Synthesis of delafossite-derived phases, RCuO2+δ 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+δ (δ≥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 δ≥0.5 per formula unit could be loaded into the lattices. Although electrical resistivity (ρ) of the oxygen-rich RCuO2+δ samples exhibited semiconducting behavior at temperatures below 325 K a couple of samples showed anomalous temperature dependences in ρ at low temperatures. Unusual temperature dependence of magnetic susceptibility (χ), was observed for Ca-doped YCuO2+δ samples, but not for the other RCuO2+δ phases. If the Curie term was subtracted from the susceptibility of (Y,Ca)CuO2+δ, the remaining part of χ decreased monotonically with decreasing temperature down to ∼100K, and then leveled off with a finite value. Assuming that this finite value could be attributed to the Pauli paramagnetism of free carriers, the χ vs T characteristics were compared with those of other layered cuprates, such as SrCu2O3, (Sr,Ca)14Cu24O41, and (La,Sr)CuO2.5, which have been reported to exhibit spin-gap behavior. copyright 1997 The American Physical Society