The non-rhombohedral low-temperature structure of PMN-10% PT

The phase diagram of the Pb(Mg_1/3Nb_2/3)O₃ and PbTiO₃ solid solution (PMN-xFT) depicts a boundary between cubic and rhombohedral phases for x ≤ 0.32. X-ray powder measurements reported by Dkhil et al show a rhombohedrally split (222) Bragg peak for PMN-10% PT at 80 K that is consistent with this phase boundary. Remarkably, our neutron data taken on a single crystal of the same compound with comparable q-resolution reveal a single resolution-limited (111) peak down to 50K, and thus no rhombohedral distortion. Given the marked difference in penetration depths between x-rays and neutrons in these lead-oxide relaxor materials, our results suggest that the bulk structure, which is nearly cubic, differs from that of the outer layer in PMN-10% PT, a situation that has also recently been observed in PZN by Xu et al.