Universal relaxor polarization in Pb(Mg1/3Nb2/3)O3 and related materials

TL;DR

This study reveals a universal relaxor polarization behavior across different compositions of Pb(Mg1/3Nb2/3)O3-based relaxor ferroelectrics, characterized by a fractional power frequency dependence and a quadratic divergence of susceptibility.

Contribution

It introduces a microscopic model explaining the universal relaxor susceptibility as polarization of polar nanoregions with freely choosing dipole orientations, supported by a spherical model analysis.

Findings

Universal relaxor dispersion exists in multiple compositions.

Susceptibility shows quadratic divergence above Tm.

Real part of susceptibility is small, but imaginary part dominates losses.

Abstract

The dielectric permittivity e at frequencies from [10^(-1) -10^(-5)] Hz to 10^5 Hz is studied in perovskite (1-x)Pb(Mg1/3Nb2/3)O3 - xPbTiO3 relaxor ferroelectric ceramics of different compositions x = 0.35, 0.25 and 0, which exhibit, below the temperature of the diffuse epsilon'(T) maximum Tm, a tetragonal ferroelectric, a rhombohedral ferroelectric and a nonergodic relaxor phase, respectively. The universal relaxor dispersion previously observed at temperatures near and above Tm in the ceramics of x=0.25, is also found to exist in other compositions. This dispersion is described by the fractional power dependence of the real and imaginary parts of susceptibility on frequency. The real part of the universal relaxor susceptibility chi'U is only a comparatively small fraction of the total permittivity epsilon', but chi"U is the dominant contribution to the losses in a wide frequency-temperature range above Tm. In the high-temperature phase a quadratic divergent temperature behavior is observed for all the three compositions studied. The universal relaxor susceptibility is attributed to the polarization of polar nanoregions, which are inherent in the relaxor ferroelectrics. A microscopic model of this polarization is proposed, according to which the dipole moments of some ('free') unit cells inside polar nanoregion can freely choose several different directions, while the direction of the total moment of the nanoregion remains the same. The ensemble of interacting polar nanoregions is described in terms of a standard spherical model, which predicts the quadratic divergence of susceptibility above the critical temperature, in agreement with the experimental results.