The effect of local dipole moments on the structure and lattice dynamics of K0.98Li0.02TaO3

TL;DR

This study investigates how local dipole moments influence the structure and lattice dynamics of K$_{1-x}$Li$_x$TaO$_3$ with low lithium content, revealing a dipolar glass state without long-range order despite relaxor-like dielectric behavior.

Contribution

It provides detailed neutron and x-ray scattering analysis showing the absence of static polar nanoregions and the behavior of phonons, advancing understanding of dipolar glasses in relaxor materials.

Findings

No evidence of static polar nanoregions between 300 K and 10 K.

The TO1 phonon mode softens but does not reach zero energy.

The material exhibits relaxor-like dielectric permittivity without long-range order.

Abstract

We present high energy x-ray (67 keV) and neutron scattering measurements on a single crystal of K$_{1-x}$Li$_x$TaO$_3$ for which the Li content ($x=0.02$) is less than $x_c = 0.022$, the critical value below which no structural phase transitions have been reported in zero field. While the crystal lattice does remain cubic down to T=10 K under both zero-field and field-cooled ($E \le 4$ kV/cm) conditions, indications of crystal symmetry lowering are seen at $T_C=63$ K where the Bragg peak intensity changes significantly. A strong and frequency-dependent dielectric permittivity is observed at ambient pressure, a defining characteristic of relaxors. However an extensive search for static polar nanoregions, which is also widely associated with relaxor materials, detected no evidence of elastic neutron diffuse scattering between 300 K and 10 K. Neutron inelastic scattering methods were used to characterize the transverse acoustic and optic phonons (TA1 and TO1 modes) near the (200) and (002) Bragg peaks. The zone center TO1 mode softens monotonically with cooling but never reaches zero energy in either zero field or in external electric fields of up to 4 kV/cm. These results are consistent with the behavior expected for a dipolar glass in which the local polar moments are frozen and exhibit no long-range order at low temperatures.