Structural and morphological evolution from a freeze-dried precursor to the La_1.85Sr_0.15 CuO₄ superconductor during thermal processing

Abstract

The crystalline phases and morphology of solids with a composition of La_1.85Sr_0.15CuO₄ were studied in terms of thermal processing conditions of freeze-dried acetates. Samples were characterized by X-ray diffraction, scanning electron microscopy and iodometric titration techniques. Results indicate that a full development of the superconducting phase only occurs after pressing sintering and annealing under oxygen of the powders treated at T _f=1253 K for 8 h. The structural evolution observed in this case involves a solid-state reaction between La_1.67Sr_0.33Cu₂O_5−δ, La(OH)₃ and CuO. The existence of transient liquid phase at temperatures around 600 K generates local composition fluctations (lanthanum deficiency) where the non-superconducting ternary compound La_1.67Sr_0.33Cu₂O_5−δ develops. The selection of high heating rates (50 K min⁻¹) below 773 K minimizes the deleterious effect of transient melting and, at the same time, ensures a high reproducibility in the morphology of the final material obtained.