In Nd3.5Sm0.5Ni3O8which has basically the same crystal structure and the similar electrical configuration (Ni+3d9/Ni2+3d8mix valence state) with high-Tccuprate, it has been found that this material shows metallic behaviour down to about 20 K by intercalation and subsequent deintercalation with sulfur. This is the first observation of the metallic state in this system. It is unclear why sulfur-intercalation and deintercalation induce the metallic state. We speculate that sulfur works as an effective getter for removing the interstitial apical oxygen which impedes the metallic conduction. However, the weak localization of carriers in the NiO2planes still remains below 20 K and the localization may be one of the obstacles to occurrence of possible superconductivity.

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