Dy0.64{Dy5[Fe2C9]}: A complex carbide with a composite structure

The ternary carbide Dy5.64[Fe2C9] has been prepared by high temperature synthesis from the elements. The compound forms as a composite structure with two partial structures, {Dy5[Fe2C9]} and {Dy0.64} crystallizing in space groups Pnam and Pbam, respectively. The lattice parameters a=2934.63(9) pm and b=1263.63(5) pm for the two partial structures are identical, whereas the c lattice parameters for {Dy5[Fe2C9]} (cH=504.14(3) pm) and {Dy0.64} (cG=1056.01(5) pm) are different. This gives rise to a modulated structure in the (3+1)D superspace group Pnam(00g) with q=cH/cG=0.4775. The refinement converged to R(F) values below 0.055 for each partial structure using harmonic and Crenel functions. The compound is a bad metallic conductor and represents a new structure type composed of Dy3+ species embedded in a matrix of discrete planar FeC(C2)2 units and edge-sharing Fe(C2)4 tetrahedra forming infinite chains Fe(C2)4/2 running along the c axis. - Graphical abstract: Dy guest atoms (green) within an carbon octahedral channel of the modulated structure together with a tetrahedral Fe(C2)4/2 chain and triangular planar FeC(C2)2 groups in the crystal structure of Dy0.64{Dy5[Fe2C9]}. Highlights: ► The first successful refinement of a large composite structure Dy0.64{Dy5[Fe2C9]}. ► The presence of two kinds of complex anions with Fe as central atoms and C-ligands within this structure. ► The oxidation state of the Fe atoms are low, they can be assigned FeI and FeII.