Phase diagrams of [001] and [110] field-cooled (FC) (1-x)Pb(Mg1/3Nb2/3O3)-xPbTiO(3) 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 not equal 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.