Weak ferroelectrics: Specific behaviour of hypersound velocity and damping

A Brillouin spectroscopic investigation was carried out on PMN-55%PT single crystals, which are known to have no chemically ordered regions and undergo a well-defined structural phase transition at T(C) ∼ 521 K. The longitudinal and transverse sound velocities probed on a right-angle scattering geometry exhibited a remarkable softening and increasing hypersonic damping on approaching T(C) from T(B) ∼ 610 K that was characterized by the deviation of the dielectric permittivity from the high-temperature Curie-Weiss behavior. The acoustic anomalies of the longitudinal acoustic mode at the backward scattering were more substantial than those observed at the right-angle scattering, which could be understood in the framework of normal acoustic dispersion considering the difference in the acoustic frequency. The softening of the transverse sound velocity was more significant than that of the longitudinal one upon cooling toward T(C), suggesting that this acoustic anomaly may be related to the local rhombohedral transformation, occurring in polar nano-regions (PNRs). The observed acoustic behaviors combined with the central peak dynamics clearly indicated the existence of dynamic polar nano-regions in PMN-55%PT where there are no chemically ordered regions, and seem to suggest that the order parameter fluctuations due to two kinds of coupling contribute to the acoustic anomalies in the temperature range of T(C) ∼ T(B): electrostrictive coupling between the acoustic waves and the dynamic PNRs, and linear coupling between the acoustic waves and the precursor polar clusters, i.e., the ordering unit responsible for the order-disorder-type slowing down behavior probed by the central peak.

Section: Introductionsupporting

confidence: 60%

Sound velocities and hypersonic dampings of Pb[(Mg_1/3Nb_2/3)_0.45Ti_0.55]O₃single crystals studied by Brillouin light scattering

Kim

Kojima

et al. 2010

“…The deviation of the temperature dependence of the LA phonon velocity from a linear dependence is observed in the vicinity of the temperature of polar nanoregion formation (T B ≈ 570 K) (figures 3(a) and (b)). For the first time this phenomenon was associated with the formation of nanoregions in the analysis of Brillouin light scattering in PMN crystals [22]. Later the effect of polar nanoregions on the behavior of acoustic phonons was also revealed in the relaxor compound lanthanum lead zirconate titanate [23].…”

Section: Resultsmentioning

confidence: 92%

Anomalous behavior of the acoustic phonon velocity and elastic constants of relaxor ferroelectric PbMg_1/3Ta_2/3O₃

Lushnikov

Fedoseev

Gvasaliya

et al. 2007

The behavior of longitudinal (LA) and transverse (TA) acoustic phonons in a cubic relaxor PbMg 1/3 Ta 2/3 O 3 (PMT) ferroelectric has been investigated by Brillouin light scattering. Analysis of the temperature dependences of phonon velocity in the temperature range from 50 to 870 К has revealed anomalies in the vicinity of Burns temperature (Т В ≈ 570К) for LA phonons and a wide frequency-dependent minimum of the velocities of LA and TA phonons in the same temperature region as the dielectric response anomaly. Using experimental data, temperature dependences of the С 11 and С 44 elastic constants have been calculated. The elastic constants have been found to be frequency-independent in the gigahertz range. The results obtained are discussed in the framework of modern ideas on the crystalline lattice dynamics of relaxor ferroelectrics.

“…The lattice dynamics of PMN and similar relaxor crystals PbMg 1/3 Ta 2/3 O 3 (PMT), PbZn 1/3 Nb 2/3 O 3 (PZN) and PbZn 1/3 Nb 2/3 O 3 -xPbTiO 3 (PZN-xPT) has been studied by means of light [8][9][10][11][12][13] and neutron scattering [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]30]. First-order Raman light scattering was observed in both PMN and PZN [8], although first-order scattering is forbidden if the crystal structure remains that of the cubic perovskites.…”

Section: Introductionmentioning

confidence: 99%

“…This anomalous behaviour of the TO phonons was attributed to the formation of PNR in both PZN and PMN [17][18][19]21], although an alternative interpretation was recently proposed that describes the results and the so-called 'waterfall' effect in terms of coupling between TA and highly damped TO phonons [24]. In addition, both light and neutron spectroscopy have reported a quasi-elastic (QE) component in the phonon spectrum of PMN [9,11,13,27,28,30]. As anharmonicity is important in relaxors, it is a priori possible that this quasi-elastic mode originates from the strong coupling between the TA and heavily damped TO phonon modes [32].…”

Section: Introductionmentioning

confidence: 99%

Quasi-elastic scattering, random fields and phonon-coupling effects in PbMg_1/3Nb_2/3O₃

Gvasaliya¹,

Roessli²,

Cowley³

et al. 2005

104

The low energy part of the vibration spectrum in PbMg 1/3 Nb 2/3 O 3 (PMN) relaxor ferroelectric has been studied by means of neutron scattering above and below the Burns temperature, T d . The transverse acoustic and the lowest transverse optic phonons are strongly coupled and we have obtained a model for this coupling. We observe that the lowest optic branch is always underdamped. A resolution-limited central peak and quasi-elastic scattering appear in the vicinity of the Burns temperature. It is shown that it is unlikely that the quasielastic scattering originates from the combined effects of coupling between TA and TO phonons with an increase of the damping of the TO phonon below T d . The quasi-elastic scattering has a peak as a function of temperature close to the peak in the dielectric constant while the intensity of the central peak scattering increases strongly below this temperature. These results are discussed in terms of a random field model for relaxors.