Phase separation and oxygen diffusion in electrochemically oxidizedLa2CuO4+δ: A static magnetic susceptibility study

The compound ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ is known to phase separate for 0.01≲δ≲0.06 below a temperature ${\mathit{T}}_{\mathrm{ps}}$∼300 K into the nearly stoichiometric antiferromagnetic compound ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4.01–4.02}$ with Néel temperature ${\mathit{T}}_{\mathit{N}}$∼250 K, and a metallic oxygen-rich phase ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{\mathrm{\approxeq }4.06}$ with superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$≊34 K. We report studies of the superconducting and normal-state static magnetic susceptibility χ of ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ samples with 0≲δ≤0.11 prepared by electrochemical oxidation or reduction of conventionally synthesized ceramic ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$. The upper limit to the miscibility gap at low T is found be δ≲0.065, in agreement with the previous work. The interstitial oxygen diffusion during the phase-separation process was studied using thermal- and magnetic-field history-dependent χ(T,t) measurements versus temperature T and time t as a probe. Phase separation is found to be suppressed by quenching at ≳100 K/s and favored by slow cooling at ∼0.5 K/min. A large thermal hysteresis of both the normal and superconducting state χ(T) was observed between data obtained after quenching to 5 K and then warming, and data obtained while or after slowly cooling from 300 K, for samples of ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ (δ≊0.030, 0.044) within the miscibility gap. Quenching reduces ${\mathit{T}}_{\mathit{c}}$ by ≊5 K relative to the value (34 K) obtained after slow cooling. A similar decrease is found for ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4.065}$ which does not phase separate, indicating the importance of oxygen-ordering effects within this single phase. A model for the excess oxygen diffusion is presented, from which the data yield a nearly T-independent activation energy for excess oxygen diffusion of (0.24±0.03) eV from 150 to 220 K apart from a possible anomaly near 210 K. © 1996 The American Physical Society.