Synthesis, characterization, phase diagrams and superconducting and normal state magnetic properties of La<sub>2-x</sub>Sr<sub>x</sub>CuO<sub>4</sub> (0 ≤ x ≤ 0.08) and electrochemically oxidized La<sub>2-x</sub>Sr<sub>x</sub>CuO<sub>4+δ</sub> (0 ≤ x ≤ 0.33, 0 ≤ δ ≤ 0.12)

Abstract

La2-xSrxCuO4 (0 ≤ x ≤ 0.15) can all be intercalated with oxygen by a novel electrochemical oxidation method. Bulk superconductivity is found with an onset Tc~ 40 K for the whole range 0.01 ≤ x ≤ 0.15; for x = 0.25 and 0.33, the electrochemical oxidation did not improve the superconducting properties. The magnetic susceptibility χ(T = 50--320 K) data for La2CuO4.11 and La1.92Sr0.08CuO4.07 are nearly identical with those of conventionally prepared La1.85Sr0.15CuO4, indicating that the hole doping level (p) in the CuO2 planes of the three compounds is nearly the same. Combined thermogravimetric analysis and iodometric titration experiments indicate that part of the intercalated oxygen has a formal valence close to -1. The maximum doped-hole concentration in the CuO2 planes that can be achieved from combined Sr-doping and electrochemical oxygen doping for 0 ≤ x ≤ 0.15 is p ~ 0.16 holes/formula unit. Oxygen can also intercalate into single crystal La2CuO4 through a slow electrochemical oxidation process. The required low current and long time for the charging process reflects that the oxygen intercalation for a single crystal is limited by its small specific surface area and long diffusion distance. The anisotropic superconducting, magnetic and transport properties are summarized and compared with those of polycrystalline La2CuO4+δ as well as of YBa2Cu3O7-δ and La2-xSrxCuO4 single crystals. The single crystal La2CuO4+δ has a maximum Tc ~ 40 K, which is lower than that (Tc ~ 42--45) of the corresponding polycrystalline samples. The magnetic phase diagram of La2-xSrxCuO4 in the antiferromagnetic (AF) regime (0 ≤ x ≤ 0.02) has been derived from 139La NQR studies from 4 to 250 K.