The first example of a molecule-based ferroelectric with barium cation: catena-(μ2-nitrito-O,O)-bi-aqua-(18-crown-6)-barium nitrite

Abstract

The molecular crystal catena-(μ2-nitrito-O,O)-bi-aqua-(18-crown-6)-barium nitrite (1) undergoes a reversible ferroelectric phase (FP) transition at Tc (= 182 K, phase transition temperature) from a paraelectric phase (PP). The symmetry breaking is changed from a high temperature centrosymmetric space group (Pnma) to a low temperature noncentrosymmetric space group (Pca21) with an Aizu notation of mmmFmm2. Symmetry breaking analysis indicates that the FP space group is not a sub-group of the PP space group, suggesting it does not obey the Curie-symmetry principle. Some physical property anomalies near Tc, such as the temperature-dependences of Cp, dielectric constant, and second-harmonic generation effects, firmly confirm that this phase transition is reversible, and symmetry breaking really occurred during the temperature cooling down process from high- to low-temperature. Finally experimental fitting based on the Curie–Weiss constant and Cp gives a spontaneous polarization (Ps), reaching about 1.2 μC cm−2.