Ferroelectric ordering in the relaxorPb(Mg1/3Nb

Abstract

Neutron scattering measurements of the lowest-energy TO phonons in the relaxor Pb(Mg 1/3 Nb 2/3 )O3 (PMN) are reported for 10 ≤ T ≤ 750 K. The soft mode, which is overdamped by the polar nanoregions below the Burns temperature T d = 620 K, surprisingly recovers below 220 K. The square of the soft mode energy (hω0) 2 increases linearly with decreasing temperature, and is consistent with the behavior of a ferroelectric soft mode. At 10 K hω0 reaches 11 meV, the same value observed in ferroelectric Pb(Zn 1/3 Nb 2/3 )O3 at low-T . An unusual broadening of the TA phonon starts at T d and disappears at 220 K, coincident with the recovery of the TO mode. These dynamics suggest that a well-developed ferroelectric state is established below 220 K.

Dynamic origin of the morphotropic phase boundary: Soft modes and phase instability in0.68Pb(Mg1/3Nb2/3Cao1, Stock2, Xu3 et al. 2008Phys. Rev. B262150View full textAdd to dashboardCite

“…Phonon damping below T Burns has previously been interpreted in terms of a condensation of PNR, which impede the propagation of long-wavelength polar modes. 19,20,27 Our results on PMN-0.32PT, combined with those from PMN and PMN-0.60PT, show that this strong damping persists in both relaxor and conventional ferroelectric phases and thus is not a feature unique to relaxors. Instead, the anomalous TO damping seems to correlate directly with the disorder introduced through the extensive mixing of the Mg 2+ , Nb 5+ , and Ti 4+ cations in these compounds.…”

“…In conventional ferroelectrics such as PbTiO 3 ͑PT͒, neutron inelastic scattering was used to show that the condensation or softening of a zone-center, transverse optic ͑TO͒ phonon mode drives the C → T ferroelectric transition, 25 while the square of the soft-mode energy ͑ប 0 ͒ 2 varies linearly with temperature above and below the Curie point ͑T C = 763 K͒ according to the Lyddane-Sachs-Teller ͑LST͒ relationship. 26 Neutron studies of the relaxors PMN, 19,20 PZN, 27 and PZN-0.08T, 28 however, revealed notable differences with PbTiO 3 in that the corresponding TO mode softens slowly and becomes heavily damped at high temperature near T d , the well-known Burns temperature where static, polar nanoregions ͑PNRs͒ first appear. 29 The soft mode remains damped down to ϳ210 K, the temperature below which a remnant polarization can be sustained in a field-cooled state, 30 but exhibits an undamped line shape at lower tem-peratures.…”

“…19,27 By contrast, the TA dispersion measured is essentially temperature independent and similar to those previously reported for other PMN-xPT compositions. 20,22,25 Some changes do occur in the TA phonon parameters with temperature, and these are discussed below. The lack of temperature dependence of the TA phonon branch, despite the significant softening of the TO mode over a broad temperature range, is actually a key observation in light of recent models of the relaxor phase transition.…”

“…Neutron scattering studies have advanced our understanding of PMN-based relaxor ferroelectrics significantly; [18][19][20][21][22][23][24] however to date comparatively few lattice dynamical measurements have been made on PMN-xPT crystals with compositions lying within the MPB. Considering the precipitous drop in the piezoelectric coefficients that occurs when crossing from the Ti-poor to the Ti-rich side of the MPB, such measurements are of obvious importance to efforts to determine the microscopic mechanism responsible for the anomalously high electromechanical response of these materials.…”

Neutron and x-ray diffraction study of cubic [111] field-cooledPb(Mg1∕3Nb2∕3)Stock1, Xu2, Gehring3 et al. 2007Phys. Rev. BSelf Cite603462View full textAdd to dashboardCite

“…The broad anomaly of the dielectric response is reflected in the phonon spectra where a soft transverse optic mode softens and then recovers with decreasing temperature. 5 Concurrent with the recovery of the soft-optic mode, strong diffuse scattering is observed along the ͗110͘ directions, indicative of short-range ferroelectric order, which persist to low temperature. Measurements using neutron pair distribution function analysis have confirmed that at low temperatures no long-range polar order is present with a maximum of ϳ1 / 3 of the sample having polar order.…”