A quadrupole coupling induced ${}^{\mathrm{47}}\mathrm{T}\mathrm{i}$ and ${}^{\mathrm{49}}\mathrm{T}\mathrm{i}$ satellite background which transforms into well-defined satellite lines below $T_c$ in the ferroelectric phase has been observed in the cubic phase of an ultrapure ${\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}\mathrm{O}}_3$ single crystal. The results demonstrate the coexistence of a displacive and order-disorder component in the phase transition mechanism and tetragonal breaking of the cubic symmetry due to biased Ti motion between off-center sites in the paraelectric phase above $T_c$.

• Received 10 September 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.037601