Polarization Mechanism in Filled Tungsten Bronze Ba_4Eu_2Ti_4Nb_6O_30 with Pinched P–E Hysteresis Loops

Pinched P–E hysteresis loops have been observed in filled tungsten bronze Ba_4Eu_2Ti_4Nb_6O_30, indicating the presence of novel polarization mechanisms. We investigate the evolution of polar order in filled tungsten bronze Ba_4Eu_2Ti_4Nb_6O_30, together with its dielectric properties over a wide temperature range, from 50 K to 773 K. The temperature dependences of the dielectric properties exhibit two low-temperature dielectric relaxations, at around 300 K (P1), and 100 K (P2), and a high temperature peak at 588 K with no frequency dispersion, indicating the ferroelectric transition temperature T_\rm c. Pinched P–E loops are observed in the temperature range between the low temperature relaxation at P1, and the ferroelectric transition. On cooling, the pinched P–E hysteresis loops open gradually, with increasing remnant polarization (P_\rm r). Two pairs of reversal electric fields indicate two types of polar reversal mechanisms, with an activated energy of 1.41 eV (E_1), and 0.94 eV (E_2), respectively. One corresponds to the field-induced transition from a nonpolar to a polar state, which dominates at a high temperature close to T_\rm c, while the other relates to the reversal of ferroelectric domains which stabilize gradually on cooling. At temperatures below 300 K, the polarization exhibits an evident decrease, probably related to the disruption of the polar order due to the dielectric relaxation at P1.