Spontaneous formation of circular and vortex ferroelectric domain structure in hexagonal YMnO₃ and YMn_0.9Fe_0.1O₃ prepared by low temperature solution synthesis

We report an experimental study of the domain structure of ferroelectric YMnO₃ and YMn_0.9Fe_0.1O₃ using polycrystalline samples prepared by direct hydrothermal crystallisation at 240 °C, well below their structural phase transition temperatures. Powder X-ray diffraction shows the expected P6₃cm space group for both samples with an increase in a and a small decrease in c with Fe incorporation, consistent with an adjustment of MnO₅ tilting, while XANES spectra at the Mn and Fe K edges show the oxidation state of both metals are maintained at +3 in the doped sample. High resolution TEM shows that curved stripe, annular and vortex domains can all be observed in the YMnO₃ crystals, proving that the structural phase transition is not the only driving force for the occurrence of the annular and vortex domains. Furthermore, the absence of the annular and vortex domains in YMn_0.9Fe_0.1O₃ indicates that the tilting state of MnO₅ bipyramids plays an important role in the domain formation. Atomic resolution STEM images confirm that the ferroelectric domain walls correspond to structural antiphase boundaries similar to the crystals made via high temperature solid-state reactions.