Forcing substitution of tantalum by copper in 1T-TaS: synthesis, structure and electronic properties of 1T-Cu Ta S.

We investigated the compound 1T-Cu Ta S with respect to its synthesis, homogeneity range, structure and electronic properties. The average structure of 1T-Cu Ta S resembles that of the high-temperature phase of the layered transition metal dichalcogenide 1T-TaS in which tantalum is partially substituted by copper. 1T-Cu Ta S readily decomposes at elevated temperatures and can only be prepared and stabilized by a sufficiently high amount of sulfur excess. XPS and NEXAFS measurements reveal that copper has the oxidation state  +I in 1T-Cu Ta S, which is supported by quantum chemical calculations. The disorder introduced by copper doping causes an Anderson-type localization of the conduction electrons as manifested by a strong increase of the electrical resistivity and a Curie-type paramagnetism at low temperatures as in other doped systems 1T-M Ta S with higher valent metals. Quantum chemical calculations support this interpretation.