Pyrochlores titanate

Voigt, J. A. Tuttle, B. A. Headley, T. J. Lamppa, D. L. 1995. The pyrochlore-to-perovskite transformation in solution-derived lead zirconate titanate thin films. In Ferroelectric Thin Films IV, edited by Tuttle, B. A. Desu, S. B. Ramesh, R. Shiosaki,T. Mat. Res. Soc. Symp. Proc. 361 395 102. 

Chen, J., Lian, J., Wang, L. M., Ewing, R. C., Farmer, J. M. Boatner, L. A. 2002. Structural alterations in titanate pyrochlores induced by ion irradiation X-ray photoelectron spectrum... 

Of these, bastnasite is the only mineral worked primarily for rare earths and both monazite and xenotime are mostly by-products of mining ilmenite, rutile, cassiterite, zircon or gold. Apatite and some multi oxide minerals like pyrochlore, euxenite, brannerite and loparite (a niobium titanate) are also commercial sources of rare earths, but production of RE from these is limited. 

Ln2Ti20j.Ln = Gd, Tb, Dy, Ho. The rare earth titanates that crystallize in the so-called pyrochlore stracture have received much attention. Here only the Ln + site is magnetic as Ti + is a 3d° ion. These four materials exemphfy the variety of exotic magnetic phenomena that can be found in frustrated pyrochlores. 

Kramer, S., Spears, M., and Tuller, H.L., Conduction in titanate pyrochlores role of dopants. Solid State Ionics, 1994, 72, 59-66. 

Among other ion-conducting phases with fluorite-like structures, note should be taken of Y4Nl)Ox s, (Y,Nb,Zr)O2 8 solid solutions, and their derivatives (see Refs. [89-91] and references cited therein). The total conductivity of Y4NbO8.5-8 is essentially independent of the oxygen partial pressure, which may suggest a dominant ionic transport [90]. However, the conductivity level in this system is rather low, and similar to that in pyrochlore-type titanates and zirconates, although some improvements can be achieved by the addition of zirconia. 

Zhang, F.X. and Saxena, S.K. (2005) Structural changes and pressure-induced amorphization in rare earth titanates RE2Ti207 (RE Gd, Sm) with pyrochlore... 

G., Paul, D.M., and Weber, W.J. (2004) Damage accumulation and amorphization in samarium titanate pyrochlore. Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact Mater. At, 218, 89-94. 

The electrical properties of the titanate-based pyrochlores can be described by point defect models in which the acceptor (A) and donor (D) impurities are compensated by oxide ion vacancies, or oxide ion interstitials, respectively. The principal defect reactions inclnde the redox reaction, Equation (5.67), the Frenkel disorder, Equation (5.57), dopant ionization, intrinsic electronic disorder, Equation (5.60), and the electroneutrality relation. For these compounds the total electroneutrality condition given by Equation (5.55) is, on the one hand, reduced, taking Equation (5.57) into account, and is, on the other hand, extended to include acceptor and donor impurities. In addition, defect association is included explicitly. 

In addition to fluorite structure electrolytes such as stabilised zirconia and ceria, there are many non-fluorite structure oxides which are potentially attractive for SOFC electrolyte application. These include perovskites like lanthanum gallate and to a lesser degree calcium titanate. Alternative oxides are the pyrochlores such as yttrium zirconate (YZr207)and gadolinium titanate (Gd2Ti207) [49,50], but these are only suitable in very limited oxygen pressure ranges. Therefore, the main discussion here focuses on the perovskites. 

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