Studies of the influence of defect distributions and mobility on ferroelectric phase transformations in lead zirconate titanate have been performed by transmission electron microscopy, high resolution electron microscopy, polarization switching measurements, and dielectric spectroscopy. Investigations focused on La3+ and K1+ modified lead zirconate titanates. Defects which could be randomly quenched-in from temperatures significantly above that of the ferroelectric phase transformation resulted in relaxor ferroelectric behavior characterized by polar nanodomains. However, defects which remained mobile until temperatures significantly below that of the phase transformation resulted in a normal ferroelectric state which was characterized by domain pinning. Mobile impurities/vacancies are revealed to order into chain fragments or clusters which diffuse to domain boundaries resulting in a pinning of the boundaries on cooling.