Abstract
The most ferroelectrics of the Rb2ZnCl4 group exhibit ferroelectricity in the a-direction. It was reported that the Rb2ZnCl4 indicates ferroelectricity also in the pseudo-hexagonal c-direction and the Rb2ZnBr4 and Rb2CoBr4 show not only antiferroelectricity along the b-direction in the intermediate phase but also ferroelectricity along the c-direction below the second Curie point. In this work, by means of a pyroelectric charge measurement, it was found that Rb2CoCl4 crystals exhibit the secondary ferroelectricity below the second Curie transition point, T C2 = 69·0 K along the pseudo-hexagonal c-direction. The temperature dependence of the spontaneous polarization in the a- and c-direction, P a F, and Pc F, was measured down to 4 K. Furthermore the temperature dependence of the dielectric constants along the three crystallographic axes was reexamined between 4 K and about 350 K. A small peak anomaly of the dielectric constant in the a-direction was observed around 20 K.