Ferroelectricity along thea- andc-Directionswith Antiferroelectricity along theb-Directionin Rb2ZnBr4and Rb2CoBr4

It was found that Rb 2 ZnBr 4 and Rb 2 CoBr 4 crystals exhibit ferroelectricity along the pseudo-hexagonal c -axis in the phases below the second Curie point 78 K and 62 K, respectively. The temperature dependence of the spontaneous polarization in the a -direction, P a F , the antiferroelectric polarization in the b -direction, P b AF , and the spontaneous polarization in the c -direction, P c F , of these single crystals was measured by means of a pyroelectric method down to 4 K. The temperature dependence of the dielectric constants along the three crystallographic axes was reexamined between 4 and 360 K. Small peak anomalies around 10 K and 56 K for Rb 2 ZnBr 4 and around 17 K for Rb 2 CoBr 4 were observed.