Quantum paraelectric behavior of pyrochlore PMN

TL;DR

This study demonstrates that pyrochlore PMN exhibits quantum paraelectric behavior characterized by a permittivity that increases with cooling and saturates below 30K, without structural phase transition, confirmed by dielectric and spectroscopic analysis.

Contribution

First demonstration of quantum paraelectric behavior in pyrochlore structured dielectric, with detailed spectroscopic evidence linking soft mode dynamics to permittivity saturation.

Findings

Permittivity increases on cooling and levels off below ~30K.

Infrared spectra reveal a soft polar optic mode responsible for dielectric behavior.

Structural analysis suggests local non-cubic symmetry.

Abstract

Pb_1.83Mg_0.29Nb_1.71O_6.39 (PMN) crystallizing in a cubic pyrochlore structure exhibits, as the first dielectrics with pyrochlore structure, typical feature of quantum paraelectrics - its permittivity continuously increases on cooling and levels off below ~ 30K, without any signature of a structural phase transition. Broad-band dielectric spectra do not show any dielectric dispersion in the real part of permittivity up to 8.8 GHz. THz and infrared spectra reveal a soft polar optic mode which is responsible for the temperature dependence of the permittivity. The leveling-off of the permittivity at low temperatures obeys the Barrett formula and the fitted vibrational zero-point energy 1/(2*k_B*T_1) corresponds to the measured soft mode frequency. The number of observed infrared phonons exceeds that predicted from the factor-group analysis which indicates that the structure is at least locally non-cubic.