Transport properties and electronic structure of new rare earth-copper chalcogenides

Authors:

• Irena Jacyna-Onyszkiewicz,
• M. Zimpel,
• M.A. Obolensky,
• Yu.B. Poltoracky,
• S. Starodub,
• Stanisław Robaszkiewicz

Abstract

New rare earth-copper chalcogenides: La₂CuS_4+δ and Y₂Cu₂S_5+δ with the chemical patterns corresponding to the extensively studied copper oxides La₂CuO_4+δ and Y₂Cu₂S₅, have been synthesised and investigated with respect to their electronic structure, electric transport and magnetic properties. These properties are found to be dependent on the technology of their production and sulphur content. Surprisingly, the sulphur-rich systems are insulators with the activation energy E_A∼0.2-0.3 eV, whereas the compounds synthesised from substrate amounts, corresponding to the stoichiometry of La₂CuS₄ and Y₂Cu₂S₅, exhibit metallic conductivity with a linear temperature dependence of resistivity in almost entire temperature range studied (4.2-300 K). For additionally soaked Y₂Cu₂S₅ samples, a broad metal-nonmetal transition taking place with increasing temperature has been detected between 170 and 210 K. XPS investigation supported by susceptibility measurements indicate that in Y₂Cu₂S_5+δ (both metallic and nonmetallic samples) a part of copper atoms occur in Cu²⁺ valence state, whereas in the metallic La₂CuS_4+δ(δ ≈ 0) the copper valence state is mainly Cu¹⁺.