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

C. Stock; P. M. Gehring; G. Xu; D. Lamago; D. Reznik; M. Russina; J. Wen; L. A. Boatner

doi:10.1103/PhysRevB.90.224302

paper

Fluctuating defects in the incipient relaxor K $_{1 - x}$ Li $_{x}$ TaO $_{3}$ (x=0.02)

arXiv:1505.05439 · doi:10.1103/PhysRevB.90.224302

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.

(13 pages, 11 figures)

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Fluctuating defects in the incipient relaxor K $_{1 - x}$ Li $_{x}$ TaO $_{3}$ (x=0.02)