Well-defined polymorph transformation sequences and novel tetragonal twin-domain patterns in the morphotropic phase boundary of Dy-modified BiFeO₃-PbTiO₃ single crystals

BiFeO₃-PbTiO₃-based perovskite materials show great potential in high-temperature piezoelectric and memory devices due to their high Curie temperature and large ferroelectric polarization. In this study, single crystals with nominal composition of 0.7(Bi_0.9Dy_0.1)FeO₃-0.3PbTiO₃ were grown successfully by high-temperature solution method. The coexistence of rhombohedral phase and tetragonal phase at room temperature indicates the nature of morphotropic phase boundary. A successive phase transformation sequence of rhombohedral-to-tetragonal-to-cubic structure was revealed by in situ XRD patterns and further supported by the first observation of two distinct dielectric anomalies. The hierarchical domain structures, i.e., large tetragonal a/c banded domains with width above 10 μm and fine lamellar domains with width below 200 nm, were revealed by combining polarized light microscopy and piezoresponse force microscopy techniques. Besides the common 90° herringbone domain structures as often observed in tetragonal ferroelectrics, novel tetragonal domain patterns containing two a/c domain stacks and complex interfaces between them were found. The detailed 3-dimensional domain structures were constructed based on mechanical compatible principle. The results of this study can not only provide an understanding of the complex phase transition behavior of BiFeO₃-PbTiO₃-based functional materials but also enrich the knowledge of domain structures in tetragonal perovskite ferroelectrics.