Order-disorder phenomena in the low-temperature phase of BaTiO3

X- and Q-band electron paramagnetic resonance measurements are reported on Mn4+-doped BaTiO3 single crystals in the rhombohedral low-temperature phase. The Mn4+ probe ion is statistically substitute for the isovalent Ti4+ ion. The critical line broadening observed when approaching the phase transition to the orthorhombic phase demonstrates the presence of order-disorder processes within the off-center Ti subsystem and the formation of dynamic precursor clusters with a structure compatible with one of the orthorhombic phase. From the data it is concluded that BaTiO3 shows a special type of phase transition where displacive and order-disorder character are not only present at the cubic-tetragonal transition, but also at the orthorhombic-rhombohedral transition at low temperatures. The disappearance of the Mn4+ spectrum in the orthorhombic, tetragonal, and cubic phases can be interpreted as the consequence of the strong line broadening caused by changes of the instantaneous off-center positions in time around the averaged off-center position along a body diagonal.