Dielectric Properties and Phase Transformation of Poled <001>-Oriented Pb(Mg1/3Nb2/3)O3-PbTiO3 Single Crystals

Abstract

The dielectric properties and phase transformation of poled <001>-oriented <TEX>$Pb(Mg_{1/3}Nb_{2/3})O_3-x%PbTiO_3$</TEX>(PMN-x%PT) single crystals with compositions of x = 20, 30, and 35 mole% are investigated for orientations both parallel and perpendicular to the [001] poling direction. An electric-field-induced monoclinic phase was observed for the initial poled PMN-30PT and PMN-35PT samples by means of high-resolution synchrotron x-ray diffraction. The monoclinic phase appears from <TEX>$-25^{\circ}C$</TEX> to <TEX>$100^{\circ}C$</TEX> and from <TEX>$-25^{\circ}C$</TEX> to <TEX>$80^{\circ}C$</TEX> for the PMN-30PT and PMN-35PT samples, respectively. The dielectric constant (<TEX>${\varepsilon}$</TEX>)-temperature (T) characteristics above the Curie temperature were found to be described by the equation<TEX>$(1/{\varepsilon}-1/{\varepsilon}_m)^{1/n}=(T-T_m)/C$</TEX>, where <TEX>${\varepsilon}_m$</TEX> is the maximum dielectric constant and <TEX>$T_m$</TEX> is the temperature giving <TEX>${\varepsilon}_m$</TEX>, and n and C are constants that change with the composition. The value of n was found to be 1.82 and 1.38 for 20PT and 35PT, respectively. The results of mesh scans and the temperature-dependence of the dielectric constant demonstrate that the initial monoclinic phase changes to a single domain tetragonal phase and a to paraelectric cubic phase. In the ferroelectric tetragonal phase with a single domain state, the dielectric constant measured perpendicular to the poling direction was dramatically higher than that measured in the parallel direction. A large dielectric constant implies easier polarization rotation away from the polar axis. This enhancement is believed to be related to dielectric softening close to the morphotropic phase boundary.