Synthesis of metastable Co4N, Co3N, Co2N, and CoO0.74N0.24 from a single azide precursor and intermediates in CoBr2 ammonolysis

Metastable cobalt nitrides Co4N, Co3N, Co2N, and cobalt oxide nitride CoO0.74N0.24 were synthesized by ammonolysis of one single metastable azide precursor, [Co(NH3)5N3]Cl2, in high purity by controlled variation of the reaction conditions (temperature, duration, gas atmosphere, and flow rate). The alternative route towards cobalt nitrides by ammonolysis of CoBr2 proceeds via [Co(NH3)6]Br2 and Co(NH3)2Br2 to exclusively yield Co3N. For Co3N1.1 a complex magnetic behavior is observed: dominating ferromagnetic ordering at low temperatures and a small coercive field of 10 mT. Co2N orders antiferromagnetically below 10 K. Electronic structure calculations [DFT, generalized-gradient approximation (GGA)+U] on CoO0.75N0.25 reveal that the experimentally observed rock-salt structure is less stable than the sphalerite-type polymorph by about 30 kJ mol-1 and possesses an antiferromagnetically ordered ground state.