An Unusual Metallic Nitride: Sr2NiN2

The synthesis of single crystals of Sr2NiN2 has been accomplished in a liquid sodium flux from strontium or strontium nitride, nickel, and sodium azide. The decomposition of sodium azide provides additional nitrogen for the synthesis. The structure of Sr2NiN2 is derived from the Na2HgO2 structure type. However, Sr2NiN2 forms with a twinned superstructure. Single crystal X-ray diffraction was used to determine the structure of the tetragonal subcell: I4/mmm, a=3.8874(3) ?, c=13.992(2) ?, Z=2, R(F)=5.2%, and Rw(F)=5.0% with the Ni and N positions split into two sites, each half occupied. The superstructure can be modeled as a 2×1×1 ordering of the subcell in Pnma. The physical properties are unusual since Sr2NiN2 displays metallic conductivity and Pauli paramagnetism even though it is comprised of isolated NiN2 units. In addition, Sr2NiN2 possesses a Ni2+ formal charge, whereas the oxidation states for nickel nitrides are typically Ni+ and Ni0.