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The thermal expansion and phase transition of unpoled and poled [0 0 1]-oriented 0.69Pb(Mg1/3Nb2/3)O3-0.31PbTiO3 crystals are investigated using thermomechanical analysis, domain evolution, and dielectric measurement. Results of thermomechanical analysis show that the unpoled crystal exhibits negative thermal expansion at a temperature range from 30 °C to 120 °C, and the lattice parameter c of the tetragonal (T) phase is larger than those of the rhombohedral (R) and cubic (C) phases, i.e., cT > cR and cT > cC. Two additional structural modifications near 50 °C and 125 °C are detected in the unpoled crystal. Compared with that of the poled crystal, the T → C phase transition temperature of the unpoled crystal is relatively higher. An increase in temperature causes the unpoled crystal to undergo R → T → C phase transition, whereas R → M → T → C phase transition sequences are observed in the poled crystal. The phase transition behaviors are further verified by domain evolution and dielectric measurement. The significant differences between poled and unpoled crystals are attributed to the differences in their domain structures. [ABSTRACT FROM AUTHOR]