Preparation of Layered Ferroelectric Bi2SrTa2O9 Single-Crystal Platelets

Layered ferroelectric Bi2SrTa2O9 is attracting much attention as a fatigue-free material for ferroelectric nonvolatile memory devices. We have successfully synthesized this single-crystal platelet without any impurity phases by the self-flux method. The largest crystals measure about 1 mm×1 mm×5 µ m, and the lattice parameters were estimated to be a=0.5528 nm and c=2.498 nm, assuming a tetragonal cell. The chemical composition is 1.92:1.10:2.00 in an atomic ratio of Bi:Sr:Ta, from ICP-AES analysis. By wet etching, the crystal structure was found to be decomposed anisotropically, leaving only the c-axis framework. The step height of the topmost surface observed by atomic force microscopy (AFM) was 1.2 to 1.3 nm, which corresponds to half of the c-axis length (about 2.5 nm). Through thermal analysis, the thermal anomaly originating in ferroelectric-paraelectric phase transition was not observed and there was little evaporation of Bi compounds.