Fluctuating defects in the incipient relaxorK1−xLixTaO3(x=

Abstract

We report neutron scattering measurements of the structural correlations associated with the apparent relaxor transition in K1−xLixTaO3 for x = 0.02 (KLT(0.02)). This compound displays a broad and frequency-dependent peak in the dielectric permittivity, which is the accepted hallmark of all relaxors. However, no evidence of elastic diffuse scattering or any soft mode anomaly is observed in KLT(0.02) [J. Wen et al., Phys. Rev. B 78, 144202 (2008)], a situation that diverges from that in other relaxors such as PbMg 1/3 Nb 2/3 O3. We resolve this dichotomy by showing that the structural correlations associated with the transition in KLT(0.02) are purely dynamic at all temperatures, having a timescale on the order of ∼ THz. These fluctuations are overdamped, non-propagating, and spatially uncorrelated. Identical measurements made on pure KTaO3 show that they are absent (within experimental error) in the undoped parent material. They exhibit a temperature dependence that correlates well with the dielectric response, which suggests that they are associated with local ferroelectric regions induced by the Li + doping. The ferroelectric transition that is induced by the introduction of Li + cations is therefore characterized by quasistatic fluctuations, which represents a stark contrast to the soft harmonic-mode-driven transition observed in conventional perovskite ferroelectrics like PbTiO3. The dynamic, glass-like, structural correlations in KLT(0.02) are much faster than those measured in random-field-based lead-based relaxors, which exhibit a frequency scale of order of ∼ GHz and are comparatively better correlated spatially. Our results support the view that random fields give rise to the relaxor phenomena, and that the glass-like dynamics observed here characterize a nascent response.

Macroscopic response and directional disorder dynamics in chemically substituted ferroelectrics

“…The shift of the permittivity maxima with respect to the corresponding T C and the dispersion in frequency of the (T ) curves around the maxima signal a broad-type transition. This kind of frequency dependence is found in agreement with that reported in the literature on lead-free relaxor ferroelectrics [5,13]: We note that it does not manifest the features of a canonical relaxor response but instead those of a sharp-transition relaxor [5] (or "incipient relaxor" of [46]). …”

“…compound ferroelectrics [46] suggest methods to evaluate the energy associated with the reorientation of the dipolar structures specifically associated with disorder. In this vein it may be possible to shed light on the mechanisms linking the inherent dipoles and the larger scale dipolar systems by analyzing the scaling of the reorientation energy as a function of structure size.…”

Thermal conductivity of K1−xLixTaO3 and KTa1−xNbxO3

“…From the phenomenological analysis of dielectric susceptibility, one is led to believe that similar polar nanoregions are present in both systems [1,2,20]. This view, which is widespread especially for KLT-x, has been questioned by the results of x-ray and neutron scattering studies [27][28][29][30][31]. For example, elastic diffuse scattering and soft-mode anomaly are either absent or extremely weak in the KTaO 3 -based systems, while these components are considered as evidence of polar nanoregions in conventional relaxors.…”

“…Understanding the difference between the relaxor behavior in conventional relaxors (such as PMN and NBT) and the relaxor-like behavior in Li-and Nb-doped KTaO 3 has been a topic of significant interest in recent years [28][29][30][31]. From the phenomenological analysis of dielectric susceptibility, one is led to believe that similar polar nanoregions are present in both systems [1,2,20].…”

Origin of the crossover between a freezing and a structural transition at low concentration in the relaxor ferroelectricK1−xLixTaO3

“…2 (c), the KLT1.8 and KLT2.6 crystals indeed show an increase in intensity at ∼48 K and ∼51 K respectively. The Bragg intensity curves reveal a thermal hysteresis indicative of a metastability region within which the cubic and tetragonal phases can coexist 20,21 . It is important to note that the Bragg intensity of the KLT2.6 crystal in Fig.…”