Synthesis, characterization, and superconducting and magnetic properties of electrochemically oxidized La 2CuO 4+δ and La 2−xSr xCuO 4+δ (0.01 ≤ x ≤ 0.33, 0.01 ≤ δ ≤ 0.36)

Abstract

La 2CuO 4 can be electrochemically oxidized in aqueous base at 295 K to form a superconductor with T c ≈44 K [Wattiaux et al., C.R. Acad. Sci. (Paris) 310 (1990) 1047]. We have confirmed and extended this study. We conclude that the electrochemical reaction forms La 2 CuO 4+ δ . Two distinct superconducting phases are found at δ≈0.03 ( T c ≈32 K) and δ 0 ̆ .08(T c≈4 K) Bulk superconductivity is found in similarly prepared La 2- x Sr x CuO 4+ δ with T c ≈40 K for 0.01≤ x≤0.15. Normal stat e magnetic susceptibility data for La 2CuO 4.11 and La 1.92Sr 0.08CuO 4.07 are nearly identical with those of conventionally prepared La 1.85Sr 0.15CuO 4, indicating that the hole doping level ( p) in the CuO 2 planes of the three compounds is nearly the same. Iodometric titration experiments confirmed that the maximum doped-hole concentration from combined Sr-doping and electrochemical oxygen doping for 0≤ x≤0.15 is p≈0.16 holes/formula unit. The electrochemical oxidation of the starting materials does not appear to be reversible.