Renormalization phenomena in the improper ferroelastic palmierite-type lead phosphate are a consequence of order parameter coupling with defect-induced conjugate fields. The influence of the lead dilution by strontium on the thermal evolution of the strain-related morphic birefringence, the plateau effect of the critical temperature and the ferroelastic domain structure is studied and compared with the influence of barium impurities. The critical behaviour of the diluted material which has been described by a three-states Potts model with order parameter components (Q1, Q2) and (Q3) was determined by Raman spectroscopy, optical birefringence studies, specific heat measurements and X-ray diffraction. The different order parameter coupling in Ba-and Sr-diluted samples leads to a pronounced Tc-plateau in (Pb1-xSrx)3 (PO4)2 and to a small plateau in (Pb1-xBax)3(PO4)2. The domain pattern in (Pb1-xSrx)3 (PO4)2 with x>0.3 represents a microtwin structure of w and w lamellae compared with the wider and regular twin pattern with needle shapes in (Pb1-xBax)3(PO4)2 for x>0.07. Metastable Ba-doped material with x≈0.096 shows a transformation to the high-temperature phase as a function of time.