Relaxor-to-ferroelectric crossover and disruption of polar order in "empty" tetragonal tungsten bronzes

Combined temperature-dependent structural and electrical characterization of a series of “empty” ferroelectric tetragonal tungsten bronzes (TTBs) of composition Ba₄(La₁- _xNd_x)_0.67□_1.33Nb₁₀O₃₀ are reported. The La-material exhibits a temperature dependent crossover from relaxor-ferroelectric to polar (but non-ferroelectric) to linear dielectric behavior. The loss of ferroelectric switching in the polar, non-ferroelectric phase is accompanied by disorder associated with structural relaxation due the significant vacancy concentration at the A1-perovskite-like site. In this disordered regime, large polarization can be re-established with application of sufficient electric field, however relaxation back into the disordered phase occurs on removal of the field as indicated by the loss of remenant polarization. The field against which “backswitching” (depolarization) occurs increases with temperature indicating increasing stability of the disordered regime. The disordered phase can be de-stabilized by substituting Nd for La at the A1-site and which reintroduces “normal” ferroelectric behavior.