Crossover in the mechanism of ferroelectric phase transition ofPb[(Mg1/3Nb2/

Abstract

The inelastic light scattering spectrum of Pb͓͑Mg 1/3 Nb 2/3 ͒ 1−x Ti x ͔O 3 ͑PMN-xPT͒ single crystals with x = 0-0.55 due to longitudinal-acoustic ͑LA͒ waves was investigated in a wide temperature range by using a Brillouin scattering technique. The acoustic frequency of LA modes of all PMN-xPT crystals showed a significant softening upon cooling in the high-temperature range where the deviation from the high-temperature linearity starts from ϳ750 K. In addition, hypersonic damping exhibited a remarkable increase upon cooling toward the freezing or ferroelectric phase-transition temperature. These acoustic anomalies were attributed to the polarization fluctuations due to the displacive polarization component arising from the condensation of transverse-optic mode, flipping of polar nanoregions, and, for PMN-xPT with large x, correlated polar nanometer-sized regions ͑PNRs͒ as ordering units. The application of the modified superparaelectric model to the measured acoustic properties revealed that the activation energy for the flipping of PNRs displayed a sharp increase upon crossing the composition of morphotropic phase boundary ͑MPB͒ of PMN-xPT. This change was attributed to the appearance of order-disorder polarization component, i.e., polar clusters in PMN-xPT with large x beyond MPB, which indicated PNRs are correlated and oriented along the same direction. These results may suggest a crossover in the mechanism of phase transition with increasing x: disordered glass state with random directions of PNRs in PMN, comparatively abrupt growth of PNRs at the ferroelectric phase transition without changing the directions of their dipole moments at small x, and order-disorder type ferroelectric transition through alignment of PNRs into single direction via order-disorder mechanism at large x. These results are consistent with broadband dielectric and quasielastic scattering data that show critical slowing down for PMN-xPT with large x.

Development of nanoscale polarization fluctuations in relaxor-based (1-x)Pb(Zn1/3Nb2/Tsukada1, Hidaka2, Kojima3 et al. 2013Phys. Rev. BSelf Cite252250View full textAdd to dashboardCite

“…This peak is also a characteristic of relaxor ferroelectrics, and its interpretation was discussed in our previous works. 17,19,20,22 To extract the frequency shift and width of the LA phonon from the light scattering spectra, we used the Voigt function. The width of a Gaussian component in the Voigt function was fixed as an instrumental function.…”

“…We can therefore conclude that the LA phonon anomaly at T * is an intrinsic effect. [18][19][20]22 We attribute the elastic anomaly in the paraelectric phase to the polarization fluctuation through the piezoelectric coupling. This is described in Eq.…”

“…However, ferroelectric materials with disorder, e.g., relaxor ferroelectrics and barium titanate-(BaTiO 3 -) based ferroelectrics show the anomaly in a much wider temperature range. [17][18][19][20]22,[31][32][33] We attribute the anomaly to a piezoelectric coupling inside the noncentrosymmetric disordered regions. Because the local structure inside the PNRs is noncentrosymmetric, 3,12,13 even though the matrix around the PNR is centrosymmetric, the polarization fluctuation can couple linearly with the strain fluctuation.…”

“…However, τ 0 determined by the Arrhenius law (∼10 −17 Hz) is too small to be meaningful for the physical phenomena. 22,40 Thus, to reproduce the linear temperature dependence of τ , we apply the modified superparaelectric model (modified Arrhenius law)…”

“…Thus, dielectric measurement, inelastic light scattering (both Brillouin and Raman scatterings), inelastic neutron scattering, and inelastic x-ray scattering have been performed in various studies. 11,[14][15][16][17][18][19][20][21][22][23] Of note, the study of acoustic phonons in ferroelectric materials is one of the most powerful approaches for investigating the polarization fluctuations related to a ferroelectric phase transition. 24 This is because acoustic properties are influenced by polarization fluctuations through strainpolarization coupling.…”