Fluorite type defects

The fact that rock salt layers can adapt to perovskite layers suggests the possibility of formation of extended defects due to a variation of the thickness of these layers in the original matrix. Such extended defects are frequently observed by high resolution electron microscopy in bismuth cuprates and especially in thallium cuprates. Moreover the relationships between the two structural types and the fluorite structure makes that fluorite type defects are also observed. 

The comparison of the different [m, n] superconducting cuprates suggests that the formation of rock salt or perovskite type defects will not affect dramatically the superconducting properties of these materials. On the opposite, one can expect a dramatic decrease of the superconductivity due to the appearence of double or triple fluorite type defects, since the corresponding regular intergrowths do not superconduct. 

Sulphides. The partially ionic alkali metal sulphides Me2S have the anti-fluorite-type structure (each Me surrounded by a tetrahedron of S, and each S atom surrounded by a cube of Me). The NaCl-structure type (6/6 coordination) is adopted by several mono-sulphides (alkaline earth, rare earth metals), whereas for instance the cubic ZnS-type structure (coordination 4/4) is observed in BeS, ZnS, CdS, HgS, etc. The hexagonal NiAs-type structure, the characteristics of which are described in 7.4.2.4.2, is observed in several mono-sulphides (and mono-selenides and tellurides) of the first-row transition metals the related Cdl2 (NiAs defect-derivative) type is formed by various di-chalcogenides. Pyrite (cP 12-FeS2 type see in 7.4.3.13 its description, and a comparison with the NaCl type) and marcasite oP6-FeS2 are structural types frequently observed in several sulphides containing the S2 unit. 

Figure 1.11. Schematic diagrams of (a) and (b) Fei j O point defect clusters (after Koch and Cohen 1969) (c) clustering in fluorite-type structures and (d) the Cap2 structure.

All the elements of the actinide series from Th to Cm form fluorite-type dioxides, and the series affords an opportunity of studying the nonstoichiometry and ordered defect phases characteristic of the structure. To date, the most complete set of results refers to uranium dioxide, and these are discussed first. 

Fluorite type oxides are particularly prone to nonstoichio-metric effects. This most commonly occurs in the form of cation nonstoichiometry induced by partial reduction of the cation or by replacement of a portion of the oxide by flnoride. Anion excess phases can occur as a result of cation oxidation or by replacement with higher valence impurities. The dominant defect in this structure involves the migration of oxygen to the large cuboidal interstice resulting in the formation of a vacancy at a normal lattice site. A vacancy of this type is called a Frenkel defect. 

Neptunium. The fluorite-type Np02 is the stable oxide formed in air when neptunium oxysalts are decomposed, but almost no studies have been carried out in the oxygen-defect region. Ackermann et al. ( ) in studying the vaporization process of NpO observed A-type NP2O3 in quenched samples which had been 70% vaporized. It is likely that the two phases were formed from a nonstoichiometric Np02. x phase by disproportionation as the sample was cooled. 

Such Frenkel type defects are known to exist in many anion excess fluorite related compounds forming clusters, at higher defect concentration, like in U409 a, [14J. This interstitial oxygen was made responsible for the excellent behaviour of ceria as an oxygen storage medium. 

Pure stoichiometric CeOj has the calcium fluoride (fluorite) type of structure with space group / m3m. It has been also well known as a Non-stoichiometri compound and there are variety of studies concerning about the defect and redox chemistry [17-19]. Among them, Stefano has already reported the electron localization in pure and defective ceria by a unified LDA+U approach as shown in Figure 4-9 [20]. Here we can see the decrease of band gap of both cerium sub oxide... 

The doping of Zr02 with alkaline earth metal cations (A ) is less effective due to a greater tendency to defect association and to a lower thermodynamic stability of the cubic fluorite-type solid solutions in Zr02-A0 systems. To date, attempts to increase the stability of Sc-containing materials by codoping, or to reduce the cost of Ln -stabilized phases by mixing them with cheaper alkaline earth dopants, have not yielded any worthwhile results [228]. 

Solid electrolytes. These correspond to soHd materials in which the ionic mobility is insured by various intrinsic and extrinsic defects and are called solid ion conductors. Common examples are ion-conducting solids with rock salt or halite-type solids with a Bl structure (e.g., a-AgI), oxygen-conducting solids with a fluorite-type Cl structure (A"02), for instance CaF and yttria-stabilized zirconia (YSZ, ZrO with 8 mol.% Y O,), a pyro-chlore structure (A BjO ), perovskite-type oxides (A"B" 03), La Mo O, or solids with the spinel-type structure such as beta-aluminas (NaAl 0 ) for which the ionic conduction is ensured by Na mobility. 

Notation of Point Defects ), such defects are called as oxygen vacancies (more precisely oxide ion vacancies). Due to the incorporation of lower valent yttrium ions instead of zirconium ions, oxide ion vacancies are generated in the cubic fluorite-type lattice. For 8 mol-% Y2O3 the composition of such substituted zirconia is Zro.84Yo.i6O1.92. 

Yttria-stabilized Z1O2, discovered by Nemst [39], is still one of the state-of-the-art SOFC electrolyte materials which was used to demonstrate the first SOFC (and the first solid electrolyte fuel cell) in 1937 at ETH-Ziirich [40]. Electronic defect concentrations are negligibly low [41]. As can be observed in Fig. 6.1, the ionic conductivity of fluorite-type oxides stabilized with hypovalent elements exhibits a maximum at a certain dopant concentration above which defect interactions occur [42-45]. As shown in Fig. 6.2, it is known that the peak value of the ionic... 

Pyrochlores and Other Fluorite-Type Oxides (Y, Nb, Zr)02S Pyrochlore-type oxides have the general formula A2 B2 07 in which A is a rare-earth element such as Gd or Y, and B is Ti or Zr. Gd2Zr207 is the typical composition of pyrochlore-type oxides and can be considered as fluorite-type in which ionic defects are regularly arranged. The defect structure and the mixed conductivity can be controlled by the value of x in, for example, Gd2(ZrxTii x)20 . which is abbreviated as GZT [69]. When the ionic radius of rare-earth elements for the A site is larger than that of Gd, the structure changes from highly defective fluorite to pyrochlore [70, 71]. 

The dihydride phase crystallizes in general in the fee fluorite-type structure, with an ideal composition of RH2. The frequently noted stoichiometric deficit, RH2-5, is caused by impurities and structural defects of various kinds such as surfaces and grain boundaries. Thus, 5 is smaller for bulk specimens than for powder and for specimens made of originally 4N material than for those from 3N it can reach values of up to 0.2, leading to pure dihydrides with a formula RHi.g. 

For zirconia doped with M rare earth elements (Me = Nd-Lu, Y, Sc), the conductivity of the fluorite-type solutions also has a maximum at 8 10 mol% MeO and decreases with the further increase in the dopant concentration because of the same defect associates [34]. 

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