Size effects in ferroelectric PbTiO3 nanomaterials observed by multi-frequency electron paramagnetic resonance spectroscopy

Lead titanate (PbTiO3) micro- and nanocrystalline powders have been prepared from metallo-oranic precursor through combined polymerisation and pyrolysis (CPP). The enhanced liquid-precursor based version of the cpp route in combination with soft milling enables an adjustment of the mean particle size up to 5 rim. A multi-frequency (X, Q, and W band) electron paramagnetic resonance study of Cr-doped micro- and nanocrystalline PbTiO3 samples was performed. Three Cr3+ centers (C1, C2, and C3) with different axial Zero Field Splitting (ZFS) parameters were identified in microcrystalline samples. The center Cl is similar to that observed in previous X band single crystal and ceramic sample measurements. The superposition model by Newman and Urban was applied to translate the ZFS data of these centers into local Cr3+ displacements inside the distorted oxygen octahedra of the microcrystalline PbTiO3 lattice. In the nanocrystalline powders only the center C1 was observed. Its EPR spectra in dependence on the mean particle size were fitted using a spin-Hamiltonian in which a Gaussian distribution of ZFS terms was assumed. The variation of the mean value of ZFS parameter D and distribution width AD was determined and the critical particle size of the size-driven phase (tetragonal-cubic) transition was estimated. In nanocrystalline powders with mean particle size d < d(cr) the tetragonal Cl spectrum is not more detectable. A new Cr3+ center spectrum, C4, consisting of a single line with an isotropic g-factor is detectable allowing the cubic phase in the nanomaterials to be quantified. Further, temperature dependent EPR measurements were made which allowed the variation in Curie temperature with mean particle size to be determined.