Crystal structure transformations induced by surface stresses in BaTiO3 and BaTiO3@SiO2 nanoparticles and ceramics
Abstract
Lattice structure transformations in nanopowders of ferroelectric BaTiO3 and BaTiO3@SiO2 core-shell nanostructured ceramics were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) at the temperatures 120-450K and particle size of 300 and 500nm. NMR spectra of all studied samples in the paraelectric phase are identical to the spectra in bulk material indicating their perfect perovskite structure without visible influence of particle surface.
However, we have found that surface of particles essentially influence the ferroelectric phase transitions detected by both NMR and EPR techniques. The strongest changes as compared to bulk material were observed in BaTiO3@SiO2 core-shell ceramics.
Thorough analysis of NMR spectra suggests that the orthorhombic-like symmetry phase coexists here with other polar phases up to the Curie temperature. Depending on temperature, its relative volume varies from 25% to 100%.
We assume that the orthorhombic-like symmetry phase is stabilized by anisotropic components of surface stresses which increase also global stability of polar state in nanoceramics to the temperature in bulk material. We summarize our results in a phase diagram.