Atomic-Scale Displacement in Ordered SmMnO₃ Nanoislands

In orthorhombic RMnO₃, multiferroicity is governed by epitaxial strain, which establishes a delicate balance between competing magnetic and ferroelectric interactions by tuning the Mn-O-Mn bond angles and Mn-O bond lengths. Here, we employed high-angle annular dark field (HAADF), atomic-resolution energy-dispersive X-ray spectroscopy (EDS), and annular bright field (ABF) imaging to resolve the structure of SmMnO₃ nanoislands grown on SrTiO₃ (001) by sol-gel spin coating. We quantified lattice constants, strain gradients, and unit-cell level polar displacements, finding polar displacements ranging from 4.26 to 20.47 pm with a marked reduction at the interface. Subunit-cell scale measurements showed Mn-O bonds shorten and Mn-O-Mn angles enlarge under ∼-3.5% in-plane compression. Electron energy loss spectroscopy (EELS) further reveals a uniform distribution of oxygen vacancies and a mixed Mn³⁺/Mn²⁺ valence state with a constant L₃/L₂ intensity ratio of ∼3.5 across the entire nanoisland. These atomic-scale measurements provide fundamental insights into the structure-property relationships governing structural polar behavior, offering valuable guidance for future functional applications.