Broadband dielectric response of Ba(Zr,Ti)O 3 ceramics: From incipient via relaxor and diffuse up to classical ferroelectric behavior

Dielectric responses of ceramics from the lead-free isovalent BaZrO₃-BaTiO₃ (BZT) system were investigated from Hz frequencies up to the infrared in a broad temperature range, 10--700 K. Pure BaZrO₃ is a displacive weak-incipient ferroelectric with a simple cubic perovskite structure down to low temperatures, whose dielectric response is fully determined by polar phonons, the lowest-frequency one being of the Last type, unlike BaTiO₃, where it is of the Slater type. BaZr0.4Ti0.6O₃ is a relaxor ferroelectric whose dielectric anomaly is caused by a strong, overdamped excitation, which softens from the THz down to MHz range according to the Arrhenius law and merges into a constant-loss background at low temperatures. Such a reponse is similar to lead-containing and heterovalent relaxors, but unlike them, the lowest-frequency TO1 polar phonon does not soften appreciably. In the case of BaZr0.2Ti0.8O₃ we have investigated the dynamic response connected with a diffuse ferroelectric phase transition. The main dielectric anomaly is again due to similar overdamped THz-microwave excitation, which, however, softens only to the GHz range near the transition temperature and below it merges with a near-constant-loss background. The picture of polar nanoregions in BZT differs from that in heterovalent relaxors, because they are pinned to the regions of the off-centered Ti⁴⁺ ions, which are frozen in our temperature range. Therefore we assign the soft relaxations to hopping of the off-centered Ti⁴⁺ ions. This is compared with the behavior of pure BaTiO₃ ceramics, in which the hopping of the off-centered Ti⁴⁺ ions also substantially contributes to the phase transition dynamics. Unlike BaTiO₃, the dynamic instability, which is responsible for the diffuse ferroelectric and relaxor behavior in BZT, is fully due to the hopping dynamics of the off-centered Ti⁴⁺ ions rather than due to soft phonons, and therefore the diffuse transition is of the order-disorder type.