Structural coherence and ferroelectric order in nanosized multiferroic YMnO ₃

Atomic-scale structure studies involving synchrotron x-ray diffraction (SXRD) and pair distribution function (PDF) analysis on a series of YMnO ₃ particles with sizes ranging from 467 ± 42 (bulk) to 10 ± 1 nm are presented. Studies reveal that while the nanoparticles retain most of the characteristics of the layered hexagonal-type structure of the bulk, substantial local atomic displacements arise with diminishing particle size. The displacements lead to a very substantial loss of structural coherence in the particles of size smaller than 100 nm. The displacements mostly affect the yttrium (Y) atoms and to a lesser extent the Mn-O sublattice in YMnO ₃. We argue that the increased displacement of Y atoms along the polar c axis of the hexagonal unit cell may result in enhanced local ferroelectric distortions with decreasing particle size. The planar, that is, a- and b-axis direction displacements of Y atoms, however, may interfere with the cooperative ferroelectricity of nanosized YMnO ₃, so future efforts to employ YMnO ₃ in nanoscale applications should take them into account.