Temperature- and electric-field-dependent phase transformations in [001]-oriented Pb(Mg13/Nb23/)0.6Ti0.4O3 single crystal

Temperature- and electric (E)-field-dependent phase transformations in a [001]-oriented Pb(Mg₁3/Nb₂3/)_0.6Ti_0.4O₃ (PMNT40%) single crystal have been investigated by temperature- and frequency-dependent dielectric permittivity and polarizing microscopy. The permittivity ε\´ exhibits a maximum near 463 K with a shoulder near 446 K and a broad step near 270 K. By using relations of crystallographic symmetry and optical extinction, polarizing microscopy data show that the unpoled crystal has tetragonal (T) phase domains with polarizations P along the [100] and [010] axes. These domains are 15 to 60 microns wide, and are separated by 90° domain walls. They coexist at 200 K with a small fraction of monoclinic (M) phase. As temperature increases toward room temperature, the T phase domain increases rapidly at the expense of the M phase. This may account for the broad peak in ε\´ and decrease in ε" near 270 K. The whole crystal becomes cubic near 464 K The E-field-dependent polarizing microscopy results at room temperature show that the polarizations begin to rotate to the tetragonal [001] (T₀₀₁) direction through the M phase and significantly change near E=11 kV/cm. As E field increases, the crystal exhibits more T₀₀₁ domain, which is associated with optical extinction at every orientation of the perpendicularly crossed polarizer/analyzer pair. The crystal becomes entirely T₀₀₁ monodomain near E=33 kV/cm. After the E field is removed, the domain structure shows irreversible behavior. It does not reestablish the broad T₁₀₀ and T₀₁₀ domains described above, but the extinction pattern is consistent with tetragonal microdomains.