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.