Abstract
Single crystals of have been studied by dynamic mechanical analysis measurements in the low-frequency range –50 Hz. The complex Young's modulus exhibits a quite rich behavior and depends strongly on the direction of the applied dynamic force. In pseudocubic direction, we found intrinsic elastic behavior as expected from the Landau theory; at the antiferroelectric transition K, a downwards cusp anomaly in accompanied by a peak in points to a quadratic/linear order parameter/strain coupling in the Landau free energy. Both anomalies are increasing with decreasing frequency showing that the measurements are performed in the limit . Frequency scans around show energy dissipation, which could result from interphase boundary motion and/or heat diffusion. Above , we observe a pronounced precursor softening, quite similar as it was found in other perovskites, which can be perfectly fitted including isotropic order parameter fluctuations. The low-frequency elastic response in direction is different. Below , we find in addition to the intrinsic anomaly a strong contribution from ferroelastic domains, which leads to an additional softening in . With decreasing temperatures this superelastic softening gradually disappears, due to an increasing relaxation time for domain wall motion, indicating glassy behavior of domain freezing in . In contrast to the direction, for forces along , we found a pronounced precursor hardening, starting at about 60 K above . Since this anomaly is of dynamic nature, starting at the same temperature as the observed birefringence and piezoelectric anomalies [Ko et al. Phys. Rev. B 87, 184110 (2013)], we conclude that it originates from slow dynamic polar clusters, which freeze at .
4 More- Received 2 August 2016
- Revised 20 October 2016
DOI:https://doi.org/10.1103/PhysRevB.94.214101
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