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

Quaternary transition metal cyanamides NaMSn(NCN) with M = Mn, Fe, Co, and Ni were prepared via solid-state metathesis reactions between NaSn(NCN) and binary transition metal fluorides MF in a 2:1 molar ratio. All phases crystallize isotypically in [NiAs]-derived structures (1) with inter- and intra-layer cation ordering over the octahedral sites. This leads to a highly asymmetric coordination of the NCN anion, resulting in a strong degree of cyanamide character, which is confirmed via IR measurements. Intriguingly, the optical properties of NaMSn(NCN) 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+ calculated energetic interval from the Fermi level to the unoccupied 3d states. Mott-Schottky analysis then goes on to characterize NaFeSn(NCN) and NaCoSn(NCN) as -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.