Investigation of the structure of the relaxor ferroelectric Pb(Fe_1/2Nb_1/2)O₃ by neutron powder diffraction

The complex perovskite lead iron niobate, Pb(Fe_1/2 Nb_1/2 )O₃(PFN), has been studied by neutron powder diffraction. Following collection of diffraction data at 300 K and at 10 K, structural refinements have been carried out by means of the Rietveld method. As expected, a straightforward unit cell of symmetry R 3mwas obtained for the 300 K structure, with the same symmetry and a similar unit cell also obtained at low temperature. Furthermore, in order to obtain a good agreement with experiment at 10 K, it was necessary to assign non-zero magnetic moments to the iron ions, these being in a collinear, antiferromagnetic alignment. This magnetic structure can be described with reference to doubled unit cell axes. The factors governing the observed structures of PFN are discussed by comparison with the related system of Pb(Mg_1/3 Nb_2/3 )O₃ .