Other ternary oxides

In this section the results on high-spin iron(II) systems are presented before those on iron(III). The latter dominate, and are ordered approximately as follows the major structural classes of Fe203/M203 solid solutions, MFe03 perovskites, MFe03 orthoferrites, M3FesOi2 garnets and other iron(III) oxides approximately in the periodic table classification of the second metal. Any quaternary oxides are included with the most appropriate ternary system. 

The quadrupole splitting of the Fe + ion in the ternary oxide FeSb204 decreases from 2-88 mm s at 50 K to 1-02 mm s at 817 K as a result of thermal population of the tzg levels [97]. The iron is in a site of 6-coordination with three mutually perpendicular twofold axes, and it is possible to describe the three lowest states as yz) , xzy, and primarily although the 

FeTi20s is derived from FeaTiOg and is a ferrous oxide. The cation distribution is not known, but the broad MQssbauer spectrum suggests that it may be partly inverse [20]. 

Early data on a sample of Alo.46Fej.54O3 showed it to be magnetically ordered and to give a room-temperature Mossbaiier spectrum comparable 

0-6 X 1-0. AlFe03 itself gives a magnetically ordered spin system at 

Goodenough and co-workers [10] made a detailed study of the solid state chemistry and electrochemistry of ruthenates of general formula Bi(2 2x)P-b2xRu20(7. v) with the pyrochlore structure and reported that the electroreduction of oxygen proceeds at low overpotentials according to the electrokinetic equation 

The kinetics of eqn. (85) should be independent of the electrode surface it it proceeds by an outer sphere mechanism. 

---

Other Ternary Copper Oxides Two other ternary oxides of copper(II) are worthy of mention. These are BaCu02 and the Ln2Cu04 series in which Ln = Nd, Sm, Eu, and Gd. The former compound, BaCu02, was first reported in 1975 by Arjomand and Machin (120), and they briefly reported on its powder pattern and magnetic properties. Further studies on BaCu02 were carried out the... 

Table 10. Other ternary oxides with the general formula, AB03, that have been examined from a water photoelectrolysis perspective.

In closing this Section, a variety of other ternary oxides (besides the SrTi03 prototype) have been examined over the years. Table 10 contains a representative listing of these compounds. 

The following subdivisions will be considered (i) binary oxides (ii) spinel oxides (iii) other ternary oxides (iv) iron(IV) oxides (v) chalcogenides (vi) silicate minerals and (vii) lunar samples. 

Because the high dielectric responses in the tetragonal phase of BaTiOs are caused by a deformation of the TiOe coordination polyhedron and consequently the creation of a permanent electric dipole, it is instmctive to consider from this viewpoint the dielectric properties in other ternary oxides. After the size effect in BaTiOa was clarified, works on the synthesis of materials with high e were focused on obtaining compounds with the maximum distortion in the crystal... 

The other ternary oxides also occur as spinel superstructures such as CaFe204 (ferrite) or potassium iron oxides which contain the spinel structure as a building block. These complex structures of the beta alumina type consist of the formal composition (K2O x 11 Fc203) and can be described as a layer structure with unit cells of spinel linked by layers of potassium ions octahedrally coordinated to the (4 -h 4) oxygen ions of two adjacent spinel units. 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

It is interesting to note that the large difference in covalency parameters for the Mn2+—0 bond in MnO Nathans et al., 1964) vs the Mn2+—O bond in MnCC>3 (Lindgard and Marshall, 1969) agrees well with the spin transfer anticipated from Rv and R. This covalency difference for an M—0 bond in ternary oxides M Y Oz, which is dependent on the nature of the Y—0 bonds [in this case (Mn—O)—Mn for MnO and (Mn—0)-C for MnCOs], was also shown to exist for other oxides (see Table 4). 

Structural Imperfections. In many respects HREM has had a greater impact upon our knowledge of the nature of the atomic reorganization at crystalline imperfections than any other single technique. One of the very first contributions of HREM as a new analytical and structural tool was described in the paper by Iijimia (42) in 1971 on 2 10 29 v -ewe< down to its b - axis. Structural faults, arising from subtle fluctuations in composition, could be clearly seen in the block-structure (based on NbO octahedra) which is a feature of this ternary oxide system. More than a decade later similar materials are yielding to active scrutiny by HREM, and Horiuchi (43), for example, has shown how point defects may be directly viewed... 

Although periodic trends in enthalpies of formation are often striking, these trends can in general not be used to estimate accurate data for compounds where experimental data are not available. Other schemes are frequently used and these estimates are often based on atomic size and electronegativity-related arguments. As an example, the enthalpy of formation of a ternary oxide from the binary constituent oxides, i.e. the enthalpy of a reaction like... 

The structures of ternary oxides such as spinels, perovskites, pyrochlores, layered cuprates (high-7 c superconductors), and other lamellar oxides are fascinating subjects by themselves and are beyond the scope of the present discussion. 

Another example of 2-dimensional infinite linking of octahedra, sharing edges in the one and comers in the other dimension, has recently been found in the ternary oxide CalrOa 262). No examples of isostructural fluorides are known, however. 

Research studies on other binary and ternary oxide systems were quite limited during the next 20 years. In a 1952 comprehensive review, entitled "A Search for New Superconducting Compounds", B. 

The only recorded hydroxide, Pu(0H)3 jcH20, rapidly oxidizes to a plutonium(IV) species, but two forms of Pu203 are known, one with the hexagonal La203 structure and the other cubic in these the Pu3+ is seven- and six-coordinate respectively. Ternary oxides, such as PuA103, are also known. 

In other cases, however, and in particular when sublattices are occupied by rather immobile components, the point defect concentrations may not be in local equilibrium during transport and reaction. For example, in ternary oxide solutions, component transport (at high temperatures) occurs almost exclusively in the cation sublattices. It is mediated by the predominant point defects, which are cation vacancies. The nearly perfect oxygen sublattice, by contrast, serves as a rigid matrix. These oxides can thus be regarded as models for closed or partially closed systems. These characteristic features make an AO-BO (or rather A, O-B, a 0) interdiffusion experiment a critical test for possible deviations from local point defect equilibrium. We therefore develop the concept and quantitative analysis using this inhomogeneous model solid solution. 

---