A zero-field spontaneous relaxor-ferroelectric transition is reported in Pb(Sc0.5Ta0.5)O3 (PST). This behavior is different from that of other relaxors, where such transitions occur only under the field. A highly disordered PST that has the wide relaxation spectrum typical of relaxors is shown to transform spontaneously into a macroscopic ferroelectric state. Introduction of defects (lead vacancies) into the material impedes the transition resulting in the usual relaxor behavior. Dielectric properties of PST, with and without defects, are analyzed. For the interpretation of the observed properties, a model invoking an additional nonpolar phase is proposed. This model does not imply a freezing in the system. At the low-frequency limit, it is possible to account for the Vogel-Fulcher (VF) law for the temperature of the maximum of the dielectric constant, using only the commonly accepted assumption of an exponentially wide relaxation time spectrum that shrinks on heating. The presented approach interprets the observed proximity between the ferroelectric phase transition temperature and that of the freezing temperature obtained from the VF relation.