Rare earth oxide systems

Ordered Phases and Nonstoichiometry in the Rare Earth Oxide System... 

Stable phases in the rare earth oxide systems are tabulated and discussed. New data on the structure of sesquioxides quenched from the melt are reported. The structural interrelations between the A, B, and C type sesquioxides and the fiuorite dioxides are pointed out. The sequences of several intermediate oxides in the CeO, PrO., and TbO, systems are observed to be related to the fluorite structure and the C form sesquioxide with respect to the metal atom positions. A hypothetical homologous series of the general formula Mn02n i, related to the fluorite structure and the A form sesquioxide with a more or less fixed oxygen lattice, is suggested. 

Table I. Some Properties of Stable Phases in Rare Earth Oxide Systems...

Lower Oxides. The question of the incorporation of oxygen into the rare earth metal lattice and the extent of oxide formation between the metal and the sesquioxide has not been systematically studied. The lower oxides reported in Table I were prepared by distilling the rare earth metals in a system of low oxygen partial pressure (9). This is one of the great unexplored regions of the rare earth oxide systems. 

Nonstoichiometry in Rare Earth Oxide Systems. As more work is done on the intermediate oxides at low temperatures, regions earlier thought to be non-stoichiometric are resolved into phases of narrow composition limit. It is possible that for carefully annealed specimens the entire range of composition will be resolved into definite compounds of narrow composition limits separated by two-phase regions. This, of course, presumes that equilibrium can be achieved. 

Theoretical calculations on the rare earth oxide systems using an intermediate coupling scheme show good agreement with experimentally observed spectra [18,19]. 

A complete isobar at 159 mm. of Hg shows the normal breaks at CmOi.8 and CmOi.7 in the reduction half of the cycle but exhibits an extreme hysteresis loop in the oxidation part of the cycle which did not close until a composition near Cm02 was reached. This behavior, to a lesser degree, is exhibited by the rare earth oxide systems discussed above. 

Sohd solubility in rare earth oxide systems has been studied extensively. There is no doubt of complete solubility, often called isomorphous replacement between oxide phases of the same symmetry as in mixtures of Pr02 and Ce02 or in the sesquioxides of one type. In these cases Vegards law appears to hold precisely (40). 

Z.-K. Huang, T.-Y. Tien, and T.-S. Yen, Subsolidus phase relationships in Si3N4-AlN-rare-earth oxide systems. J. Am. Ceram. Soc. 69 (10), C241-242 (1986). 

In [138] a number of siliea- and alumina-supported Ln203 samples are investigated by means of TPD of ehemisorbed CO2 and pyridine, FTIR of adsorbed pyridine, as well as eatalytie assays of a-pinene isomerization and 2-butanol decomposition. Particular attention is paid to the supported ytterbia systems. In good agreement with the results commented on above, supported rare earth oxide systems... 

Yahiro, H., Eguchi, K., and Aral, H. Electrical properties and reducibihties of ceiia-rare earth oxide systems and their apphcation to solid oxide fuel cell. Solid State Ionics 1989, 36, 71-75. 

On Rare-Earth Oxide Systems, L. Eyring. In J.F. Nachman and C.E. Lundin (Eds.), Rare Earth Research, Gordon and Breach, New York, pp. 339-354,1962. 

Ordered Phases and Non-Stoichometry in the Rare Earth Oxide Systems, L. Eyring and B. Holmberg. In Advances in Chemistry Series, No. 39, American Chemical Society, Washington, DC, pp. 46-57, 1963. 

A Thermodynamic Model of Hysteresis in Phase Transitions and its Application to Rare Earth Oxide Systems, D.R. Knittel, S.P. Pack, S.H. Lin and L. Eyring, J. Chem. Phys., 67, 134 (1977). 

Eyring, L. and Bo Holmberg, 1963, Ordered Phases and Nonstoichiometry in the Rare Earth Oxide Systems, in Gould, R.F. ed.. Advances in Chemistry Series 39 (Amer. 

Although perovskite phases as such involving rare earth oxides are dealt with separately in this volume (ch. 29) there are two important series of compounds based on the perovskite structure which occur in mixed rare earth oxide systems and are treated here. 

Bevan, D.J.M., W.W. Barker, R.L. Martin and T.C. Parks, 1965, Mixed Oxides of the Type M02(Fluorite)-M203, Part 2, Non-stoichiometry in ternary rare-earth oxide systems, in Eyring, L., ed., Proc. of the Fourth Conference on Rare Earth Research, (Gordon and Breach, New York), pp. 441-468. 

Another characteristic of the solvent extraction system is the high solute concentration in both aqueous and organic phases, which influences greatly the size of the required installation. Concentrations of rare-earth oxides (REO) higher than 100 g/L are often reached in both phases. The process therefore requires only relatively small equipment. 

IR spectrometers have the same components as UY/visible, except the materials need to be specially selected for their transmission properties in the IR (e.g., NaCl prisms for the monochromators). The radiation source is simply an inert substance heated to about 1500 °C (e.g., the Nernst glower, which uses a cylinder composed of rare earth oxides). Detection is usually by a thermal detector, such as a simple thermocouple, or some similar device. Two-beam system instruments often work on the null principle, in which the power of the reference beam is mechanically attenuated by the gradual insertion of a wedge-shaped absorber inserted into the beam, until it matches the power in the sample beam. In a simple ( flatbed ) system with a chart recorder, the movement of the mechanical attenuator is directly linked to the chart recorder. The output spectrum is essentially a record of the degree of... 

Alumina, alkaline-earth oxides, mixed oxides (spinels), rare-earth oxides, and lanthanide ores are known additives capable of sorbing S-impurities. The properties of these materials can be manipulated to produce catalysts capable of reducing up to -80% S-emissions and meet the refiner needs. It is, however, unlikely that these systems will be capable of satisfying the more stringent environmental S-emission standards expected in the future. Details of the reaction mechanism by which additives and promoters catalyze the oxidative sorption of S-impurities and details of catalyst deactivation have not yet been proposed. This work could provide useful information to help design more efficient S-transfer catalysts. The catalytic control of S-emissions from FCC units has been described in detail in two papers appearing in this volume (46,47) and in the references given (59). 

Insoluble silica residues are removed by filtration. The solution now contains beryllium, iron, yttrium, and the rare earths. The solution is treated with oxalic acid to precipitate yttrium and the rare earths. The precipitate is calcined at 800°C to form rare earth oxides. The oxide mixture is dissolved in an acid from which yttrium and the rare earths are separated by the ion-exchange as above. Caustic fusion may be carried out instead of acid digestion to open the ore. Under this condition sihca converts to sodium sihcate and is leached with water. The insoluble residue containing rare earths and yttrium is dissolved in an acid. The acid solution is fed to an ion exchange system for separating thuhum from other rare earths. 

Oxides or salts may play the part of promoters or additives in a heterogeneous catalyst. Their function in various catalyst systems can vary widely and is too complicated to have been adequately elucidated so far. However, we have found that they often operate as surface modifiers. For example, adding a small amount of rare earth oxide such as La203 to the methanation catalyst Ni/y-Al203 can significantly increase its activity and thermal stability (34). 

Fig. 8. The electrical conductivities of binary rare-earth oxide fluorides, Ln2Ln 203F6 produced from Nd203-LnF3 and Y203-LnF3 systems. The electrical conductivity was measured at 650°C under an oxygen partial pressure of 3.3 x lO 1 Pa.

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