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

Barium titanate (BaTiO3) ultrafine powders have been prepared from a monome ric metallo-organic precursor through combined solid-state polymerization a nd pyrolysis. This particular preparation route enables an adjustment of th e mean particle size in a wide range from a few nanometers up to micrometer size by choosing an appropriate reaction temperature and tempering atmosph ere. Doping by paramagnetic probe ions such as Mn2+ is readily done by addi ng the corresponding metal acetates. The ultrafine doped BaTiO3 powder samp les with an average grain size varying from 15 nm to 155 nm were studied by x-ray diffraction (XRD) and electron paramagnetic resonance (EPR) techniqu es, taking EPR spectra in the X(9.5 GHz), Q (34.2 GHz), and W (94.1 GHz) fr equency bands. The determined quantities such as grain size, unit cell defo rmation c/a - 1, the axial fine structure parameter D, the distribution wid th Delta D of the latter, and the EPR intensify ratio of cubic and tetragon al Mn-Ti(2+) sites are used to develop a more comprehensive insight into sm all particles. Sufficiently large crystallites consist of a regular ferroel ectric core with a tetragonality gradient towards the outer surface range, surrounded by a peripheral particle layer with strongly distorted translati onal symmetry. In particles smaller than about 40 nm the regular core no lo nger exists at room temperature. EPR spectroscopy along with second-harmoni c generation and Raman measurements at room temperature reveals the occurre nce of a locally acentric structure in very small particles whereas XRD mea surements can only indicate an averaged cubic structure. In agreement with findings in the literature, the presented results give evidence of a regula r grain core undergoing a size-driven transition into the paraelectric phas e, 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 the ory to a particular BaTiO3 particle brings out a critical size of about 50 nm at room temperature whereas in the size-distributed nanopowder a conside rably lower mean size (similar to 25 nm) represents the effective threshold for the size-driven transition into the paraelectric phase.