SYNTHESIS OF DELAFOSSITE-DERIVED PHASES, RCUO2-GAP-LIKE BEHAVIOR(DELTA WITH R=Y, LA, PR, ND, SM, AND EU, AND OBSERVATION OF SPIN)

The structural and physical properties of layered cuprates of a RCuO2 delafossite structure have been investigated for various rare-earth el ements R: Y, La, Pr, Nd, Sm, and Eu. An oxygen-rich delafossite-derive d phase, RCuO2+delta (delta greater than or equal to 0.5), as previous ly reported by Cava et al. for R = Y and La was found for all the othe r R presently studied. Each as-sintered RCuO2 sample appeared to have a nearly stoichiometric oxygen content, but by thermal oxidation exces s oxygen atoms corresponding to delta greater than or equal to 0.5 per formula unit could be loaded into the lattices. Although electrical r esistivity (rho) of the oxygen-rich RCuO2+delta samples exhibited semi conducting behavior at temperatures below 325 K a couple of samples sh owed anomalous temperature dependences in rho at low temperatures. Unu sual temperature dependence of magnetic susceptibility (chi), was obse rved for Ca-doped YCuO2+delta samples, but not for the other RCuO2+del ta phases. If the Curie term was subtracted from the susceptibility of (Y,Ca)CuO2+delta, the remaining part of chi decreased monotonically w ith decreasing temperature down to similar to 100 K, and then leveled off with a finite value. Assuming that this finite value could be attr ibuted to the Pauli paramagnetism of free carriers, the chi vs T chara cteristics were compared with those of other layered cuprates, such as SrCu2O3, (Sr,Ca)(14)Cu24O41, and (La,Sr)CuO2.5, which have been repor ted to exhibit spin-gap behavior.