Size effects in PbTiO3 nanocrystals:: Effect of particle size on spontaneous polarization and strains -: art. no. 084305

The spontaneous polarization (P-s) and spontaneous strains (x(i)) in mechanically unclamped and surface charge compensated PbTiO3 nanocrystals were determined as a function of particle size in the range < 150 nm by differential scanning calorimetry and x-ray powder diffraction, respectively. Significant deviations from bulk order parameters (P,x(i)) have been observed as the particle size decreased below similar to 100 nm. The critical size (r(c)) below which the ferroelectric tetragonal phase transforms to the paraelectric cubic phase was determined as similar to 15 nm. The depression in transition temperature with particle size is 14 degrees C at 28 nm. No change in the order of m3m -> 4mm ferrodistortive phase transition is observed. A simple analysis showed that Delta H-tr/(k(B)T)similar to 10(3) at 25 degrees C for r=16 nm, indicating that the stabilization of the cubic phase at r(c) cannot be linked to an instability in dipolar ordering due to thermal agitations. Comparison of the spontaneous volumetric strains with the strain induced by surface stress indicated that the effect of surface stress on ferroelectric phase stability was negligible. Anomalies in electrostrictive properties were determined for r -> r(c). The observed size dependence of P-S is attributed to the reduced extent of long-range dipole-dipole interactions that arise due to the changes in bonding characteristics of ions with decreasing particle size in the perovskite lattice, in conformity with a recent study by Tsunekawa [Phys. Rev. Lett. 85 (16), 4340 (2000)]. (C) 2005 American Institute of Physics.