Quasi-elastic scattering, random fields and phonon-coupling effects in PbMg_1/3Nb_2/3O₃

Abstract

The low energy part of the vibration spectrum in PbMg 1/3 Nb 2/3 O 3 (PMN) relaxor ferroelectric has been studied by means of neutron scattering above and below the Burns temperature, T d . The transverse acoustic and the lowest transverse optic phonons are strongly coupled and we have obtained a model for this coupling. We observe that the lowest optic branch is always underdamped. A resolution-limited central peak and quasi-elastic scattering appear in the vicinity of the Burns temperature. It is shown that it is unlikely that the quasielastic scattering originates from the combined effects of coupling between TA and TO phonons with an increase of the damping of the TO phonon below T d . The quasi-elastic scattering has a peak as a function of temperature close to the peak in the dielectric constant while the intensity of the central peak scattering increases strongly below this temperature. These results are discussed in terms of a random field model for relaxors.

Structure factor of a relaxor ferroelectric

“…In the absence of disorder and in the classical limit ( → 0), we recover the standard results for conventional ferroelectrics. 31 The line shape of the structure factor (13) resembles that of the well-known Lorentzian plus Lorentzian squared for disordered magenetic systems 38 with the important distinction that the relevant interactions in our problem are long-ranged and anisotropic dipolar forces rather than short-ranged isotropic exchange interactions.…”

“…We now compare the static structure factor S q with that measured by neutron scattering experiments 13,14 . Without compositional disorder (∆ = 0), simple inspection shows that S q=0 diverges as the mode frequency softens in the vicinity of the critical point T c where long-range ferroelectric order sets in.…”

Neutron and x-ray diffraction study of cubic [111] field-cooledPb(Mg1∕3Nb2∕3)Stock1, Xu2, Gehring3 et al. 2007Phys. Rev. B603462View full textAdd to dashboardCite

“…8 Dielectric measurements have found the presence of a frozen glassy state at low temperatures and have led to theories of random bond and field effects in analogy to the case of spin glasses. [9][10][11][12] Recent neutron and x-ray scattering studies conducted on PMN and PZN in zero field have found no long-range structural distortion in the bulk at low temperatures. This new phase ͑referred to as phase X in the original measurements͒ was first measured and presented based on neutron diffraction measurements on PZN doped with 8% PT.…”

Evolution of the neutron quasielastic scattering through the ferroelectric phase transition in 93%PbZn1∕3Nb2∕3O3–7%PbTiO3

“…6,7 For PMN the intensity associated with the QE scattering was found to follow the temperature dependence of the dielectric permittivity. 8 The central peak ͑CP͒ and the QE component are also observed in doped PMN by neutron scattering, 9 although the CP intensity is weaker than in pure PMN. In ͑1−x͒PMN− xPT with x = 0.32 the susceptibility of the QE scattering increases below the Burns temperature and has a broad maximum at the temperature where the cubic-totetragonal phase transition occurs.…”

“…We present an analysis of the evolution of the intensity and of the linewidth of this component as a function of the temperature and compare the results with those obtained in ͑1−x͒PMN− xPT ͑x = 0 and x = 0.32͒. 8,9 We used the cold neutron three-axis spectrometer TASP ͑Ref. 14͒ ͑SINQ, 15 PSI͒ operated in the constant k f -mode with k f = 1.64 Å −1 .…”