Fluctuating defects in the incipient relaxor K$_{1-x}$Li$_x$TaO$_3$ (x=0.02)

TL;DR

This study uses neutron scattering to reveal that the relaxor transition in K$_{1-x}$Li$_x$TaO$_3$ at x=0.02 involves purely dynamic, overdamped fluctuations without soft mode anomalies, differing from typical relaxors and indicating local ferroelectric regions.

Contribution

It demonstrates that the relaxor behavior in KLT(0.02) is driven by dynamic fluctuations rather than soft modes, contrasting with conventional ferroelectrics and other relaxors.

Findings

Structural correlations are purely dynamic and overdamped.

Fluctuations are faster than in lead-based relaxors.

Doping induces local ferroelectric regions without soft mode softening.

Abstract

We report neutron scattering measurements of the structural correlations associated with the apparent relaxor transition in K$_{1-x}$Li$_x$TaO$_3$ 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$_{2/3}$O$_3$. 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 $\sim$THz. These fluctuations are overdamped, non-propagating, and spatially uncorrelated. Identical measurements made on pure KTaO$_3$ 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 PbTiO$_3$. 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 $\sim$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.