Theory of Relaxor Ferroelectrics
Relaxor ferroelectric crystals are characterized by critical fluctuations of polarization that extend over a wide region of temperatures with unusual line shapes. Despite more than 50 years of intense research, a satisfactory theory of relaxor ferroelectricity has remained elusive. We present a theory of the fluctuations of relaxors with a model of polarizable unit cells with dipolar forces, local anharmonic forces, and local random fields.
We find that (i) arbitrarily small compositional disorder together with dipolar forces extend the region of critical fluctuations down to absolute zero temperature and (ii) the correlation functions of polarization are highly anisotropic and slowly varying with a power law component. We compare our results to the elastic diffuse scattering measured by neutron experiments and frequency dependent dielectric constant measurements.