Abstract
The observation of first order satellites in the NMR spectra of $^{47}\mathrm{Ti}$ and $^{49}\mathrm{Ti}$ in the cubic phases of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ as well as $^{18}\mathrm{O}$ enriched $\mathrm{Sr}\mathrm{Ti}^{18}\mathrm{O}_{3}$ demonstrates the presence of nonzero quadrupole coupling at the Ti sites in all these systems. This is incompatible with the central position of Ti in the oxygen octahedron and requires the presence of off-center Ti sites and dynamic disorder even in the paraelectric cubic phase. The Ti potential surface does not seem to change significantly at the cubic-tetragonal transitions. The pure 8-site ``order-disorder'' model however does not agree with the experiment. A good agreement with the experimental data is obtained if in addition to the dynamic Ti disorder the ``displacive'' soft mode induced deformation of the unit cell shape in the dynamic tetragonal nanodomains, existing in the cubic phase, is taken into account. The simultaneous presence of the Ti disorder and the soft mode leads to a special type of phase transition with combined ``displacive'' and ``order-disorder'' character.
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