Half-Filled Honeycomb Transition-Metal Cyanamide Fe₃Cr₂(NCN)₆

We report the existence of Fe₃Cr₂(NCN)₆, a ternary cyanamide featuring two distinct paramagnetic centers, Fe²⁺ and Cr³⁺, bridged by NCN^2- anions, and adopting a layered motif. The compound was synthesized via solid-state metathesis using ZnNCN as carbodiimide source and the corresponding binary metal chlorides. Initial crystal structure determination was undertaken from powder X-ray diffraction data, revealing an orthorhombic Pnnm structure. Complementary neutron diffraction measurements enabled an unambiguous atomic assignment to all respective crystallographic sites. Moreover, the oxidation states of divalent Fe and trivalent Cr were confirmed by XANES measurements. The crystal structure of Fe₃Cr₂(NCN)₆ derives from NiAs and features hexagonally packed NCN^2- anions with cations occupying five-sixths of the octahedral voids in a vacancy-ordered framework, resulting in a novel ordering scheme: honeycomb and triangular motifs of Fe²⁺/Cr³⁺ decorate a mixed-substructure pattern, motivating an investigation of the magnetic properties. It reveals predominantly antiferromagnetic (AFM) interactions at high temperature, followed by a magnetic transition at 160 K marking the onset of long-range order. Below this temperature the emergence of hysteresis with a weak remanent magnetization point toward either ferrimagnetism, due to uncompensated Fe²⁺ and Cr³⁺ spins, or weak ferromagnetism as a result of spin canting.