High dielectric constant and frozen macroscopic polarization in dense nanocrystallineBaTiO3ceramics

Theoretical models for small ferroelectric particles predict a progressive decrease of the Curie temperature, spontaneous lattice strain, and polarization until the critical size corresponding to transition to the cubic phase and disappearance of ferroelectricity is reached. In contrast, the behavior of nanocrystalline $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ ceramics with a grain size of $\ensuremath{\approx}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ is dominated by extrinsic effects related to the grain boundaries which mask the expected downscaling of properties. While the noncubic crystal structure, the high dielectric constant $(\ensuremath{\approx}1600)$ and the variation of permittivity with temperature suggest a ferroelectric behavior, very slim, and nearly linear polarization hysteresis loops are observed. Evidence for the existence of a ferroelectric domain structure with domains extending over several grains and of polarization switching at local scale is given by piezoresponse force microscopy. The suppression of macroscopic ferroelectric hysteresis and switching originates from a frozen domain structure stable under an external field owing to the effects exerted by the grain boundaries, such as the clamping of the domain walls and the hindrance of polarization switching. Furthermore, the depolarization field originated by the low-permittivity nonferroelectric grain boundaries can cause a significant reduction of polarization. If the grain size is small enough, the ceramic is expected to undergo a ``phase transition'' to a polar phase with nonswitchable polarization. The $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ ceramics with grain size of $30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ investigated in the present study are deemed to be close to this transition.