Dynamics and ferroelectric phase transition of (C3N2H5)5Bi2Br11 by means of ac calorimetry and 1H NMR relaxometry

► The excess entropy clearly confirms an 'order-disorder' mechanism assigned to the dynamics of the imidazolium cations. ► 1H NMR conformed dynamical heterogeneity of the imidazolium cations in low temperature phases. ► The paraelectric-ferroelectric transition at 155 K is close to the tricritical one.