Is NiCo2S4 really a semiconductor?

Chuan Xia, Peng Li, Appala Naidu Gandi, Udo Schwingenschlögl, and Husam N. Alshareef, "Is NiCo2S4 really a semiconductor?​"
Chem. Mater., DOI: 10.1021/acs.chemmater.5b01843
Chuan Xia, Peng Li, Appala Naidu Gandi, Udo Schwingenschlögl, and Husam N. Alshareef
NiCo2S4, metallic
NiCo2S4 is a technologically important electrode material that has recently achieved remarkable performance in pseu-docapacitor, catalysis, and dye-synthesized solar cell applications.[1-5] Essentially, all reports on this material have pre-sumed it to be semiconducting, like many of the chalcogenides, with a reported band-gap in the range of 1.2-1.7 eV.[6,7] In this report, we have conducted detailed experimental and theoretical studies, most of which done for the first time, which overwhelmingly show that NiCo2S4 is in fact a metal. We have also calculated the Raman spectrum of this material and experimentally verified it for the first time, hence clarifying inconsistent Raman spectra reports. Some of the key results that support our conclusions include: (1) the measured carrier density in NiCo2S4 is 3.18×1022 cm-3, (2) Ni-Co2S4 has a room temperature resistivity of around 103 µΩ cm which increases with temperature, (3) NiCo2S4 exhibits a quadratic dependence of the magnetoresistance on magnetic field, (4) thermopower measurements show an extremely low Seebeck coefficient of 5 µV K-1, (5) first principles calculations confirm that NiCo2S4 is a metal. These results suggest that it is time to re-think the presumed semiconducting nature of this promising material. They also suggest that the metallic conductivity is another reason (besides the known significant redox activity) behind the excellent perfor-mance reported for this material.