Ferroelectric precursor behavior in PbSc0.5Ta0.5O3detected by fi

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 ∼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 onset , of 425 K. Differences between the RPS and RUS responses can be understood if the PST structure in the precursor regime between T onset and the transition point, T 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 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.

Ferroelectric precursor behavior of highly cation-ordered PbSc0.5Ta0.5O3 detected by acoustic emission: Tweed and polar nanoregions

“…The transition is now sharp with no frequency dispersion of the dielectric response, 45 Previous studies using transmission electron microscopy have shown that structural modulations and complex strain pattern exist in all PST samples examined, independent of their degree of order. [46][47][48][49] These structural modulations are related to the relaxor behavior in disordered PST as PNRs.…”

“…23 Below 425 K the thermal expansion increases when tweed precursor ordering of the ferroelectric transition commences. 45 In the precursor regime the sample is locally polar similar to other ferroelectric materials like BaTiO 3 , 55 as well as alloys and ferroelastics. 51,52 Disordered PST is a RFE undergoing a spontaneous cubic-to-rhombohedral (P m3m to R3) phase transition at Curie temperature T c just below T m .…”

“…PST with s = 65% (PST-65 for short) cation order has been studied using dielectric, x-ray and resonant piezoelectric spectroscopy (RPS). 45 The dielectric constant exhibits a sharp, frequency independent maximum at the phase transition point at T trans = 300 K. Thermal expansion of the lattice constant a, derived from x-ray diffraction, is constant between 700 K and 425 K in contrast with disordered PST. 23 Below 425 K the thermal expansion increases when tweed precursor ordering of the ferroelectric transition commences.…”

“…57 Details of the AE technique employed in this study were described before. 26 The PST samples 45 with the sizes of 4 × 5 × 1 mm 3 were coated with fired silver contacts and pasted with a silver epoxy onto the polished side of a fused silica acoustic rod waveguide. A PZT-19 disk piezoelectric sensor was attached to the rear end of the waveguide.…”

Elastic and anelastic relaxation behaviour of perovskite multiferroics I: PbZr0.53Ti0.47O3 (PZT)–PbFe0.5Nb0.5O3 (PFN)

“…This was found to provide good descriptions of precursor softening of the shear modulus in PbMg 1/3 Nb 2/3 O 3 [141] and PbSc 0.5 Ta 0.5 O 3 [142], and it also describes softening ahead of the cubic-tetragonal transition in BaTiO 3 [143]. These models have been tested for PZTFN6 but neither provides a quantitative fit to the precursor softening.…”