Investigation of Local A-site Chemistry in Barium Strontium Titanate Using Aberration Corrected STEM, EELS and EDS

Barium strontium titante (BST) is an important dielectric material because of its high tunability (dielectric constant as a function of applied voltage) and low loss, with specific applications in tunable microwave circuits. However, regardless of processing technique, additives, or synthesis temperatures some remnant ferroelectricity is always observed beyond the paraelectric phase transition [1]. One possible explanation for this remnant ferroelectricity, which has been hotly debated, is that the local stoichiometry of the A-site cations (Ba and Sr) is not globally and locally homogeneous. This hypothesis surmises that local areas, which are barium rich, may result in remnant polarization [1]. However, this hypothesis has never been experimentally shown. In this work, we use an aberration corrected scanning transmission electron microscope (STEM), electron energy loss spectroscopy (EELS), and energy dispersive X-ray spectroscopy (EDS) to investigate the local stoichiometry of arguably the bestengineered barium strontium titanate samples ever fabricated [2] in comparison to BST thin films showing some of the best tunability in thin-film form.