The disordered structure of the complex perovskite Pb(Fe0.5Ta0.5)O-3

The structure of the complex perovskite PbFe0.5Ta0.5O3(PFT), which belongs, according to the literature, to the ferroic species m(3) over barm$ F3m, has been refined by single-crystal and powder x-ray diffraction at room temperature, i.e. within the range of stability of the cubic phase. Attention has been paid to the possible presence of tetragonal spontaneous strain, suggested by the anomalous uniaxial birefringence of each (100)(cub) pyramidal growth sector. A crystal cut from a single growth sector was measured and compared with an as-grown one. Several models, both cubic (<Pm(3)over bar m>) and tetragonal (P4/mmm), these latter used in refining the single growth sector, are compared. The main feature of all the refined structures is the positional disorder at the Pb site, the importance of which in connection with the ferroic transitions is briefly discussed.