Size Effects in Radiospectroscopy Spectra of Ferroelectric Nanopowders

Abstract

The theoretical and experimental investigation of ferroelectric nanopowders is performed. The manifestation in radiospectroscopy spectra of size driven ferroelectric-paraelectric phase transition at some critical particle average size R = R c was the main goal of the consideration. In theoretical part the size effect for the materials with ferroelectric tetragonal phase at room temperature and cubic paraelectric phase was considered allowing for the spontaneous polarization inhomogeneity inside a particle and distribution of particle sizes. In ESR the transformation of the spectra from tetragonal symmetry to cubic symmetry lines with decrease in nanoparticle sizes was calculated. The method of Rc value extraction from the ratio of the different symmetry lines intensities in the absorption spectra was proposed. Measurements of Fe 3+ ESR spectra in nanopowder of BaTiO3 were carried out at room temperature. The samples were prepared by rate-controlled method with different particle sizes, which depend on annealing temperature. The decrease in intensity of tetragonal symmetry ESR lines of Fe 3+ and appearance of cubic symmetry line with asymmetry of the shoulders was observed with the average sizes decrease with complete disappearance of tetragonal spectrum at R ≤ 40 nm. The comparison of the theory with experiment was carried out. The theory fits experimental data pretty good. The value of critical size R c ≈ 40 nm was extracted from ESR data. The asymmetry and broadening of right hand side shoulder of ESR cubic symmetry line was shown to be related to contribution of paramagnetic centers in the vicinity of the particles surface with lower than cubic symmetry. The deconvolution of the cubic line allowed to show that this region size is about 3 nm.

Giant magnetoelectric effect induced by intrinsic surface stress in ferroic nanorods

“…The core properties are practically the same as those in the bulk. Investigation of ferroelectric nanoparticles by electron-spin-resonance method had shown 28 that the shell sizes are in the region from several to tens of nanometers. The characteristic feature of the shell is the absence of spatial inversion symmetry and so the existence of piezoelectric effect even for cubic symmetry in the bulk.…”

Investigation of ferroelectric nanopowders by EPR method

“…As an example in Figure 1 is shown EPR spectra recorded at different temperatures for the sample annealed at 1100 0 C. The same behavior was observed for all samples, however, the shift of the Fe 3+ tetragonal symmetry fine transition lines to the cubic ones and Mn 2+ spectra appearance started at different temperatures. The main features of the fine structure lines have been already calculated and observed in both tetragonal and cubic symmetry for the Fe 3+ ion (S=5/2) in BaTiO 3 nanopowders [9].…”

“…The local properties therefore becomes of primary importance to exactly determine the effect of a size distribution and fully understand the volume smoothed properties. For this purpose, the application of radiospectroscopic methods (nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR)) were shown to be particularly useful [8,9]. In Ref.…”

“…In our previous paper [9] we used the local polarization and its dependence on the particle size as well as particle size distribution. This allows to obtain more accurate value of the critical radius and the size of the region sensitive to surface influence in the framework of the core and shell model [3].…”

Low temperature dielectric relaxation and charged defects in ferroelectric thin films

“…[19][20][21][22] Within this context, the approach applied for ceramics and nanopowders is reasonably extending to thin films. Just like in the case of BT-based films in this study, the BT thin films in Ref.…”