Abstract: This paper studied the evolution progress and feature of domain configuration while the
composition and structure changed in relaxor ferroelectric single crystals Pb(Mg_1/3Nb_2/3)O₃-PbTiO_₃. It was found that
with increasing PbTiO_3 concentration in the rhombohedral phase region of PMN-PT system, domain configuration exhibited a transition
process, i.e. microdomains-(sub-microdomains)-irregular macrodomains-regular macrodomains. And then, if PbTiO₃ increased further,
rhombohedral-tetragonal phase transition could take place, giving rise to the change from 71°(or 109°) domains to 90° ones. 90° domains demonstrated more regular or
typical configuration than 71°(or 109°) domains. In light of the showing feature of domain configuration, it was proposed
that the largest birefractive index of PMNT crystals could be taken as an optical parameter measuring their relaxor degree in ferroelectric behavior.
Moreover, the phenomena of non-uniform distribution of domain structures within a plate were observed, which arose from composition heterogeneity
and structure fluctuation. Domains with substructure were also visualized, which could be interpreted by multistage martensitic transformation. In addition, the
paper discussed the imaging features of domain configuration under the observation means including polarizing microscopes, differential
interference contrast microscopes (DIC), scanning electronic microscopes (SEM) and scanning electronic acoustic microscopes (SEAM).

Key words: relaxor ferroelectrics, piezocrystals, PMNT, domain configuration, microdomain-macrodomain transformation