Neutron and x-ray diffraction study of cubic [111] field-cooled Pb(Mg_1∕3Nb_2∕3)O₃

Neutron and x-ray diffraction techniques have been used to study the competing long- and short-range polar order in the relaxor ferroelectric Pb(Mg_1∕3Nb_2∕3)O₃ (PMN) under a [111] applied electric field. Despite reports of a structural transition from a cubic phase to a rhombohedral phase for fields E>1.7 kV∕cm, we find that the bulk unit cell remains cubic (within a sensitivity of 90°−α=0.03°) for fields up to 8 kV∕cm. Furthermore, we observe a structural transition confined to the near surface volume or “skin” of the crystal where the cubic cell is transformed to a rhombohedral unit cell at T_c=210 K for E>4 kV∕cm, for which 90°−α=0.08±0.03° below 50 K. While the bulk unit cell remains cubic, a suppression of the diffuse scattering and concomitant enhancement of the Bragg peak intensity is observed below T_c=210 K, indicating a more ordered structure with increasing electric field yet an absence of a long-range ferroelectric ground state in the bulk. The electric field strength has little effect on the diffuse scattering above T_c, however, below T_c the diffuse scattering is reduced in intensity and adopts an asymmetric line shape in reciprocal space. The absence of hysteresis in our neutron measurements (on the bulk) and the presence of two distinct temperature scales suggests that the ground state of PMN is not a frozen glassy phase as suggested by some theories but is better understood in terms of random fields introduced through the presence of structural disorder. Based on these results, we also suggest that PMN represents an extreme example of the two-length scale problem, and that the presence of a distinct skin may be necessary for a relaxor ground state.