Dynamic origin of the morphotropic phase boundary: Soft modes and phase instability in0.68Pb(Mg1/3Nb2/3

Abstract

We report neutron inelastic scattering and high-resolution x-ray diffraction measurements on single crystal 0.68Pb͑Mg 1/3 Nb 2/3 O 3 ͒-0.32PbTiO 3 ͑PMN-0.32PT͒, a relaxor ferroelectric material that lies within the compositional range of the morphotropic phase boundary ͑MPB͒ where the piezoelectric properties of PMN-xPT compounds are close to maximal. Data were obtained between 100 and 600 K under zero and nonzero electric field applied along the cubic ͓001͔ direction. The lowest-energy, zone-center, transverse optic phonon is strongly damped and softens slowly at high temperature; however, the square of the soft-mode energy begins to increase linearly with temperature as in a conventional ferroelectric, which we term the soft-mode "recovery," upon cooling into the tetragonal phase at T C . Our data show that the soft mode in PMN-0.32PT behaves almost identically to that in pure PMN, exhibiting the same temperature dependence and recovery temperature even though PMN exhibits no well-defined structural transition ͑no T C ͒. The temperature dependence of the soft mode in PMN-0.32PT is also similar to that in PMN-0.60PT; however, in PMN-0.60PT the recovery temperature equals T C . These results suggest that the temperature dependence and the energy scale of the soft-mode dynamics in PMN-xPT are independent of concentration on the Ti-poor side of the MPB, but scale with T C for Ti-rich compositions. Thus the MPB may be defined in lattice dynamical terms as the concentration where T C first matches the recovery temperature of the soft mode. High-resolution x-ray studies show that the cubic-to-ferroelectric phase boundary shifts to higher temperatures by an abnormal amount within the MPB region in the presence of an electric field. This suggests that an unusual instability exists within the apparently cubic phase at the MPB.

Relaxing with relaxors: a review of relaxor ferroelectrics

“…Two experiments have been reported for a concentration of PT with x = 0.32 [201,202]. Both experiments measured the phonons and the conclusions are that the phonons were qualitatively similar both with one another and with the results for PMN, especially at a temperature of 650 K near the Burns temperature.…”

“…At lower temperatures the data showed similar effects to those discussed for PMN in Sections 3.4-3.6, namely that to understand the zone centre results for the TO mode the damping had to be increased drastically compared with measurements taken at finite wavevectors. Cao et al [202] extrapolated their experimental data observed well away from the zone centre to the zone centre so as to obtain the frequency of the TO mode. This procedure gives interesting results but does not always give the frequency of the zone centre modes, because no account is taken of any additional damping at the zone centre.…”

Three-mode coupling interference patterns in the dynamic structure factor of a relaxor ferroelectric

“…[12]. The TO and TA phonon dispersion are also similar to those measured previously in PMN-32%PT [33,34]. An extra soft optic mode was also identified previously in PMN [35][36][37], but that mode only appears at high temperatures [36] and was only at wave vectors near the zone center beneath the TO mode.…”

Interplay between static and dynamic polar correlations in relaxorPb(Mg1/3Nb2/3Stock1, Eijck2, Fouquet3 et al. 2010Phys. Rev. BSelf Cite628681View full textAdd to dashboardCite

“…[21][22][23][24] Third, on doping PMN with PT, both the piezoelectric properties and the total diffuse scattering increase but then drop sharply above PT concentrations near 32%, at which point the diffuse scattering is replaced by a well-defined structural transition. 25,26 While the role of strain and strong polarization-deformation coupling must influence many of these results, and has been demonstrated by studying the acoustic phonons ͑Refs. 5 and 27͒, there exists a basic connection between the diffusescattering cross section and the underlying polar structure.…”