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 CoN 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 CoN is 216(18) GPa, which is comparable to that of RhN. This indicates that the interatomic distance of the N-N dimer in marcasite-type CoN is short because of weak orbital interaction between cobalt and nitrogen atoms, as in RhN. Surprisingly, a first-principles electronic band calculation suggests that the NiAs-type FeN and CoN and marcasite-type CoN exhibit metallic characteristics with magnetic moments of 3.4, 0.6, and 1.2 μ, respectively. The ferromagnetic NiAs-type structure originates from the anisotropic arrangement of transition atoms stacked along the c axis.

The high‐pressure nitridation of nickel was investigated using a laser‐heated diamond‐anvil cell. Marcasite‐type nickel pernitride (NiN2) was synthesized at approximately 40 GPa, and it transformed into the tetragonal phase at approximately 3 GPa along with the decompression. The structural refinement of marcasite‐type NiN2 at 36 GPa gives an N‐N distance of 1.24 Å. The first‐principles calculation reveals that the marcasite‐type NiN2 is a narrow‐gap semiconductor, and high‐pressure in‐situ X‐ray diffraction measurements revealed a zero‐pressure bulk modulus of 172(6) GPa. The axial ratios (c/a and b/a) of marcasite‐type NiN2 are close to the upper‐limit values of marcasite‐type structures, which suggests that the stability of marcasite‐type transition metal pernitrides strongly depends on the d‐electrons.

High-pressure stability, ambient metastability, and high-pressure crystal chemistry of chemical bonds of marcasite-type RhN2 have been investigated using a laser-heated diamond-anvil cell up to a pressure of 70.6 GPa. High-pressure in-situ X-ray diffraction and Raman scattering measurements revealed that the marcasite-type RhN2 structure is stable up to 70.6 GPa and exhibited an order of axial compressibility of βc > βb > βa. This indicates that single bonded nitrogen dimer (N-N) plays an important role in the incompressibility of a- and b-axes than in that of the c-axis and stabilizes the marcasite-type structure at high-pressure. Field emission scanning electron microscopic analysis in combination with the energy dispersive X-ray spectroscopic measurements and the result of our previous study indicates that the marcasite-type RhN2 can be quenched to ambient pressure when the grain size is less than 100 nm. Our study together with other previous studies indicates that the ambient metastability of 4d platinum group pernitrides (RuN2, RhN2, and PdN2) decreases from ruthenium to palladium.

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