The Phase Diagrams and Doped-Hole Segregation in La2CuO4+δ and La2−xSrxCuO4+δ (x ≤ 0.15, δ ≤ 0.12)

Abstract

The magnetic and structural phase diagrams of the title systems are reviewed, with an emphasis on our recent results obtained from magnetic and structural neutron diffraction, thermogravimetric analysis, iodometric titration, magnetic susceptibility x(T), and 139La nuclear quadrupole resonance (NQR) measurements. From measurements on electrochemically oxidized polycrystalline samples, the known miscibility gap in the system La2CuO4+δ is found to lie between δ ≈ 0.01 and 0.06 at low temperatures, with a maximum phase separation temperature T ps ≈ 415 K. Within the miscibility gap, the superconducting transition temperature T c of the oxygen-rich phase and the Neel temperature T N of the oxygen- deficient phase are constant at ≈ 32 K and 250 K, respectively. Neutron diffraction measurements showed T ps = 260(5) K and T N = 245(3) K for a single crystal. Beyond the miscibility gap, two distinct superconducting phases are found in polycrystalline samples at δ ≈ 0.06–0.08 (T c ≈ 32–34 K) and 0.11–0.12 (T c ≈ 42–45 K), separated by another two-phase region. The doped-hole concentration in the CuO2 planes is found to be p ≈ 0.08 holes/Cu and ≈ 0.16 holes/Cu for the two phases, respectively. These data suggest that a large fraction of the excess oxygen atoms participate in oxygen-oxygen bonding in both phases. Measurements of T c versus pressure suggest pressure-induced changes in the doped-hole concentration in the CuO2 layers. Superstructure reflections observed in the neutron diffraction patterns of both phases suggest spatial ordering of the excess oxygen atoms. In the La2−xSrxCuO4+δ system, the phase separation disappears by x = 0.03 for δ ≈ 0.03. Bulk superconductivity is found below T c = 40 K for 0.01 ≤ x ≤ 0.15 for maximally oxidized samples. For δ = 0 and 0 < x < 0.08, our 139La NQR and X(T) data indicate that the doped holes condense into walls separating undoped nanoscopic domains, consistent with theory based on an electronic mechanism for phase separation in these systems. In the antiferromagnetic regime (x < 0.02) below TN, the doped-hole spins are found to freeze at a temperature T f = (815 K)x, whereas in the spin-glass regime 0.02 < x < 0.08, the spin-glass transition at T g ∝ 1/x is found to arise from cooperative freezing of the dynamically-ordered mesoscopic undoped domains.