Neutron diffraction study of the relaxor-ferroelectric phase transition in disordered Pb(Sc_1/2Nb_1/2)O₃

The crystal structure of chemically disordered lead scandium niobate, Pb(Sc_1/2Nb_1/2)O₃, is studied by the high-resolution neutron powder diffraction method. The data, collected over the temperature range 2-653 K, are analysed using Rietveld refinement. From 653 K to 378 K, the symmetry of PSN is found to be cubic (space group Pmm) with positional disorder on the lead and oxygen crystallographic sites. For temperatures close to the ferroelectric phase transition (~378 K), the formation of polar domains in the paraelectric phase is deduced from the observation of the diffuse scattering around Bragg peaks. For the ferroelectric phase, between 368 K and down to 2 K, the symmetry of PSN is determined to be trigonal (with space group R3m) and a positional disorder on the Pb site is evidenced perpendicular to the ferroelectric axis.