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 (Pb 0.97 La 0.03 )(Zr 0.90 Ti 0.10 ) 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.