Avalanche-like hybrid-improper ferroelectric transition in SrBi2Nb2O9

Improper-ferroelectrics (FE) are the backbone to development towards room-temperature multiferroics and carry the immense technological potential for next-generation data-storage and energy conversion. This class of FEs differs from displacive FEs (i.e. BaTiO3) as ferroelectricity is induced by anharmonic coupling between a (multiple) non-polar and a polar mode. With the proposed work, we aim to measure the condensation of non-polar and polar modes and confirm the theoretically proposed transition path. These polar and non-polar modes involve motions of heavy (Bi) and light (O) atoms, consequently, their energy scales are vastly different allowing for separation of dynamics on powder samples. Detailed T-dependent phonon measurements will also enable us to probe unusual T-dependence of polarization, which deviates strongly from square-root law because of the anharmonic coupling.