Fragile phase stability in (1−x)Pb(Mg_1∕3Nb_2∕3O₃)-xPbTiO₃ crystals: A comparison of [001] and [110] field-cooled phase diagrams

Phase diagrams of [001] and [110] field-cooled (FC) (1−x)Pb(Mg_1∕3Nb_2∕3O₃)-xPbTiO₃ or PMN-xPT (0.15⩽x⩽0.38) crystals have been constructed, based on high-resolution x-ray diffraction data. Comparisons reveal several interesting findings. First, a region of abnormal thermal expansion (c≠a) above the dielectric maximum was found, whose stability range extended to higher temperatures on application of electric field (E). Second, the rhombohedral (R) phase of the zero-field-cooled (ZFC) state was replaced by monoclinic M_A in the [001] FC diagram, but with monoclinic M_B in the [110] FC. Third, the monoclinic M_C phase in the ZFC and [001] FC diagram was replaced by an orthorhombic (O) phase in the [110] FC diagram. Finally, in the [001] FC diagram, the phase boundary between tetragonal (T) and M_A phases was extended to lower PT content (x=0.25); whereas in the [110] FC diagram, this extended region was entirely replaced by the O phase. These results clearly demonstrate that the phase stability of PMN-xPT crystals is quite fragile—depending not only on modest changes in E, but also on the direction along which E is applied.