Forcing substitution of tantalum by copper in 1T-TaS2: synthesis, structure and electronic properties of 1T-Cu x Ta1-x S2

We investigated the compound 1T-Cu _x Ta_1-x S₂ with respect to its synthesis, homogeneity range, structure and electronic properties. The average structure of 1T-Cu _x Ta_1-x S₂ resembles that of the high-temperature phase of the layered transition metal dichalcogenide 1T-TaS₂ in which tantalum is partially substituted by copper. 1T-Cu _x Ta_1-x S₂ 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 _x Ta_1-x S₂, 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 _x Ta_1-x S₂ with higher valent metals. Quantum chemical calculations support this interpretation.