Second harmonic generation and X-ray diffraction studies of pretransitional region in relaxor K(1-x) LixTaO3

TL;DR

This study investigates the pretransitional region in relaxor K(1-x)LixTaO3 using SHG and X-ray diffraction, revealing polar micro-region nucleation, lattice deformation, and phase transition characteristics related to Li concentration.

Contribution

It provides detailed experimental evidence of the pretransitional region, micro-region evolution, and phase transition behavior in relaxor K(1-x)LixTaO3, combining optical and diffraction techniques.

Findings

Pretransitional region exists between Td and Tp with tetragonal symmetry.

Polar micro-regions nucleate around Td and grow towards Tp.

Ferroelectric micro-domains form below Tp and are field-induced.

Abstract

Optical second-harmonic-generation(SHG) observations and precise X-ray diffraction experiments have been performed on quantum paraelectrics KTaO3 (KTO) and relaxor K(1-x)LixTaO3 with x=3% (KLT-3) and 7% (KLT-7). It is found in KLT-3 and KLT-7 that a pretransitional region exists between two characteristic temperatures Td and Tp (<Td). The average symmetry of the region is tetragonal with a weak lattice-deformation but non-polar in average. The temperature interval between Td and Tp is consistent with the interval on which neutron diffuse scatterings have been previously reported. These facts strongly suggest that polar micro-regions (PMRs) nucleate around Td and grow toward Tp. Below Tp, a larger deformation and a field-induced SH intensity start to develop, while no significant SHG appear in zero-field cooling process. The temperature dependence of the SH intensity below Tp coincides well with that of the tetragonality determined from the lattice deformation. The Landau-Devonshire phenomenological approach suggests that the ferroelectric phase transition at Tp is of first order and that it approaches the second order transition with the decrease of Li concentration. A marked increase of neutron diffraction intensities below Tp indicates that PMRs are transformed to ferroelectric micro-domains at Tp, and the micro-domains change to macroscopic ones under the electric field below Tp.