The Mixed Transition-Metal Cyanamide MnCr₂(NCN)₄

The ternary transition-metal cyanamide MnCr₂(NCN)₄ was synthesized by a solid-state metathesis reaction between MnCl₂, CrCl₃, and ZnNCN. Powder X-ray diffraction reveals that MnCr₂(NCN)₄ adopts an orthorhombic [NiAs]-derived structure with Pbcn symmetry, featuring a hexagonally close-packed array of NCN^2- with metal cations in 3/4 of the octahedral interstitial holes. The question of cation order was addressed via the combinatorial use of X-ray powder diffraction, neutron powder diffraction, electron diffraction, and HAADF-STEM measurements. These studies support an average structure with complete ordering of Mn²⁺ and Cr³⁺ in single and double corrugated rows of like cations, respectively. HAADF images, however, suggest locally a degree of cation disorder, especially at the manganese site. UV-vis and XANES measurements confirm the assignment of divalent manganese and trivalent chromium, whereas IR spectroscopy reveals the presence of cyanamide-type NCN^2- moieties with a degree of single and triple bond character due to the asymmetric coordination that is distorted away from the regular trigonal prismatic coordination encountered in the CoNCN aristotype due to vacancy and cation order. Finally, SQUID magnetometry unearths predominantly antiferromagnetic interactions with a transition to what appears to be a ferrimagnetically ordered state below 160 K.