Origin of Ferroelectricity in Aurivillius Compounds
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Origin of Ferroelectricity in Aurivillius Compounds
Donají Y. Suárez Ian M. Reaney, and William E. Lee Department of Engineering Materials, University of Sheffield, Mappin St, Sheffield, S1 3JD, United Kingdom
ABSTRACT
The structures and microstructures of a range of Aurivillius phases have been investigated using transmission electron microscopy. Superlattice reflections arising from rotations of octahedra around the c-axis have been identified and their intensity at room temperature has been shown to diminish as the tolerance factor of the perovskite blocks increases. The paraelectric to ferroelectric phase transition temperature (Tc) has been monitored using permittivity measurements as a function of temperature and Tc has also been shown to decrease as tolerance factor increases. It is proposed that the onset of octahedral tilting and Tc are related in Aurivillius phases.
INTRODUCTION Aurivillius phases, general formula Bi2O22+(Mn-1RnO3n+1)2-, were first characterised in the 1950’s and are composed of perovskite blocks, (Mn-1RnO3n+1)2-, 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 (Figure 1a) and undergoes a paraelectric-ferroelectric (PE-FE) phase transition (Tc) at ~672°C on cooling [2]. Room temperature structural refinements for ferroelectric Bi4Ti3O12 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 polarisation vector is parallel with the a axis, but there is also a small component along c (long axis) [3]. The addition of MTiO3, e.g. SrTiO3 and BaTiO3 to Bi4Ti3O12 results in the formation of compounds with a larger number of perovskite units [1,4]. Compounds with fewer perovskite units can also be obtained by substitutions of cations onto the Ti-site with a higher valence state than Ti4+ such as Nb5+ and W6+, e.g. PbBi2Nb2O9 and Bi3TiNbO9 [2]. In the early 1970’s, Newnham et al. [2], and Dorrian et al.[3] refined the structure of Bi4Ti3O12 and related compounds and demonstrated that the unit cell not only contained, FE cation displacements but that the octahedra in between the Bi-oxide layers were tilted. In Bi4Ti3O12, n=3 (odd), tilting occurs in antiphase around the c-axis and in phase around the a-axis [5]. Antiphase rotations (+/-7.5°) are not present in the central octahedra but in those adjacent to the Bi oxide layer. This structural effect results in a doubling of the ab plane such that a and b of the room temperature orthorhombic/monoclinic cell are approximately √2 times and at 45° to the GG11.9.1
(a)
(b) 0 tilt layer Rocksalt layer a c
c a
0 tilt layer
b
a
Figure 1. (a) Prototype unit cell of Bi4Ti3O12 (b) Schematic showing the tilting of octahedra. prototype tetragonal lattice parameters (~0
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