Relation between tolerance factor and T c in Aurivillius compounds
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he structures and microstructures of a range of Aurivillius phases were investigated by transmission electron microscopy. Systematic rows of superlattice reflections arising from tilting of octahedra around the c axis were identified, and their intensities at room temperature were shown to diminish as the tolerance factor (t) of the perovskite blocks increased. For compounds with t’s approaching 1, no superlattice reflections were observed. the paraelectric-to-ferroelectric phase transition temperature (Tc) was monitored through permittivity measurements as a function of temperature, and Tc was also shown to decrease as t increased. Consequently, the onset of octahedral tilting and Tc appeared to be strongly related Aurivillius phases. Planar defects arising from the structural phase transitions and from stacking irregularities were also discussed.
I. INTRODUCTION
Aurivillius phases, general formula Bi 2 O 2 2 + (Mn−1RnO3n+1)2−, were first discovered in the 1950s and are composed of perovskite blocks, (Mn−1RnO3n+1), separated and sheared along 1⁄2[111] by rock salt structured Bi2O22+ layers.1 The archetype compound is Bi4Ti3O12 which has two perovskite/three octahedral units in between the Bi-oxide layers. It has tetragonal prototype symmetry, space group I4/mmm (Fig. 1) and undergoes a paraelectric–ferroelectric (PE–FE) phase transition (T c ) at approximately 672 °C. 2 Roomtemperature structural refinements fit well to an orthorhombic cell with space group B2cb and lattice parameters, a ⳱ 0.545 nm, b ⳱ 0.541 nm, and c ⳱ 3.28 nm, but optical microscopy has revealed a domain structure only possible if Bi4Ti3O12 is monoclinic (Pc).2 The main polarization vector is parallel to the a axis, but there is also a small component along the long axis, c.3 Other members of the Aurivillius family are generally orthorhombic and can be formed by cation substitutions onto either the Bi3+ or Ti4+ perovskite sites.1 Typically, on the Bi-site, Pb, Ba, Sr, and Ca can be added to form compounds with a larger number of perovskite units interleaved with rock salt layers, such as Pb2Bi4Ti5O18.4 Compounds with fewer perovskite units between the layers of Bi-oxide can be obtained by substitutions of cations onto the B-site with a higher valence state than Ti4+ such as Nb5+ and W6+, e.g., PbBi2Nb2O9 and Bi3TiNbO9.2 Aurivillius phases also occur as mixed-layer compounds with the general formula Bi4M2m−1R2m+1O6m+9,5 where M is a divalent species and R is the octahedral J. Mater. Res., Vol. 16, No. 11, Nov 2001
cation. Using x-ray diffraction (XRD), Kikuchi showed that these phases are built up of intergrowths of two half-unit cells of conventional Aurivillius phases, e.g., SrBi8Ti7O27, comprising alternate half unit cells of Bi4Ti3O12 and SrBi4Ti4O15 (Fig. 2).5 Reaney et al.6 confirmed this structure using high-resolution transmission electron microscopy (TEM) and suggested a symmetry for the high-temperature phase based on the space group P4/mmm, with a PE-FE phase transition halfway between those of Bi4Ti3O12 and SrBi4Ti4O15 at approximat
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