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Communication Dans Un Congrès Année : 2008

The structural origin of the antiferroelectric properties and relaxor behavior of Na0.5 Bi0.5 TiO3

Résumé

Na0.5Bi0.5TiO3 (NBT) has been thoroughly investigated during the past forty years, mainly for its interesting ferroelectric and dielectric properties. At ambient temperature, this ferroelectric perovskite compound is rhombohedral with the polar R3c space group. It is known to transform to a tetragonal phase over a large domain of temperature of about 150°C during which the two phases coexist, which makes NBT an unusual compound exhibiting very specific behavior. In addition, if we compare the temperatures of phase transition deduced on one hand from the structural studies and on the others from the numerous experimental physical properties obtained over the last forty years, no clear correspondence exists. Then, between 200°C and 320°C, the correlation between the properties and the structure is not yet understood and a temperature insitu analysis was carried out by transmission electron microscopy (TEM) to reconsider this phase transition. This study shows for the first time that the rhombohedral to tetragonal phase transition in NBT is in fact a two steps phase transitions. The transformation begins by a first order phase transition involving the reconstructive transformation of the rhombohedral (a-a-a- tilt system) phase into an orthorhombic one, through the formation of an intermediate modulated phase. This phase transition begins slightly over 200°C by the disappearance of the ferroelectric-ferroelastic domains. The intermediate modulated phase is observed up to 300°C, temperature at which it disappears. It is formed of Pnma orthorhombic sheets, appearing within the R3c matrix and exhibiting a-a+a- octahedra tilting system. These sheets are twin boundaries between two R3c ferroelectric domains. As the temperature increases, a micro-twining process takes place, generating more and more Pnma sheets. The increasing amount of Pnma sheets, in which the cations are displaced along , leads to a re-orientation of the polar vector within the R3c perovskite blocks initially orientated along . Thus, due to the pseudo merohedral twining law, two successive ferroelastic-ferroelectric R3c domains present polar vectors orientated in opposite direction, in a plane perpendicular to the modulated direction. The modulated phase explains the antiferroelectric property of NBT in this temperature range. The modulated phase is also at the origin of the relaxor behavior of NBT. In the close neighborhood of the Pnma sheets, the cations may occupy two kinds of atomic positions which are nearly equivalent from the point of view of their energy. These two positions are defined by the local structure of the R3c blocks ( displacement) and the Pnma sheets ( displacement). The cations could then alternatively jump from one position to the other by a flipflop mechanism, giving rise to the so-called relaxor behavior of NBT. This study reconciles the structural properties together with the electrical behavior, showing that NBT is not so peculiar compound as it seemed.
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Dates et versions

hal-00326241 , version 1 (02-10-2008)

Identifiants

  • HAL Id : hal-00326241 , version 1

Citer

Vincent Dorcet, Gilles Trolliard, P. Boullay. The structural origin of the antiferroelectric properties and relaxor behavior of Na0.5 Bi0.5 TiO3. E-MRS Spring Meeting 2008, May 2008, Strasbourg, France. ⟨hal-00326241⟩
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