Negative charge transfer in Ternary Nitride Antiferromagnets

Nitrides display unique chemical and structural characteristics rendering them attractive for a wide variety of applications that complement those based on transition metal oxides. However, limited by the inherent stability of N₂, only a small number of complex ternary nitrides have been synthesized. Such disparity presents an extensive opportunity for exploration. Here, we investigate the magnetic and electronic properties of the ternary nitride Ca₆FeN₅ employing first-principles density functional theory (DFT) calculations. Although compounds containing Fe³⁺ ions with a high-spin d⁵ electronic configuration and exhibiting antiferromagnetic ordering are typically associated with Slater insulating behavior, band theory reveals fractional band filling at the Fermi level. Our DFT results attribute this unexpected metallic character to a negative charge transfer mechanism and a more complex spin state. However, the narrow bandwidth of the N-derived states at the Fermi level suggests that Ca₆FeN₅ is more accurately described as a small-gap semiconductor. The material may represent a new promising material platform for carrier- or pressure-induced metal-insulator transition.