Synthesis of delafossite-derived phases, RCuO2+delta with R=Y, La, Pr, Nd, Sm, and Eu, and observation of spin-gap-like behavior

The structural and physical properties of layered cuprates of a RCuO2 delafossite structure have been investigated for various rare-earth elements R: Y, La, Pr, Nd, Sm, and Eu. An oxygen-rich delafossite-derived phase, RCuO2+delta (delta greater than or equal to 0.5), as previously reported by Cava et al. for R = Y and La was found for all the other R presently studied. Each as-sintered RCuO2 sample appeared to have a nearly stoichiometric oxygen content, but by thermal oxidation excess oxygen atoms corresponding to delta greater than or equal to 0.5 per formula unit could be loaded into the lattices. Although electrical resistivity (rho) of the oxygen-rich RCuO2+delta samples exhibited semiconducting behavior at temperatures below 325 K a couple of samples showed anomalous temperature dependences in rho at low temperatures. Unusual temperature dependence of magnetic susceptibility (chi), was observed for Ca-doped YCuO2+delta samples, but not for the other RCuO2+delta phases. If the Curie term was subtracted from the susceptibility of (Y,Ca)CuO2+delta, the remaining part of chi decreased monotonically with decreasing temperature down to similar to 100 K, and then leveled off with a finite value. Assuming that this finite value could be attributed to the Pauli paramagnetism of free carriers, the chi vs T characteristics were compared with those of other layered cuprates, such as SrCu2O3, (Sr,Ca)(14)Cu24O41, and (La,Sr)CuO2.5, which have been reported to exhibit spin-gap behavior.