Dielectric Properties of Ferroelectric Hexagonal BaTiO3

Dielectric properties of hexagonal BaTiO 3 have been studied under a variety of conditions. Anomalies are found in the dielectric constant e ' c along the hexagonal c -axis: a cusp-like anomaly at T 0 =222 K and a peak at the ferroelectric Curie point of T c =74 K. After a dc field cooling treatment applied along the c -axis below T 0 , the e ' c increases appreciably at T c , and even in the non-ferroelectric intermediate phase ( T c < T < T 0 ) some increase of e ' c is found. The Curie-Weiss law e ' c = e ' c (∞)+ C /( T - T p ) holds above T c with C =1.9×10 4 K, T p =70 K and e ' c (∞)=280. The estimated spontaneous polarization at 50 K is about 2×10 -2 C/m 2 . The dielectric anomaly at T c disappears if the sample is mechanically clamped. A complex twin configuration appears below T 0 and a rearrangement of twins occur if an intense dc biasis applied. A ferroelastic nature of the 74 K ferroelectric transition and effects of bias field on the twin configuration are discussed.