Phase transition behavior of the layered perovskite CsBi0.6La0.4Nb2O7 : a hybrid improper ferroelectric

Abstract

The phase behavior of the layered perovskite CsBi0.6La0.4Nb2O7, of the Dion-Jacobson family, has been studied by high-resolution powder neutron diffraction between temperatures 25 < T < 850 °C. At ambient temperature this material adopts the polar space group P21am; this represents an example of hybrid improper ferroelectricity caused by the interaction of two distinct octahedral tilt modes. Within the limits of our data resolution the thermal evolution of the crystal structure is consistent with a first-order transition between 725 and 750 °C, with both tilt modes vanishing simultaneously, leading to the aristotype space group P4/mmm. This apparent ‘avalanche transition’ behavior resembles that seen in the related Aurivillius phase SrBi2Nb2O9.