Fluorites related structures

The isomorphous diiodides of Ce, Pr and Gd stand apart from all the other, salt-like, dihalides. These three, like LaH, are notable for their metallic lustre and very high conductivities and are best formulated as (Ln ,2I",e", the electron being in a delocalized conduction band. Besides the dihalides, other reduced species have been obtained such as LnsCln (Ln = Sm, Gd, Ho). They have fluorite-related structures (p. 118) in which the anionic sublattice is partially rearranged to accommodate additional anions. 

There are large numbers of anion excess fluorite-related structures known, a small number of which are listed in Table 4.4. The defect chemistry of these phases is enormously complex, deserving of far more space than can be allocated here. The defect structures can be roughly divided into three categories random interstitials, which in... 

Anion conduction, particularly oxide and fluoride ion conduction, is found in materials with the fluorite structure. Examples are Cap2 and Zr02 which, when doped with aliovalent impurities. Fig. 2.2, schemes 2 and 4, are F and 0 ion conductors, respectively, at high temperature. The 3 polymorph of 61303 has a fluorite-related structure with a large number of oxide vacancies. It has the highest oxide ion conductivity found to date at high temperatures, > 660 °C. 

The most well-studied and useful materials to date are those with fluorite-related structures, especially ones based on ZrOj, ThOj, CeOj and Bi203 (Steele, 1989). To achieve high oxide ion conductivity in ZrOj, CeOj and ThOj, aliovalent dopants are required that lead to creation of oxide vacancies. Fig. 2.2, scheme 4. The dopants are usually alkaline earth or trivalent rare earth oxides. 

Lithium-containing ternary nitrides most commonly form anti-fluorite related structures (anti-CaF2).9 Nitrogen occupies the 8-coordinate calcium... 

Fast anionic conduction is found mainly in sohds of the fluorite (CaF2) and fluorite-related structures. It is also observed in sohds with the perovskite, YF3, tysonate (LaFs), and simple cubic structures (for these structures, see Oxides Solid-state Chemistry aoA Fluorides Solid-state Chemistry). The smaller anions (r 1.4 A) and F (r 1.2 A) show the fastest conduction however, good anionic conductivity is also found for Cl (r 1.7 A), Br (r 1.8 A), I (r 2.1 A), and for (r 1.7 A). 

For the last twenty years, the research effort on ceria-based catalytic materials has steeply increased (I). An important part of this effort has been devoted to M/Ce02 and closely related systems, i.e. catalysts consisting of transition metals supported on the higher rare earth oxides (Ce02, Pi02-x Tb02-it), and ceria-containing mixed oxides, all of them with fluorite-related structure. 

Oxides exhibiting only high ion conductivity are mainly fluorite-related structures based on zirconia or ceria. Zirconia-based electrolytes are currently used in solid oxide fuel cells (SOFCs). The MIEC oxides are more attractive for separative membrane applications, and these oxides mainly belong to the following types fluorite-related oxides doped to improve their electron conduction, - ... 

The Kang-Eyring system interpreting fluorite related anion deficient structures has recently been extended to a wide range of fluorite related structures with vacancies in both anion and cation sites [28, 29]. 

A structural essay on anion-excess, fluorite-related structures... 

From these relations, from the fundamental principle of anion-excess fluorite-related structures (see section 2.2), and from the chemical compositions, it is possible to establish the chemical formulae of the different superstructures as shown in table 11 for AF2-RF3 systems. In the case of RF2-RF3, A must be replaced by R". All observed phases belong to one or two homologous series, M F2n+5 and M F2 +6. The mineral tveitite (rhy) and the phase rhjS can be... 

Tranquillityite is from the Sea of Tranquillity on the moon (Lovering et al. 1971). The chemical formula suggests an interesting substitution between zirconium and yttrium. The crystal structure is unknown. Gatehouse et al. (1977) found that tranquillityite has a fluorite-related structure from their single crystal work using reconstituted samples in both air and vacuum, and that the structure is related to the structure of a MnZrTi-silicate synthetic phase. 

Laval and co-workers have examined numerous lanthanide and actinide fluorite-related structures. They prepared defect solid solutions of Cai, tR,F2+,j, x = 0.32 R = La, Nd, Tb, Ho, Er, Yb, Lu, by heating the mixed fluorides in sealed Ni tubes at 1000°C for two days with subsequent quench and examining them by room tempera-... 

The fluorite-related lanthanide oxides exhibit unusual diffusional properties. The conventional rule-of-thumb is that atomic mobility in a solid does not become significant until one-half of the melting point temperature (the Tammann temperature) is reached. In these oxides this value is about 1200°C. At the Tammann temperature the metal atoms in lanthanide oxides just begin to become mobile as confirmed by the temperatures required for solid-state reactions. The oxygen substructure, to the contrary, is mobile below 300°C. This leads to a situation where equilibration and reaction must be considered for each substructure separately (see Bevan and Summerville 1979). This places the lanthanide oxides with fluorite-related structures in the category of fast-ion conductors along with, e.g., calcia-stabilized zirconia as indicated in table 18. 

In addition, the solid solution of Y4NbOg.s and (Y, Nb, Zr)02 8 also has the fluorite-related structure, and these materials have stimulated research interest due to their oxide ion conduction [77, 78]. Electrical conductivity of Y4NbOg is independent of the oxygen partial pressure, and it is recognized as an ioific conductor [78]. The ionic conductivity is low, enhancement of conductivity by the addition of Zr was reported, and further improvement of ionic conductivity may be expected [77],... 

Fig. 32.13. The fluorite and fluorite-related structures of the dihalides. (See text for identification of symbols.)...

Most of LeRoy s research at Arizona State University was involved with the higher, predominantly non-stoichiometric lanthanide oxides, of Ce, Pr, and Tb (see below), but it also included non-rare earth oxides of Ti, Zr, Pb, Cm and Bk, mixed rare earth-non-rare earth oxides (especially with fluorite related structures) and rare earth carbonates. A complete list of his publications, awards and other notable achievements is given in the Appendix of the Dedication. 

Rare-Earth Oxides with Fluorite-Related Structures Their Systematic Investigation Using HREM Images, Image Simulations and Electron Diffraction Pattern Simulations, C. Lopez-Cartes, J.A. Perez-Omil, J.M. Pintado, J.J. Calvino, Z.C. Kang and L. Eyring, Ultramicroscopy, 80, 19-39 (1999). 

Proton solubility and conduction are known also for many non-perovskite classes of oxides, comprising mainly fluorite-related structures and structures with oxide ion tetrahedra. 

Fluorite-related structures 489 x,y,z = fractional atomic coordinates... 

Fluorite-related structures in rare earth oxide-fluorides... 

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