Dilution of the lead positions in improper ferroelastic lead phosphate and lead arsenate by cations without lone pair electron leads to strong re-normalization phenomena. The influence of Ba and Sr impurities on the transition behaviour is studied using optical birefringence, synchrotron X-ray diffraction and diffuse scattering. W and W' twin-walls appear in metastable ferroelastic phases where structural modulations with long lifetime show isothermal relaxations. Strain-induced order parameter coupling is observed in (Pb1-xBax)(3)(AsO4)(2).