Structural origin of recovered ferroelectricity in BaTiO3 nanoparticles

Nanoscale $\mathrm{BaTi}{\mathrm{O}}_{3}$ particles $(\ensuremath{\approx}10$ nm) prepared by ball-milling a mixture of oleic acid and heptane have been reported to have an electric polarization several times larger than that for bulk $\mathrm{BaTi}{\mathrm{O}}_{3}$. In this work, detailed local, intermediate, and long-range structural studies are combined with spectroscopic measurements to develop a model structure of these materials. The x-ray spectroscopic measurements reveal large Ti off-centering as the key factor producing the large spontaneous polarization in the nanoparticles. Temperature-dependent lattice parameter changes manifest sharpening of the structural phase transitions in these $\mathrm{BaTi}{\mathrm{O}}_{3}$ nanoparticles compared to the pure nanoparticle systems. Sharp crystalline-type peaks in the barium oleate Raman spectra suggest that this component in the composite core-shell matrix, a product of mechanochemical synthesis, stabilizes an enhanced polar structural phase of the $\mathrm{BaTi}{\mathrm{O}}_{3}$ core nanoparticles.