Shape Controllable Synthesis of Bi-Based Perovskite Superconductor Microcrystals via a Mild Hydrothermal Method

In this communication, we report two feasible routes to controllably synthesize Bi-based perovskite structure superconductor crystals with tunable shapes that expose well-defined crystal facets. The shape of as-synthesized crystals is determined by the relative growing speed along <001> and <111> crystallographic directions that correlate to the ratio of the propagation tendency of the A-O layer to BiO6 octahedral units. We have found two routes to control this process by either decreasing the growing speed of the A-O layer (NH4+ route) or increasing that of the BO6 octahedral units (Bi(V)/Bi(III) route). The crystal growth and shape formation mechanism were analyzed based on the E–pH diagrams and crystal structure characteristics according to thermodynamic data of the Bi–Ba–K–Na–H2O system and the Bravais–Friedel–Donnay–Harker (BFDH) theory. The temperature-dependent magnetization measurement indicates that the diamagnetic and superconductive performance of the samples is determined by the crystal shape and their exposed facets. Truncated cubic shape crystals show the highest transition temperature (∼30 K), while cubic shape crystals show the largest shielding volume fraction (up to 49.2%). The increasing exposed ratio of {111} facets on the surface of crystals reduces the shielding volume fraction.