Highly Coordinated Iron and Cobalt Nitrides Synthesized at High Pressures and High Temperatures

Highly coordinated iron and cobalt nitrides were successfully synthesized via direct chemical reaction between a transition metal and molecular nitrogen at pressures above approximately 30 GPa using a laser-heated diamond anvil cell. The synthesized novel transition metal nitrides were found to crystallize into the NiAs-type or marcasite-type structure. NiAs-type FeN could be quenched at ambient pressure, although it was gradually converted to the ZnS-type structure after the pressure was released. On the other hand, CoN was recovered with ZnS-type structure through a phase transition from NiAs-type structure at approximately a few gigapascals during decompression. Marcasite-type CoN₂ was also synthesized at pressures above approximately 30 GPa. High-pressure in situ X-ray diffraction measurement showed that the zero-pressure bulk modulus of marcasite-type CoN₂ is 216(18) GPa, which is comparable to that of RhN₂. This indicates that the interatomic distance of the N-N dimer in marcasite-type CoN₂ is short because of weak orbital interaction between cobalt and nitrogen atoms, as in RhN₂. Surprisingly, a first-principles electronic band calculation suggests that the NiAs-type FeN and CoN and marcasite-type CoN₂ exhibit metallic characteristics with magnetic moments of 3.4, 0.6, and 1.2 μ_B, respectively. The ferromagnetic NiAs-type structure originates from the anisotropic arrangement of transition atoms stacked along the c axis.