Phase stability and Landau phenomenological model of relaxor ferroelectric single crystals 0.78Pb(Mg_1/3Nb_2/3)O₃-0.22PbTiO₃

Relaxor-based ferroelectric single crystal with lower PbTiO₃ content has complex phase transitions and mixed ferroelectric and relaxor characteristics, making it very challenging for the construction of Landau free energy expansion crucially used for theoretical investigation of this system. In this work, relaxor behavior in 0.78Pb(Mg_1/3Nb_2/3)O₃-0.22PbTiO₃ (PMN-0.22PT) single crystals was well suppressed through careful experimental design, leaving preserved single domain state for the Landau parameter extraction in this rhombohedral-phase composition. Using these extracted Landau free energy expansion parameters, energy profile and phase coexistence were theoretically studied and compared with those of PMN-0.30PT and PMN-0.36PT single crystals. Results show that rhombohedral phase dominates in PMN-0.22PT, and tetragonal phase dominates in PMN-0.36PT around room temperature. While, rhombohedral phase and tetragonal phase coexist in MPB composition PMN-0.30PT with small discrepancy of both free energies and polarization vectors.