Is Barrett Equation Universally Applicable for Explaining Dielectric Constant of Ferroelectrics and Ferroelectric Thin Films?

Slater's formula for perovskite ferroelectrics was modified by Barrett to include the quantum effect. Dielectric constant vs temperature (ε' vs. T) data for Strontium titanate (ST) and Potassium tantalate (KT) in single crystals and ceramics fitted well, while the data (ε' vs. T) for Barium titanate (BT) in single crystals and ceramics did not fit at all to Barrett's equation. That is, ferroelectrics like ST, which show only Curie-Weiss temperature but does not show a ferroelectric transition, obey Barrett's equation. The Curie-Weiss law fitted the data of BT well. That KT a cubic and consequently a paraelectric down to 13 K obeys Barrett's equation is intriguing. ε' vs T values of Barium strontium titanate (BaxSr1-xTiO3), thin films could be fitted to the Curie-Weiss law, thus eliminating the existence of quantum effect in thin films. [ABSTRACT FROM AUTHOR]