Emergence of central mode in the paraelectric phase of ferroelectric   perovskites
  THz-range dielectric spectroscopy and first-principle-based effective-Hamiltonian molecular dynamics simulations were employed to elucidate the dielectric response in the paraelectric phase of (Ba,Sr)TiO3 solid solutions. Analysis of the resulting dielectric spectra suggests the existence of a crossover between two different regimes: a higher-temperature regime governed by the soft mode only versus a lower-temperature regime exhibiting a coupled soft mode/central mode dynamics. Interestingly, a single phenomenological coupling model can be used to adjust the THz dielectric response in the entire range of the paraelectric phase (i.e., even at high temperature). We conclude that the central peak is associated with thermally activated processes, and that it cannot be discerned anymore in the dielectric spectra when the rate of these thermally activated processes exceeds certain characteristic frequency of the system. 
J. Weerasinghe, L. Bellaiche, T. Ostapchuk, P. Kuzel, C. Kadlec, S.   Lisenkov, I. Ponomareva and J. Hlinka
cond-mat.mtrl-sci cond-mat.mes-hall cond-mat.stat-mech