Lanthanum incorporation in lead zirconate titanate (PLZT) increases the stability range of the antiferroelectric phase in the Zr-rich side of the phase diagram. The paper presents the analysis, which has been carried out on (Pb0.97La0.03) (Zr0.90Ti0.10)(0.9925)O-3 ceramic system since it exhibits interesting results with ferroelectric (FE) and antiferroelectric (AFE) phases, and coexistence of both properties in some temperature range. Transmission electron microscopy evidences a complex domain structure, where nanodomains appear within larger ones. The data are consistent with the possibility to have in this material a ferroelastic phase of novel type in which the coexistence mechanism between FE and AFE properties is associated with the oscillation in space of the polarization. The possibility for a determining role of a rigid unit mode in structural instabilities, dependence on composition and doping, and pretransitional effects in the perovskite family is underlined by the obtained results for the studied system.