Neutron diffraction study of the relaxor-ferroelectric phase transition indisordered Pb(Sc1/2Nb1/2)O-3

The crystal structure of chemically disordered lead scandium niobate, Pb(Sc 1/2Nb1/2)O-3, is studied by the high-resolution neutron powder diffraction method. The data, collected over the temperature range 2-653 K, are analyse d using Rietveld refinement. From 653 K to 378 K, the symmetry of PSN is fo und to be cubic (space group Pm (3) over bar m) with positional disorder on the lead and oxygen crystallographic sites. For temperatures close to the ferroelectric phase transition (similar to 378 K), the formation of polar d omains in the paraelectric phase is deduced from the observation of the dif fuse scattering around Bragg peaks. For the ferroelectric phase, between 36 8 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 per pendicular to the ferroelectric axis.