SIZE-INDUCED DIFFUSE PHASE-TRANSITION IN THE NANOCRYSTALLINE FERROELECTRIC PBTIO3

We report a detailed study of the size dependence of the ferroelectric transition in an ensemble of PbTiO3 nanoparticles produced by copreci pitation. The phase transition was monitored by dielectric measurement s, variable temperature x-ray diffraction, and differential scanning c alorimetry (DSC). Size effects were found to become important only bel ow approximate to 100 nm (coherently diffracting x-ray domain size). T he tetragonal distortion of the unit cell (which is related to the spo ntaneous polarization) decreases exponentially with size and vanishes at 7 nm. The T-c decreases gradually but the transition becomes increa singly diffuse with a decrease in the size from 80 to 30 nm, below whi ch there is no peak in the dielectric constant or the heat flow (DSC), though ferroelectric ordering probably persists down to approximate t o 7 nm.