Local structure and dynamics in relaxor-ferroelectric PbSc1/2Nb1/2O3 and PbSc1/2Ta1/2O3single crystals

Abstract

Local structural arrangements and corresponding dynamicaleffects in relaxor ferroelectrics PbSc1/2Nb1/2O3and PbSc1/2Ta1/2O3 were studied by Ramanspectroscopy. Polarized Raman spectra of single crystals weremeasured at different temperatures in several scattering configurations. The peaks observed are assigned on the basis ofnormal-mode calculations. The effect of small structuraldeviations on the atomic vibrations is analysed by calculatingthe modes of structural units at various atomic positions. Itis shown that doubling the structural unit is always possible ona local scale of a few unit cells. The presence of two Ramanpeaks originating from the F2u mode shows thatelectron-phonon coupling occurs in PbSc1/2B''1/2O3, thus leading to dynamical off-centre-symmetrical structural fluctuations. The structuralperturbation consists of the non-coplanarity of Pb and Oatoms in the planes perpendicular to the body diagonal anddeviations of B cations from the centre of BO6 octahedra.The intensity ratio of the two peaks is sensitive to the degree of orientational ordering of the lone pairs of Pb atoms withinthe Pb-O planes, which is related to the degree ofcompositional B'/B'' ordering. The temperature dependence of the peaks arising from asymmetrical O-B-Obending, BO3 translation and Pb-localized modesshows that on cooling different structural changes arepreferential in the two materials studied. InPbSc1/2Nb1/2O3 the lowering of the temperaturefavours the off-centre deviations of the B cations. InPbSc1/2Ta1/2O3 the temperature decreasestabilizes the non-coplanarity of the Pb and O atoms in theplanes perpendicular to the body diagonal and enhances thecorrelation between the electronic lone pairs of Pb.