Cation disorder in 3-layer Aurivillius phases: structural studies of Bi_2-xSr_xTi_1-xNb_2+xO₁₂ (0<x<0.8) and Bi_4-xLa_xTi₃O₁₂ (x=1 and 2)

The solid solutions Bi2-xSr2+xTi1-xNb2+xO12 (0 &lt; x &lt; 0.6). (1) and Bi4-xLaxTi3O12 (x = 1 and 2) (2) have been analyzed in detail by a combination of powder X-ray and neutron diffraction, Both solid solutions adopt a tetragonal variant of the archetypal three-layer Aurivillius phase structure of the parent phase Bi4Ti3O12. The X-ray studies of (1) clearly show that, contrary to earlier assumptions, Sr2+ partially substitutes into the [Bi2O2] layers, even at the stoichiometric composition Bi2Sr2TiNb2O12, the maximum amount of substitution being about 30% at x = 0.6. The neutron study demonstrates that the Sr2+ in this site adopts a significantly more isotropic coordination environment than Bi3+, thus stabilizing this unexpected disorder. The disorder is shown to be driven by the requirement for size matching of the fluorite-like and perovskite-like layers. A similar level of disorder is found in Bi2La2Ti3O12, and this is shown to be due to the requirements for optimum local coordination environment at the La3+/Bi3+ sites. (C) 2000 Academic Press.</p

Ferroelectric phase transitions in SrBi₂Nb₂O₉ and Bi₅Ti₃FeO₁₅:A powder neutron diffraction study

Structural phase transitions in the Aurivillius phase ferroelectrics SrBi2Nb2O9 and Bi5Ti3FeO15, containing two and four perovskite layers, respectively, have been studied using powder neutron diffraction. At temperatures below the ferroelectric Curie temperature both materials crystallize in the polar orthorhombic space group A2(1)am. On passing through T-C, both phases appear to transform directly to a tetragonal paraelectric phase, space group I4/mmm. This behavior contrasts with that of the analogs Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15, both of which have been shown unambiguously to transform via an intermediate paraelectric orthorhombic phase, space group Amam.</p

Two high-temperature paraelectric phases in Sr_0.85Bi_2.1Ta₂O₉

Sr0.85Bi2.1Ta2O9 is shown, by Rietveld refinement of powder neutron-diffraction data, to undergo two phase transitions at elevated temperature. The low-temperature ferroelectric phase (space group A2(1)am) transforms to a paraelectric phase (Amam) at T(c)similar to 375 degreesC. This phase does not transform to the expected tetragonal phase (I4/mmm) until T similar to 550 degreesC. The transition from A2(1)am to Amam involves not only loss of the displacive mode along the polar a axis, but also loss of a TaO6 octahedral tilt mode around the c axis.</p

Structural behaviour of the 4-layer Aurivillius phase ferroelectrics SrBi₄Ti₄O₁₅ and Bi₅Ti₃FeO₁₅

Rietveld refinement of powder neutron diffraction data has been used to study the crystal structures of the four-layer Aurivillius-phase ferroelectrics Bi5Ti3FeO15 (at 251C) and SrBi4Ti4O15 (at a series of temperatures up to 8001C). At 251C both materials adopt the polar orthorhombic space group A21am, in common with two-layer analogues such as SrBi2- Ta2O9. At temperatures well above the ferroelectric Curie temperature (i.e., at temperatures of 6501C and above, with TcB5501C) SrBi4Ti4O15 transforms to the centrosymmetric tetragonal space group I4/mmm. However, there is good evidence from the raw diffraction data of a very subtle intermediate paraelectric orthorhombic phase, of Amam symmetry, in the region 55046501C. The distortion in the ferroelectric phase can be traced to displacements of the cations in the A site of the perovskite block, with cooperative tilting of the BO6 octahedra. The nature of the octahedral tilt system, cation disorder at the perovskite A and B sites, and the phase transition sequence in SrBi4Ti4O15, which parallels that found in SrBi2Ta2O9, are discussed