Synthetic Engineering in Na₂MSn₂(NCN)₆ (M = Mn, Fe, Co, and Ni) Based on Electronic Structure Theory

Quaternary transition metal cyanamides Na₂MSn₂(NCN)₆ with M = Mn, Fe, Co, and Ni were prepared via solid-state metathesis reactions between Na₂Sn(NCN)₃ and binary transition metal fluorides MF₂ in a 2:1 molar ratio. All phases crystallize isotypically in [NiAs]-derived structures (P3̅1m) with inter- and intra-layer cation ordering over the octahedral sites. This leads to a highly asymmetric coordination of the NCN^2- anion, resulting in a strong degree of cyanamide character, which is confirmed via IR measurements. Intriguingly, the optical properties of Na₂MSn₂(NCN)₆ phases change markedly as the nature of the divalent transition metal is varied, and UV-vis measurements evidence a band gap reduction from Mn (3.43 eV) via Fe (1.90 eV) to Co (1.75 eV), which broadly mirrors the DFT+U calculated energetic interval from the Fermi level to the unoccupied 3d states. Mott-Schottky analysis then goes on to characterize Na₂FeSn₂(NCN)₆ and Na₂CoSn₂(NCN)₆ as n-type semiconductors with flat-band potentials of 0.46 and -0.24 eV, respectively, vs RHE. This study demonstrates the utility of transition metal substitutions, within a flexible cyanamide framework, to electronically tune this growing family of pseudo-oxides.