Abstract
Dielectric responses of ceramics from the lead-free isovalent BaZrO-BaTiO (BZT) system were investigated from Hz frequencies up to the infrared in a broad temperature range, 10–700 K. Pure BaZrO 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 BaTiO, where it is of the Slater type. BaZrTiO 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 BaZrTiO 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 Ti ions, which are frozen in our temperature range. Therefore we assign the soft relaxations to hopping of the off-centered Ti ions. This is compared with the behavior of pure BaTiO ceramics, in which the hopping of the off-centered Ti ions also substantially contributes to the phase transition dynamics. Unlike BaTiO, 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 Ti ions rather than due to soft phonons, and therefore the diffuse transition is of the order-disorder type.
4 More- Received 4 May 2012
DOI:https://doi.org/10.1103/PhysRevB.86.014106
©2012 American Physical Society