Barium Titanate BT

On the other hand, the A positions can be shared by dissimilar ions such as Na and Bi, yielding the complex ferroelectric relaxor ceramic BNT (bismuth sodium titanate) (Hosono et al., 2001 Pookmanee et al., 2001 Pookmanee et al, 2003 see Section 8.3.3). 

(b) Ti-O-chain with spontaneous polarization in z-direction below To The large circles represent oxygen ions, the small circles represent the B-type Ti ions. 

The stability of the perovskite structure depends on Goldschmidt s tolerance factor (rA +ro)/ (rB +ro), with 0.7 t 1.2. For tolerance factors outside this range nonperovskite structure are stable, including pyrochlore-type structures A2B2X6Z, AuriviUius phases (AuriviUius, 1949), or tetragonal tungsten bronze (TTB)-type structures Ba2MTi2X30i5 (M = Ln,Bi X = Nb,Ta) (Stennett et al., 2005). 

AuriviUius phases of the type Bi2A iB 03,, 3 (n = 1-5) are layered bismuth oxides that may be described structuraUy as intergrowths of fluorite-type (Bi202) layers with perovskite (A i B 03 +3) layers (Blake et al., 1997). Typical examples are Bi2W06 n = 1), Bi2SrTa209 (n = 2), and Bi4Ti30i2 (n = 3). 

The structural relationships in ferroelectric ceramics can be explained more clearly using the ferroelectric (and piezoelectric) solid solution system of Pb(Zrj,Tii J03 

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Figure 8.25 Dielectric properties of barium titanate (BT) with surplus Ti02 (O Bryan and Thomson, 1974 Nishigaki et al., 1986).

PZN)-barium titanate (BT) ceramics from mechanochemically synthesized powders. Mater. Res. Bull., 37 (15), 2491-2498. 

If the cations of interest, Ba2+ and Ti4+ in the case of barium titanate, can be obtained in solution in the correct ratio then precursors to the final required product can be formed. Subsequent heating of the precursors leads to the formation of BaTi03 (BT) involving the minimum of ion diffusion distances. Because of this the oxides can be formed at relatively low temperatures, for example 500 °C rather than the usual > 1000 °C necessary to effect the solid state reaction between mixed oxides. 

This is a relatively low-temperature, water-based route capable of producing submicron, spherical and uniform sized particles of either high purity or chemically modified BT. Essentially barium, titanium and dopant compounds are reacted in a basic aqueous medium to form hydroxides. Under the hydrothermal conditions, typically in the temperature and pressure ranges 100-250 °C and 100kPa-3MPa ( —30 atm) respectively, sub-micron particles of either pure or modified barium titanate are precipitated. There are many variables which need careful control, especially the reactive areas of the precursors and the degrees of supersaturation of the various species. 

Mason WP, Matthias BT (1948) Theoretictil model for explaining the ferroelectric effect in Barium Titanate. Phys Rev 74(11) 1622-1636... 

An aqueous precursorfor 0.75Pb(Zni/3- Nb2/3)O3-0.25BaTiOs solid solution was synthesized by Van Werde et al. (2001). They used acetates of lead and zinc, and Nb(V) peroxocitrato complex. The solution of lead zincate niobate was prepared by adding the compounds into aqueous solution of ammonium citrate. The aqueous solution was stable when pH was higher than 8 and the citrate acid/cation mole ratio was higher than 1.5. The aqueous precursor for barium titanate was prepared by using titanium(IV) peroxocitrato complex and barium acetate. They found that the solid solution of PZN and BT is crystallized as a perovskite phase at temperatures as low as 800°C. 

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