Abstract
Polar instabilities are well known to be suppressed on scaling materials down to the nanoscale, when the electrostatic energy increase at surfaces exceeds lowering of the bulk polarization energy. Surprisingly, here we report an emergent low symmetry polar phase arising in nanoscale powders of CaTiO3, the original mineral named perovskite discovered in 1839 and considered nominally nonpolar at any finite temperature in the bulk. Using nonlinear optics and spectroscopy, X-ray diffraction, and microscopy studies, we discover a well-defined polar to non-polar transition at a TC = 350 K in these powders. The same polar phase is also seen as a surface layer in bulk CaTiO3 single crystals, forming striking domains with in-plane polarization orientations. Density functional theory reveals that oxygen octahedral distortions in the surface layer lead to the stabilization of the observed monoclinic polar phase. These results reveal new ways of overcoming the scaling limits to polarization in perovskites.
Highlights
S scitation.org/journal/apm has been shown in SrTiO3 thin films.22 Interestingly, these defect dipoles can align when scaled to smaller dimensions giving rise to ferroelectricity in a few nanometer thick SrTiO3 films,23 an example of a reverse thickness scaling effect
We pose the following question: can room temperature polarization be unlocked in such paraelectric materials in powder and single crystal form, rather than as thin films? can it be achieved without intentionally doping, inducing non-stoichiometry, or external straining? Here, using complementary optical probe techniques including second harmonic generation (SHG), Raman spectroscopy, photoluminescence, and X-ray spectroscopy, we demonstrate that a low symmetry polar phase is stabilized in CaTiO3 nanoparticles with a Tc above room temperature (350 K)
Hysteresis is observed in the SHG intensity upon heating and cooling, which is commonly observed during ferroelectric phase transitions
Summary
S scitation.org/journal/apm has been shown in SrTiO3 thin films.22 Interestingly, these defect dipoles can align when scaled to smaller dimensions giving rise to ferroelectricity in a few nanometer thick SrTiO3 films,23 an example of a reverse thickness scaling effect. The same polar phase is seen as a surface layer in bulk CaTiO3 single crystals, forming striking domains with in-plane polarization orientations.
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