Abstract :
Substitutional charge disorder giving rise to quenched electric random-fields (RF s) is probably
at the origin of the peculiar behavior of relaxor ferroelectrics, which are primarily characterized
by their strong frequency dispersion of the dielectric response and by an apparent lack of
macroscopic symmetry breaking at the phase transition. Spatial fluctuations of the RF s
correlate the dipolar fluctuations and give rise to polar nanoregions in the paraelectric regime
as has been evidenced by piezoresponse force microscopy (PFM) at the nanoscale. The
dimension of the order parameter decides upon whether the ferroelectric phase transition is
destroyed (e.g. in cubic PbMg1/3Nb2/3O3, PMN) or modified towards RF Ising model behavior
(e.g. in tetragonal Sr1−xBaxNb2O6, SBN, x ≈ 0.4). Frustrated interaction between the polar
nanoregions in cubic relaxors gives rise to cluster glass states as evidenced by strong pressure
dependence, typical dipolar slowing-down and theoretically treated within a spherical random
bond-RF model. On the other hand, freezing into a domain state takes place in uniaxial relaxors.
While at Tc non-classical critical behavior with critical exponents γ ≈ 1.8, β ≈ 0.1 and α ≈ 0 is
encountered in accordance with the RF Ising model, below Tc ≈ 350 K RF pinning of the walls of
frozen-in nanodomains gives rise to non-Debye dielectric response. It is relaxation- and
creep-like at radio and very low frequencies, respectively.
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