Ferroelectric precursor behavior in PbSc0.5Ta0.5O3detected by field-induced resonant piezoelectric spectroscopy

Abstract

A novel experimental technique, resonant piezoelectric spectroscopy (RPS), has been applied to investigate polar precursor effects in highly (65$%$) B-site ordered PbSc${}_{0.5}$Ta${}_{0.5}$O${}_{3}$ (PST), which undergoes a ferroelectric phase transition near 300 K. The cubic-rhombohedral transition is weakly first order, with a coexistence interval of $\ensuremath{\sim}$4 K, and is accompanied by a significant elastic anomaly over a wide temperature interval. Precursor polarity in the cubic phase was detected as elastic vibrations generated by local piezoelectric excitations in the frequency range 250--710 kHz. The RPS resonance frequencies follow exactly the frequencies of elastic resonances generated by conventional resonant ultrasound spectroscopy (RUS) but RPS signals disappear on heating beyond an onset temperature, ${T}_{\mathrm{onset}}$, of 425 K. Differences between the RPS and RUS responses can be understood if the PST structure in the precursor regime between ${T}_{\mathrm{onset}}$ and the transition point, ${T}_{\mathrm{trans}}=300$ K, has locally polar symmetry even while it remains macroscopically cubic. It is proposed that this precursor behavior could involve the development of a tweed microstructure arising by coupling between strain and multiple order parameters, which can be understood from the perspective of Landau theory. As a function of temperature the transition is driven by the polar displacement $P$ and the order parameter for cation ordering on the crystallographic B site ${Q}_{\mathrm{od}}$. Results in the literature show that, as a function of pressure, there is a separate instability driven by octahedral tilting for which the assigned order parameter is $Q$. The two mainly displacive order parameters, $P$ and $Q$, are unfavorably coupled via a biquadratic term ${Q}^{2}{P}^{2}$, and complex tweedlike fluctuations in the precursor regime would be expected to combine aspects of all the order parameters. This would be different from the development of polar nanoregions, which are more usually evoked to explain relaxor ferroelectric behavior, such as occurs in PST with a lower degree of B-site order.