The electronic structure of the marcasite-type NiN2, newly synthesized under high pressures and high temperatures, was investigated using synchrotron-radiation infrared (IR) microspectroscopy and first-principles calculations based on the density functional theory (DFT). The IR spectrum of the marcasite-type NiN2 showed characteristic low reflectance of the semiconductor with spectral features probably due to defects or lattice vibrations and free carriers. The effective free-carrier concentration in the marcasite-type NiN2 was estimated by fitting a Drude-Lorentz model for the optical response to be 10(24-25) m(3), much less than those of normal metals (10(28-29) m(-3)) and comparable to the estimated concentration of the thermally-excited carrier in the conduction band. The DFT predictions could semi-quantitatively explain the experimental findings; at 36 GPa, the marcasite-type NiN2 had a narrow gap between the Ni 3d a(sigma) bands split in the distorted octahedral coordination of N neighbors.
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