$X$- and $Q$-band electron paramagnetic resonance measurements are reported on ${\mathrm{Mn}}^{4+}$-doped $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ single crystals in the rhombohedral low-temperature phase. The ${\mathrm{Mn}}^{4+}$ probe ion is statistically substitute for the isovalent ${\mathrm{Ti}}^{4+}$ 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 $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ 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 ${\mathrm{Mn}}^{4+}$ 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.

• Received 21 April 2007

DOI:https://doi.org/10.1103/PhysRevB.76.094105