TRANSPORT-PROPERTIES AND ELECTRONIC-STRUCTURE OF NEW RARE EARTH-COPPER CHALCOGENIDES

New rare earth-copper chalcogenides: La2CuS4+delta and Y2Cu2S5+delta w ith the chemical patterns corresponding to the extensively studied cop per oxides La2CuO4+delta and Y2Cu2S5, have been synthesised and invest igated 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 similar to 0.2-0.3 eV, whereas the compounds synthesised from substr ate amounts, corresponding to the stoichiometry of La2CuS4 and Y2Cu2S5 , exhibit metallic conductivity with a linear temperature dependence o f resistivity in almost entire temperature range studied (4.2-300 K). For additionally soaked Y2Cu2S5 samples, a broad metal-nonmetal transi tion taking place with increasing temperature has been detected betwee n 170 and 210 K. XPS investigation supported by susceptibility measure ments indicate that in Y2Cu2S5+delta (both metallic and nonmetallic sa mples) a part of copper atoms occur in Cu2+ valence state, whereas in the metallic La2CuS4+delta(delta approximate to 0) the copper valence state is mainly Cu1+.