The effect of chemical variations on the structural polarity of relaxor ferroelectrics studied by resonance Raman spectroscopy

Abstract

Resonance Raman spectroscopy was applied to doped PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3 relaxor ferroelectrics, to better understand the effect of composition disorder on the mesoscopic-scale polar order in complex perovskite-type (ABO3) ferroelectrics. The excitation photon energy used was 3.8 eV, which is slightly above the energy gap and corresponds to the maximum of the optical dielectric permittivity. Group-theory analysis reveals that the resonance Raman scattering (RRS) observed under these conditions is allowed only in polar crystal classes. Therefore, RRS is dominated by the atomic dynamics of nanoregions with coherent polar distortions, which considerably facilitates the comparison of polar order in various compounds. The results show that A-site doping (Ba(2+), Sr(2+), La(3+), Bi(3+)) has significantly stronger effect on the structural polarity than the introduction of a third element at the B site (Nb(5+) or Sn(4+) doped in PbSc0.5Ta0.5O3). The A-site substitution by cations that in contrast to Pb(2+) have isotropic outermost electron shells disturbs the system of lone-pair electrons, thus reducing the correlation length of coupled polar distortions and the strength of the electric field associated with the mean polarization of polar nanoregions. A-site doping with larger cations (Ba(2+)) augments the polar deformation of the individual BO6 octahedra due to local elastic fields. As a result, such A-site doping intensifies the initial structural polarity at high temperatures and prevails the enlargement of the polar fraction at low temperatures. A-site doping with smaller cations (Sr(2+), La(3+)), regardless if they are isovalent or aliovalent to Pb(2+), increases the correlation length of antiferrodistortive order (BO6 tilts), which in turn assists the development of double-perovskite structure with coherent local polar distortions. A-site doping with aliovalent cations (Bi(3+)) having the same outermost electron shell and ionic radius as the host A-site Pb(2+) cations leads to stronger coupling along the -B-O-B- bond linkages due to enhanced random local electric fields.

Atomistic insight into lithospheric conductivity revealed by phonon–electron excitations in hydrous iron-bearing silicates

“…Therefore, the optical phonon modes that are Ramanactive under non-resonance conditions cannot participate in Fröhlich interactions because they do not carry polarity and can become forbidden under resonance conditions (i.e., when electron transitions are activated by a photon or another external stimulus) 31 . Moreover, the RRS owing to Fröhlich interactions is selective; a resonance enhancement is expected only for those LO polar phonons that are related to the local structural species in which the electron transition occurs and therefore can couple with the excited electron [31][32][33] .…”

Properties of Na0.5Bi0.5TiO3 Ceramics Modified with Fe and Mn

“…Moreover, their temperature changes were characterized by the existence of slightly changeable broad bands in a very large temperature interval and some spectrum modifications near the phase transformations similar to that obtained for the pure NBT [42,43]. This feature is characteristic for relaxors and materials with a diffused phase transition [44]. Temperature-induced broadening of the main modes was visible, which could be the result of the evolution of the rhombohedral/tetragonal and tetragonal/cubic phases in the low-and high-temperature intervals, respectively.…”

“…However, the Mn-doped NBT ceramics exhibited enhanced piezoelectric properties d 33 ~89 and 97pC/N and k 33 ~24 and 27% for 0.5 and 1% mol-doped NBT, respectively, compared with those of pure NBT (73 pC/N and 21%). This could be related to lattice distortion and perturbations of the local polarization and strain, which makes the reorientation of domains easier and leads to heterogeneous polar regions and additional interfacial energies [8,44].…”

Фазовые Диаграммы Твердых Растворов Сегнетоэлектриков-Релаксоров По Данным Диэлектрической Спектроскопии

“…Замещение комплекса [B 2+ 1/3 Nb 5+ 2/3 ] 4+ ионами Ti 4+ способствует уменьшению композиционного беспорядка в системе, подавлению случайных электрических полей и, как следствие, переходу в нормальное сегнетоэлектрическое состояние. Усиление релаксорных свойств, при изовалентном замещении в A-подрешетке ионов Pb 2+ на ионы Ba 2+ , возможно за счет искажения октаэдров BO 6 вблизи атомов бария с большим ионным радиусом в сравнении с Pb 2+ [25,26]. В результате возникают локальные упругие поля, которые в случае неупорядоченного распределения ионов в A-подрешетке могут выступать в качестве аналога случайных электрических полей, способствуя подавлению дальнего сегнетоэлектрического порядка.…”