  A first-principles-based model is developed to investigate the influence of
lead vacancies on the properties of relaxor ferroelectric Pb(Sc1/2Nb1/2)O3
(PSN). Lead vacancies generate large, inhomogeneous, electric fields that
reduce barriers between energy minima for different polarization directions.
This naturally explains why relaxors with significant lead vacancy
concentrations have broadened dielectric peaks at lower temperatures, and why
lead vacancies smear properties in the neighborhood of the ferroelectric
transition in PSN. We also reconsider the conventional wisdom that lead
vacancies reduce the magnitude of dielectric response.
