Freezing in relaxor ferroelectrics and dipolar glasses

Abstract

A recently proposed semi-phenomenological model of freezing in relaxor ferroelectrics, based on the concept of polar nanoregions (PNRs) embedded in a polarizable medium, is reviewed. A generalized Landau-type free energy for the medium is discussed, where the medium polarization couples linearly to the PNR polarization. When the fourth-order Landau coefficient is negative (b < 0), the correlation radius r_c, which measures the PNR size, depends on the temperature T and the applied field E. As T is lowered or E increased, r_c increases and the volume of a cluster of PNRs grows until the percolation limit is reached. This leads to a generalized expression for the Vogel-Fulcher (VF) relaxation time with a field-dependent VF freezing temperature T₀(E). The case b > 0, in which the percolation mechanism cannot be realized, is considered to be appropriate for dipolar glasses.

Keywords:

• relaxor ferroelectrics
• dipolar glasses
• polar nanoregions
• correlation radius
• Vogel-Fulcher law
• electric-field effects

Note

Note that Equation (4(4) $$a\left(T\right)=\frac{T+T_1}{T_{ϵ}},$$(4) ) describes only the properties of the medium and not the global behavior of the relaxor system.

Additional information

Funding

This work was supported by the Slovenian Research Agency under programs P1-0125 and P1-0044, and by the NAMASTE Centre of Excellence.