Crystalline ferroelectrics with a glassy polarization phase

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Abstract
It is found that, far above the ferroelectric transition temperature, Tc, certain ferroelectric systems show a temperature dependent index of refraction, n(T), that is qualitatively different from ordinary ferroelectrics. We review measurements in these systems and show how some aspects of this n(T) data can be quantitatively understood. This is done by considering the polarization in these materials to arise from very small localized clusters of a few unit cells far above Tc. The strongly broken long range symmetry allows these effects to occur. Furthermore, we find a one-to-one correspondence between ferroelectrics that show this unusual high temperature behavior and those that show glass-like excitations at very low temperatures; this clarifies the ambiguities in the low temperature heat capacity and thermal conductivity results.