Emergence of central mode in the paraelectric phase of ferroelectric perovskites
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esearch Letters
Emergence of central mode in the paraelectric phase of ferroelectric perovskites Jeevaka Weerasinghe and L. Bellaiche, Department of Physics and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701 T. Ostapchuk, P. Kužel, and C. Kadlec, Institute of Physics, Academy of Sciences of the Czech Republic, 18221 Prague 8, Czech Republic S. Lisenkov and I. Ponomareva, Department of Physics, University of South Florida, Tampa, Florida 33620 J. Hlinka, Institute of Physics, Academy of Sciences of the Czech Republic, 18221 Prague 8, Czech Republic Address all correspondence to J. Hlinka at [email protected] (Received 26 November 2012; accepted 28 January 2013)
Abstract THz-range dielectric spectroscopy and first-principle-based effective-Hamiltonian molecular dynamics simulations were used to elucidate the dielectric response in the paraelectric phase of (Ba, Sr)TiO3 solid solutions. Our analysis suggests a crossover between two regimes: a highertemperature regime governed by the soft mode only versus a lower-temperature regime exhibiting a coupled soft mode/central mode dynamics. Interestingly, a single model can be used to adjust the THz dielectric response in the entire range of the paraelectric phase. The central peak cannot be discerned anymore in the dielectric spectra when the rate of underlying thermally activated processes exceeds certain characteristic frequency of the system.
It is well known that the static permittivity of ferroelectric materials is related to frequencies of all polar phonon modes through the Lyddane–Sachs–Teller formula.[1] Near the phase transition, however, an additional low-frequency mode has to be often taken into account—the so-called central mode (CM).[2–5] A generic reason for this additional Debye-type excitation seems to be large-amplitude fluctuations between quasi-stable off-center ionic positions. Existence of such intrinsic CM could be very clearly demonstrated, e.g., below the cubic-tetragonal phase transition TC of BaTiO3.[6] Similar CM is also known to exist in the paraelectric phase. Phenomenological theories of the paraelectric CM have been developed by several authors.[2–5,7] All these approaches led to a coupled relaxator–oscillator dielectric response. However, an important question has been left open so far: whether the CM persists up to the highest temperatures, or rather it progressively disappears, or whether it disappears at some well-defined temperature TCM (>TC). Unfortunately, it is much more difficult to obtain a clear-cut experimental evidence for the dielectric CM in the cubic perovskite phase.[2,4,8,9] The characteristic frequencies of the soft phonon-oscillator and CM in KNbO3 and BaTiO3 are so broad and close together that they can hardly be disentangled. Here, we describe a combined experimental and theoretical study of the technologically relevant mixed BaxSr(1–x)TiO3 system (BST). Established characteristic temperature trends in the model parameters of the relaxator–oscillator dielectric response allow one
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