Pyrochlore solid solutions

Another oxide family involving zirconium oxide has been examined as a possible alternative to YSZ as electrolytes in SOFCs. They are the pyrochlore solid solutions of the general formula M2Zr207, where M is a rare-earth metal [25-29]. Zirconium can be substituted by a transition metal such as titanium. Moon et al. [29] have in-... 

Rare earths are found in pyrochlores other than the simple R O7 formulation. A trivial example would be an Rli Ax M2O7 solid solution where A might be Bi, T1 or In. In fact, such solid solutions where A is Bi or T1 have been of some interest because Bi M O7 and Tl O7 pyrochlores are metallic when M is Ru or Ir, whereas analogous R2M2O7 pyrochlores are semiconducting (Sleight and Bouchard 1972). Pyrochlore solid solutions of the type R2 xBi Ti2 07 have been reported by Bamberger et al. (1985). 

Several workers have attempted to find the metal-insulator transition in Bi2 R Ru207 pyrochlore solid solutions. Spectroscopic studies (Cox et al. 1986) on the Bi2- cGd Ru207 system indicate a transition at about x = 1.55, and electrical resistivity data on the Bi2, jY Ru2 07 system suggest a very similar value (Kanno... 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

Ample studies on pyrochlore oxide electrolytes have been carried out, particularly on Gd2Ti207- and Gd2Zr207-based conductors, where the Gd2Ti. Zrx07 solid solution is of great interest because the x=0 member is an ionic insulator whereas the x = 1 end member is a good oxide ion conductor [96,97],... 

Oxygen (O2-) anion conductors - stabilized zir-conia, stabilized - bismuth oxide, - BIMEVOX, doped cerium dioxide, numerous perovskite-type - solid solutions derived from Ln(A)B (B") 03 (A = Ca, Sr, Ba B = Ga, Al, In B" = Mg, Ni, Co, Fe), La2Mo207 and its derivatives, pyrochlores based on Ln2Ti07. 

Doping of CrF, with Fe(III) or Mg(II), would be expected to result in the formation of solid solutions. However, depending on the Fe(III) concentration, there are two distinct regions, a concentration range from 0 to 41% Fe(III) with the pyrochlore structure and a second range from 65 to 100% Fe with the HTB structure [93], The surface areas of the latter samples are about double those of the former. At about 65% Fe(III) a maximum in the acidity of the samples was observed, acidity decreases with further increase of Fe(III). These phases however, exhibit only average catalytic activity. 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as tf-type Fe2 Ty03 and type Ni1 LyO. Thick film resistors are also made from transition-metal oxide solid solutions. Glass-bonded By 2 Pb2yRu207 having the pyrochlore [12174-36-6] structure is typical. 

The cubic pyrochlore-based ceria-zirconia solid solution is a good material. However, it has been reported that the reduction temperature has become higher after the high-temperature oxidation.This is due to the transformation from pyrochlore-based-cubic to r -tetragonal phase which is more stable than it, and r ... 

Recently one of the solutions to overcome this problem has been proposed.This does concern surface modification of the pyrochlore-based oxide.s. It is known that cerium and zirconium chlorides provide vapor phase complexes with aluminum chloride at elevated temperatures.The new surface modification technique utilizes the formation of these vapor complexes to remove and modify the top surface of the pyrochlore ceria-zirconia solid solution. This method is named "chemical filing". Application of the above complexes formation has already been demonstrated for the vapor phase extraction and mutual separation of rare earths based on the so-called chemical vapor transport (CVT). ... 

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. 

The most often found types of mixed oxides are perovskites (RBO3), K2NiF4-type oxides (R2BO4), R 1 B C>2 +1 ( = 2 or 3), lamellar perovskites, pyrochlores, spinels, and oxide solid solutions. Perovskite oxides are, by far, the most commonly used oxides. Therefore, this chapter will reflect this situation by putting more emphasis on this type of materials. Within this section the structure, preparation methods and general characteristics of the mixed oxides will be discussed. Note that R stands for rare-earth elements while A includes all types of elements. 

There are a few catalytic applications in which other mixed oxide structures are cited. Among them are pyrochlore compounds such as La2Zr207, spinels such as AB2O4, and lanthanum beta aluminates, all of them with well-defined crystalline structures. There are also a few examples of oxide solid solutions made up of a rare-earth oxide and a transition-metal oxide. 

Table 31.7 lists some other ferroelectric ceramics, although it does not include the large number of solid-solution phases that are ferroelectric. Many ferroelectric ceramics have a perovskite structure above 0c, but this is not a prerequisite. For example, LiNbOs has an ilmenite (FeTiOs) structure and Cd2Nb207 has a pyrochlore structure (the mineral pyrochlore is CaNaNb20eF). 

Another group of complex pyrochlores containing rare earth cations might be viewed as solid solutions between R Mf"O7 and A2 M 07 (table 4). 

The only R2V2O7 pyrochlores reported are for R = Tm, Yb and Lu (Shin-ike et al. 1977). These three compounds are ferromagnetic, as indicated in table 6. In addition, magnetic properties have been investigated in the solid solutions Lu2-,cY,cV2 07, Lu2- Sc V207, and R2V2 j,Mo,j07 (table 6). 

---