Fluorite-related solids

At least six different intermediate phases are reported. The fluorite-related solid solution (Ba, R)F2+8 (ca) occurs in all systems and becomes significantly narrower at lower temperatures, (Ba, R)F2.o-2.4 (Ba, R)F2.o-2.2- Two fluorite-related superstructure phases are observed Ba2RF7 (t ) for R = Y, Dy-Lu and... 

There are no systematic phase investigations on BeF2-Rp3 systems like those of the other AF2-RF3 systems. Some information is available, however, for ternary systems, e.g. KF-BeF2-Yp3 by Borzenkova et al. (1970). These studies are mainly carried out with respect to the formation of glasses. A general conclusion which can be drawn is that BeF2-Rp3 systems are of simple eutectic character and no intermediate phases have been observed. In connection with laser studies, some information is available on fluorite-related solid solutions (Cd, R)p2+8. but systematic phase studies on CdF2-Rp3 systems are absent. 

One of the most eye-catching structural features in RF2-RF3 (see section 2.3) and AF2-RF3 (see sections 3.2-3.4) systems are the fluorite-related solid solutions ca and the related superstructure phases t, rha/rha, rh/3, rhy, and cj3. This structural essay is devoted to their common structural chemistry, but both speculative models and experimental facts are included. The main emphasis is to... 

Information on the more recent literature of systematic phase studies of the title systems is compiled in table 12. In the following sections, attention is mainly focused upon intermediate phases and their properties. All presently known types of compounds are listed in table 13. Known phases also include fluorite-related solid solutions (Na, R)F2 6 and (K, R)F2+s. In the first case, ordered phases with the postulated formulae Na5RgF32 are reported. The appearance of ARF4 phases for the alkali metals is very controversial. 

Thermal stabilities, composition ranges, and variation of lattice parameters of fluorite-related solid solutions (A, R)F2 with Fm3m and Z = 4 (references are cited in the text). 

RF2-MF3 Two intermediate phases have been found in the system EuFt-AIF3 (Ravez and Dumors, 1969). A cubic fluorite-related solid solution, ca, exists for (Eu, A1)F2.oo-244 with a = 5.836 A to a = 5.787 A EuAlFs is tetragonal (SrAlFs-type, 14, a = 14.12 A, c = 7.185 A). 

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). 

There has been a strong effort to rationalise and elucidate a structural principle which will account for all the anion-deficienl, fluorite-related, mixed-valent binary oxides of cerium, praseodymium and terbium. This is a key step not only for the solid-state chemistry of these materials but also for a large class of fluorite-related materials involved in applications such as fast oxygen conductors and as catalysts. The two main theoretical approaches to the problem were developed by Martin and by Kang and Eyring, and will be illustrated in the following sections. 

Cerium, praseodymium, and terbium oxides display homologous series of ordered phases of narrow composition range, disordered phases of wide composition range, and the phenomenon of chemical hysteresis among phases which are structurally related to the fluorite-type dioxides. Hence they must play an essential role in the satisfactory development of a comprehensive theory of the solid state. All the actinide elements form fluorite-related oxides, and the trend from ThOx to CmOx is toward behavior similar to that of the lanthanides already mentioned. The relationships among all these fluorite-related oxides must be recognized and clarified to provide the broad base on which a satisfactory theory can be built. 

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 topic of mixed conduction in nonstoichiometric oxides was reviewed by Tuller [24], and his comprehensive paper is recommended to the reader interested in more detail concerning the role of multivalent dopants on the defect chemistry of fluorite and fluorite-related oxides, and corresponding transport properties. Equations which express the oxygen flux in solid solutions of, e.g.. 

The principles behind this membrane technology originate from solid-state electrochemistry. Conventional electrochemical halfceU reactions can be written for chemical processes occurring on each respective membrane surface. Since the general chemistry under discussion here is thermodynamically downhill, one might view these devices as short-circuited solid oxide fuel cells (SOFCs), although the ceramics used for oxygen transport are often quite different. SOFCs most frequently use fluorite-based solid electrolytes - often yttria stabUized zirco-nia (YSZ) and sometimes ceria. In comparison, dense ceramics for membrane applications most often possess a perovskite-related lattice. The key fundamental... 

Fig. 15. Tentative phase diagram for the fluorite-related ca solid solutions (A, R)F2+s Md their corresponding superstructure phases of the AF2-RF3 systems (after Greis, 1980b).

Phases of the composition ARF4 are reported for A = Li, Na, Ag, K, Rb, and Cs. From a structural view-point, they can all be considered as metal difluorides with a disordered or partially/fully ordered cation sublattice. Disordered phases can be expected mainly if A and have similar radii and in fact, cubic fluorite-related (Na, R)F2 phases are observed at high temperatures as well as a few (K, R)F2 phases. In most cases, they are part of solid solutions (A, R)F2-j, which are structurally related to yttrofluorites as far as the anion-excess phases... 

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. 

---