This work is devoted to the study of heterophase states in (1-x)PbA(1/3)Nb(2/3)O(3)-xPbTiO(3) or PAN-xPT (A=Mg or Zn) single crystals under an electric field E parallel to[001]. The possibility of elastically matched phases in the transformational sequences of Pm3m -> P4mm -> Cm, Pm3m -> Cm, and P4mm -> Cm are considered with regards to non-180 degrees domains and changes in lattice parameters. Systems of diagrams linking domain types with interfaces between coexisting polydomain phases in PMN-0.28PT and PZN-0.045PT crystals are given. An important role of an intermediate P4mm phase in achieving stress accommodation has been identified by analysis of lattice parameters of PMN-0.28PT, and the elastic compatibility conditions between non-180 degrees domain structures in the P4mm and Cm phases have been determined for various heterophase states. Two scenarios of changes in the lattice parameter a(T) in the P4mm phase of PMN-0.28PT have first been discussed to reveal possibilities of stress accommodation in heterophase states. It has been shown that the correlation between domain structures in different heterophase states of studied crystals stems from coordinated unit-cell behavior under E parallel to[001].