Rietveld refinements of the paraelectric and ferroelectric structures of PbFe0.5Ta0.5O3

The high symmetry parent phase and the two derived low symmetry phases of the complex perovskite PbFe0.5Ta0.5O3 have been refined by the Rietveld method from neutron powder diffraction data. The analysed powders were obtained by grinding single crystals. Owing to the Very small distortions from the cubic structure, the lattice symmetry of the derived phases was determined by means of synchrotron radiation powder diffraction. At 350 K the cubic phase (which is known to be stable for T > 270 K) is characterized by positional disorder or anharmonic thermal motion of lead atoms, as happens in most Pb-based complex perovskites. It was refined in space group Pm (3) over bar m, with strongly anisotropic thermal motion of oxygen atoms. The synchrotron powder diffraction pattern of the intermediate phase (stable for 220 K < T < 270 K), collected at 230 K, agrees with a small tetragonal distortion. Neutron data at 230 K were refined in symmetry P4mm. Only oxygen atoms are significantly displaced from the cubic positions. The analysis of line broadening and splitting in the synchrotron radiation patterns collected at 130 K and 15 K indicate the low temperature symmetry to be monoclinic. Neutron data at 15 K were refined in space group Cm.