The crystal structure of chemically disordered lead scandium niobate, Pb(Sc1/2Nb1/2)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 Pm3̄m) 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.

The influence of chemical B-site ordering between Sc 3+ and Nb 5+ cations on the properties of the Pb(Sc 1/2 Nb 1/2)O 3 (PSN) relaxor has been investigated. Depending of the degree of ordering, PSN exhibits different behaviours. For a completely disordered material, a relaxor-ferroelectric phase transition is observed at 379 K by DSC. Ordering between Sc 3+ and Nb 5+ cations on the B-site of the perovskite structure leads to a non-homogeneous material constituted of two phases: ordered and disordered phases. The phase transition temperature of the ordered phase is confirmed to be lower (346 K) than that of the disordered phase. It appears that the phase transition of the disordered phase is shifted to lower temperature owing to the presence of the ordered phase. Moreover, an additional short-range structural ordering is shown to exist in disordered PSN. This additional ordering, for which antiparallel displacements of Pb atoms could be involved, is favoured by chemical ordering between Sc 3+ and Nb 5+ and is long range in ordered PSN.