A neutron powder diffraction study of the ferroelectric relaxor Pb(Fe1/2Ta1/2)O-3

The complex perovskite lead iron niobate, Pb(Fe1/2Ta1/2)O-3 (PFT), 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. A straightforward unit cell of symmetry R3m was obtained for the 300 K structure, with the same symmetry, and a similar unit cell also obtained at the low temperature. In both cases, the iron an d titanium ions were found to be disordered over the perovskite B-sites. Fu rthermore, 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 b eing in a collinear, antiferromagnetic arrangement. This magnetic structure can be described with reference to doubled unit cell axes. The factors gov erning the observed structures of PFT are discussed by comparison with the related system of Pb(Fe1/2Nb1/2)O-3.