Thermal expansion anomalies in polar and ferroelectric crystals

Abstract

The physical nature of the phenomenon of negative thermal expansion coefficient seen in the polar crystals and ferroelectrics is considered. Usually it is observed at low temperatures but especially evident in vicinity of ferroelectric phase transitions. This phenomenon is associated with fundamental nature of non-centrosymmetric crystals, caused by electronegativity difference of adjoined ions forming polar crystal. Negative thermal expansion coefficient indicates a change in the nature of forces of the attraction at close proximity of ions forming polar bond. The thermal expansion minimum is specified by the configurational entropy, conditioned by polar bonds self-ordering in pyroelectrics and ferroelectrics.