Ground state of the relaxor ferroelectric Pb(Zn_1/3Nb_2/3)O₃

High energy x-ray diffraction measurements on Pb(Zn_1/3Nb_2/3)O₃ (PZN) single crystals show that the system does not have a rhombohedral symmetry at room temperature as previously believed. The phase (X) in the bulk of the crystal gives Bragg peaks similar to that of a nearly cubic lattice with a slight tetragonal distortion. The Bragg profile remains sharp with no evidence of size broadening due to the polar microcrystals. However, in our preliminary studies of the skin, we have found the expected rhombohedral (R) phase as a surface state. On the other hand, studies on an electric-field poled PZN single crystal clearly indicate a rhombohedral phase at room temperature.