Condensed-Matter Physics & Materials Science Seminar

Relaxor ferroelectrics have a frequency-dependent peak in the dielectric permittivity,
which typically extends over hundreds of degrees and is not directly related to any macroscopic
changes of the symmetry. Relaxors have been numerously studied and models have been
developed to describe them, but a coherent description of all the properties is still lacking. One
of the reasons for the difficulties is that the various anomalies in the properties are observed over
a broad temperature range. We consider the properties of two model cubic relaxor ferroelectrics,
PbMg1/3Nb2/3O3 (PMN) and PbMg1/3Ta2/3O3 (PMT). They have on average the structure of a
simple cubic perovskite, but below ~900 K first-order Raman scattering is observed suggesting
deviations from the perovskite structure. As the temperature is decreased further, there are
anomalies in the macroscopic properties such as the refractive index, the dielectric permittivity,
the optical polarization, the hyper-sound propagation and the specific heat.
This talk will be focused on the progress achieved in understanding the relaxors with the
help of neutron scattering. The results for PMN show that the lowest transverse optic phonon is
not the soft phonon of a classic ferroelectric transition. Diffuse scattering is observed in addition
to the phonons. Above the Burns temperature this scattering is weak and its distribution in wave-
vector suggests it is Huang scattering. Below the Burns temperature there is additional dynamic
quasi-elastic scattering, QE. This QE scattering is intense around some Bragg peaks, but can
hardly be detected around the other Bragg reflections. As the temperature further decreases
strictly elastic diffuse scattering is observed in the neutron spectra from both PMN and PMT.
This scattering is broader in wave vector than the Bragg peaks and cannot be described by a
Lorentzian function.