal lattice (D8(8) structure, Nowotny phase, space group P6(3)/mcm), occupying every second 2(b) void located within the chain of Mn[6(g)] octahedra and setting the limit for the carbon uptake into the Mn5Ge3 lattice at x = 0.5. Moreover, the NMR data indicate that the chains of carbon-filled voids are correlated also in plane. Eventually, for the end concentration of x = 0.5, a highly ordered superstructure is proposed, responsible for the observed two magnetically inequivalent positions within the Mn[4(d)] sublattice.