Solid Solutions with BaTiO

FIGURE 31.16 Effect of substitution on 0c for BaTiOs solid solutions with SrTiOs and PbTiOa. 

BaTiOs is rarely used in its pure form because, as mentioned in the previous section, high k occurs only over a very short temperature range that is far from room temperature. Solid solutions with an isostructural compound can broaden 0c as well as shifting it to lower temperatures. One important solid-solution phase is that formed between BaTiOs and SrTiOs. These solid solutions are often referred to as BST Solid solutions of BaTiOs and PbTiOs lead to an increase in 0c over that of pure BaTiOs. The effect on the substitution of either Sr or Pb for Ba " in BaTiOs is shown in Figure 31.16. 

There are a whole range of lead-containing relaxors based on lead zinc niobate (PZN), lead iron niobate (PFN), lead iron tungstate (PFW), and solid solutions with each other and with BaTiOs (BT), PbTi03 (PT), and SrTiOs (ST). Some of the solid-solution phases are PMN-PT, PMN-PT-PZN, PMN-PZN, PFN-PFW, PFN-PMN, and PFW-PT. 

Figure 7 shows the dielectric constant-temperature s — T) characteristics of the Pb doped BST (BPST) thin films at IMHz. For comparison, the - T behavior of BST films is also shown in the inset of Figure 7. It is noticed that the dielectric constant of BPST exhibits a maximum at the temperature of 7 °C, whereas, in the case of BST, the maximum occurs at -79 C. The temperature corresponding to this peak of dielectric constant is considered to be Tc- As is known, BST is a continuous solid solution between BaTiOs (BTO) and STO. PbTiOs (PTO) has a higher Tc of 485 °C than that of BTO of 120 °C. That s why, with the adding of PTO into BST system, the Tc of the solid solution BPST could be adjusted to shift to a higher temperature.

Titanates are double oxides of the form MeTiOa or Me2Ti04. Barium titanate BaTiOa and its solid solution crystals with other titanates are especially well-known. BaTiOs crystallizes in the perovskite structure. Its technical importance results from its ferroelectric and associated piezoelectric properties, its high dielectric constant at room temperature, and the interesting semiconducting properties which it exhibits when doped [13]. The remarkable temperature dependence of the electrical resistance of such doped material (the temperature coefficient can be metal-like) is used to advantage in control and circuit devices. 

Fourier transforms of the mixing potential and thus the free energy for temperature induced partly disordered structures to be determined. Using the 12.5% Sr-doped LaMnOs a thermodynamic analysis was performed [754]. It was predicted, in particular, that disordering of this phase with respect to the decomposition into the heterogeneous mixture of LaMnOs and SrMnOs can occur only at temperatures above the melting point. This is in contrast to a similar study of isostructural BacSr(i c)TiOs solid solution where below a certain temperature Ba impurities in SrTiOs tend to form BaTiOs nanoclusters. 

The literature data on kinetics and mechanisms of BaTiOs formation under hydrothermal conditions are contradictory. In general, the precursor of Ba is completely soluble in aqueous solutions, while the Ti precursor usually forms solid gel-like phases with low solubility. Since Ba + is not a gel forming ion, it initially adsorbs on the particles surface and dissolves in the gel of titanium compounds, destroying Ti-O-Ti bonds. This is accompanied by dehydration, followed by crystallization of perovskite phase via heterogeneous nucleation and growth [82, 83, 89-93]. 

Macdonald J.R., L.D.P. A flexible procedure for analyzing impedance spectroscopy results— description and illustrations. Solid State Ionics 1987 24 61-79 Maissel L.I. Electrical properties of metallic thin films. In Handbook of Thin Film Technology, Maissel L.I., Glang R., ed. New York McGraw-Hill, 1970, pp. 13/11—33 Matsuda H., Mizushima T, Kuwabara M. Low-temperature synthesis and electrical properties of semiconducting BaTiOs ceramics by the sol-gel method with high concentration alkoxide solutions. J. Ceram. Soc. Jap. 1999 107 290-292... 

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