Size effect in Mn2+-doped BaTiO3 nanopowders observed by electron paramagnetic resonance

Barium titanate (BaTiO3) ultrafine powders have been prepared from a monomeric metallo-organic precursor through combined solid-state polymerization and pyrolysis. This particular preparation route enables an adjustment of the mean particle size in a wide range from a few nanometers up to micrometer size by choosing an appropriate reaction temperature and tempering atmosphere. Doping by paramagnetic probe ions such as Mn2+ is readily done by adding the corresponding metal acetates. The ultrafine doped BaTiO3 powder samples with an average grain size varying from 15 nm to 155 nm were studied by x-ray diffraction (XRD) and electron paramagnetic resonance (EPR) techniques, taking EPR spectra in the X(9.5 GHz), Q (34.2 GHz), and W (94.1 GHz) frequency bands. The determined quantities such as grain size, unit cell deformation c/a - 1, the axial fine structure parameter D, the distribution width Delta D of the latter, and the EPR intensify ratio of cubic and tetragonal Mn-Ti(2+) sites are used to develop a more comprehensive insight into small particles. Sufficiently large crystallites consist of a regular ferroelectric core with a tetragonality gradient towards the outer surface range, surrounded by a peripheral particle layer with strongly distorted translational symmetry. In particles smaller than about 40 nm the regular core no longer exists at room temperature. EPR spectroscopy along with second-harmonic generation and Raman measurements at room temperature reveals the occurrence of a locally acentric structure in very small particles whereas XRD measurements can only indicate an averaged cubic structure. In agreement with findings in the literature, the presented results give evidence of a regular grain core undergoing a size-driven transition into the paraelectric phase, and they lay stress on a heavily distorted surface layer which does not participate in this transition. The size-driven transition is considerably smeared out by the grain-size distribution. Thus, application of Landau theory to a particular BaTiO3 particle brings out a critical size of about 50 nm at room temperature whereas in the size-distributed nanopowder a considerably lower mean size (similar to 25 nm) represents the effective threshold for the size-driven transition into the paraelectric phase.