Oxygen-vacancy ordering in multiferroic Bi

Multiferroic Bi1−xCaxFeO3−x/2 (0.10 ⩽ x ⩽ 0.50) materials have been synthesized via a high-temperature sintering method. The structural properties have been studied using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The EELS measurements reveal that the oxidation state of Fe ions in these compounds is Fe+3, and oxygen vacancies are created as Ca substitutions for Bi. A series of superstructure modulations appear along the a-axis direction, and their wavelength can be written as L = na (n = 4, 5, 6 and 7) depending on the Ca contents. Based on our structural analysis, we interpret these superstructures in terms of oxygen-vacancy ordering associated with local structural distortions