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.

Marcasite-type rhodium nitride was successfully synthesized in a direct chemical reaction between a rhodium metal and molecular nitrogen at 43.2 GPa using a laser-heated diamond-anvil cell. This material shows a low zero-pressure bulk modulus of K0 = 235(13) GPa, which is much lower than those of other platinum group nitrides. This finding is due to the weaker bonding interaction between metal atoms and quasi-molecular dinitrogen units in the marcasite-type structure, as proposed by theoretical studies.

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The last remaining marcasite-type RuN2 was successfully synthesized by direct chemical reaction between ruthenium and molecular nitrogen above the pressure of 32 GPa. For the first time, we found that Ru 4d is weakly hybridized with N 2p in the structure by using transmission electron microscopy equipped with electron-energy-loss spectroscopy. Our finding give important knowledge about the platinum-group pernitride with respect to the chemical bonding between platinum-group element and nitrogen.

The chemical reaction of group-14 elements with molecular nitrogen at pressures above 60 GPa successfully leads to the formation of pyrite-type pernitrides. These new crystalline group-14 elemental pernitrides were found to show a bulk modulus higher than the other known group-14 elemental nitrides. Our results offer significant new progress in the solid-state chemistry of nitrides.