Rietveld refinements of the paraelectric and ferroelectric structures of PbFe0.5Nb0.5O3

The aristotype and the two ettotypes of the complex perovskite PbFe(0.5)Nbo(0.5)O(3) have been refined by the Rietveld method from neutron and x-ray powder diffraction data. To avoid errors due to sample off-stoichiometry or inhomogeneity, only powders obtained by grinding single crystals were analysed. At 523 K the cubic phase (space group P/m (3) over bar m), which is known to be stable for T > 376 K, has the lattice parameter a(c) = 4.010(1) Angstrom and is characterized by disorder of the lead and oxygen atoms, as in most Pb-based complex perovskites. At 363 K the intermediate phase, stable for 355 K < T < 376 K, is confirmed to be tetragonal, space group P4mm, with lattice parameters a(t) = 4.007(1) Angstrom and c(t) = 4.013(1) Angstrom. The best refinement for the low-temperature structure (T < 355 K) is obtained, both at 250 and 80 K, in monoclinic symmetry, space group Cm. The monoclinic cell at 250 K has parameters a(m) = 5.674(1) Angstrom, b(m) = 5.663(1) Angstrom, c(m) = 4.013(1) Angstrom and beta = 89.84 degrees(2).