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Motivated by the existence of superconductivity in pyrite-structure CuS$_2$,we explore the possibility of ionic-liquid-gating-induced superconductivity inthe proximal antiferromagnetic Mott insulator NiS$_2$. A clear gating-inducedtransition from a two-dimensional insulating state to a three-dimensionalmetallic state is observed at positive gate bias on single crystal surfaces. Noevidence for superconductivity is observed down to the lowest measuredtemperature of 0.45 K, however. Based on transport, energy-dispersive X-rayspectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, andother techniques, we deduce an electrochemical gating mechanism involving asubstantial decrease in the S:Ni ratio (over hundreds of nm), which is bothnon-volatile and irreversible. This is in striking contrast to the reversible,volatile, surface-limited, electrostatic gate effect in pyrite FeS$_2$. Weattribute this stark difference in electrochemical vs. electrostatic gatingresponse in NiS$_2$ and FeS$_2$ to the much larger S diffusion coefficient inNiS$_2$, analogous to the different behaviors observed among electrolyte-gatedoxides with differing O-vacancy diffusivities. The gating irreversibility, onthe other hand, is associated with the lack of atmospheric S; this is incontrast to the better understood oxide case, where electrolysis of atmosphericH$_2$O provides an O reservoir. This study of NiS$_2$ thus provides new insightinto electrolyte gating mechanisms in functional materials, in a previouslyunexplored limit.

Electrochemically-driven insulator-metal transition in ionic-liquid-gated antiferromagnetic Mott-insulating NiS$_2$ single crystals