Nature of the low-temperature dielectric anomalies in (CH3NH3)(5)Bi(2)X(11) (X=Cl, Br)

The low-temperature dielectric anomalies of two isomorphic ferroelectrics (CH3NH3)(5)Bi2Cl11 (MAPCB) and (CH3NH3)(5)Bi2Br11 (MAPBB) have been studied in detail. The anomaly in the dielectric constant of MAPBB at 77 K shows a thermal hysteresis along all three crystallographic axes. The broad peak of the dielectric constant along the alpha-direction of MAPCB around 160 K becomes flatter and more diffuse as pressure increases. It is concluded that the 77 K anomaly in MAPBB represents a first-order phase transition whereas the 160 K anomaly in MAPCB is an over-critical phenomenon of an isomorphous transition.