Phase diagram cerium

The applications and the limits (for instance in the case of cerium and in some cases regarding the formation of the 6 phase) of these concepts have been discussed by Gschneidner and Calderwood (1986) and high-pressure generalized phase diagrams have also been presented. 

Figure 21. Calculated Ce—O—S phase diagram at 1073 K. The circles and triangles correspond to conditions for which Ce202S and cerium oxide were experimentally determined stable. (Reprinted with permission from ref 175. Copyright 2000 Elsevier.)...

Figure 1. Cerium-iron phase diagram showing complete solubility in the liquid phase, zero solubility in the solid phases and one, single, low-melting point eutectic 92.5% Ce.

The principle of the method can be seen by referring to Fig. 23, which is a partial phase diagram of the cerium hydrogen system. Molten cerium metal is placed within a thermal gradient such that the lowest temperature is above the peritectic temperature (ca. 1010°C., from the diagram), and hydrogen is slowly dissolved in... 

Careful analyses of the thermodynamic data of the praseodymium and terbium oxides led to the construction of their RCL-O2 phase diagrams (Hyde et al., 1966 Hyde and Eyring, 1965). The cerium oxides were studied by means of X-ray powder diffraction (Bevan, 1955) and the CeOx-02 phase diagram was constructed from equilibrium reaction studies at oxygen pressures down to 10-24 atm and temperatures up to 1200 °C (Bevan and Kordis, 1964 Ricken et al., 1984). It is worth to notice that the phase diagrams of Ce0x-02, PrCb-Cb, and Tb0x-02 systems... 

Collected Phase Information for Cerium-Cadmium Alloys. A partial phase diagram for the cerium-cadmium system is presented in Figure 1. The room temperature results are based on x-ray studies by Iandelli and Ferro (9), who studied slowly cooled samples. However, their CeCd6 is shown as a dashed line because nuclear magnetic resonance studies by Jackson and the authors (10) have shown that the compound is unstable at room temperature and can decompose to metallic cadmium and CeCd 4i5. Because of probable kinetic barriers, the absence of compounds intermediate between cadmium and CeCd 6 does not indicate that these intermediates are unstable at room temperature. 

Our recent work on the bismuth-cerium molybdate catalyst system has shown that it can serve as a tractable model for the study of the solid state mechanism of selective olefin oxidation by multicomponent molybdate catalysts. Although compositionally and structurally quite simple compared to other multiphase molybdate catalyst systems, bismuth-cerium molybdate catalysts are extremely effective for the selective ammoxidation of propylene to acrylonitrile (16). In particular, we have found that the addition of cerium to bismuth molybdate significantly enhances its catalytic activity for the selective ammoxidation of propylene to acrylonitrile. Maximum catalytic activity was observed for specific compositions in the single phase and two phase regions of the phase diagram (17). These characteristics of this catalyst system afford the opportunity to understand the physical basis for synergies in multiphase catalysts. In addition to this previously published work, we also include some of our most recent results on the bismuth-cerium molybdate system. As such, the present account represents a summary of our interpretations of the data on this system. 

The maximum in catalytic activity observed for the multiphase region of the phase diagram necessarily arises from interactions between the separate phases. The bismuth rich and cerium rich solid solutions can readily form coherent interfaces at the phase boundaries due to the structural similarities between the two phases which can permit epitaxial nucleation and growth. A good lattice match exists between the [010] faces of the compounds, this match is displayed in Figure 6. We have also shown that regions of an [010] face of a Ce doped bismuth molybdate crystal resembles cerium molybdate compos tionally. This means that the interface between the two compounds need not have sharp composition gradients. It is structurally possible for the Bi-rich phase to possess a metal stiochiometry at the surface that matches that of the Ce-rich phase. 

Fig. 3-3 shows the phase diagrams for the cerium, praseodymium and terbium oxides. It reveals several phases in each of the systems with their compositions depending on temperature and oxygen partial pressure. The different phases have been named as i = MO1.714, C, = MOi.ysg, e = MOi.goo, 5 = MO1.8I8 and (3 = MO1.833. Electron diffraction patterns (see Fig. 3-21) revealed the unit cells of those phases and their relationship to the... 

Because the tripositive ions are the most stable for all the rare earth elements in almost all compounds, the thermochemistry of the solid (crystalline) rare earth sesquioxides dominates this chapter. Some rare earths have divalent or tetravalent states, so the chemistry of solid monoxides and dioxides are included. There are also many nonstoichiometric binary oxides of cerium, praseodymium, and terbium. As much as possible, the thermochemistry of these nonstoichiometric binary oxides is included. The stability, phase diagrams, and structures of ternary and polynary... 

Gschneidner Jr., K.A. and M.E. Verkade, 1974, Selected Cerium Phase Diagrams, Document lS-RlC-7, Rare Earth Information Center, Iowa State Univ., Ames, I A, USA. 

R. Vogel in Germany and G. Canneri in Italy were the principal early pioneers in determining phase diagrams of the rare earth metals. Vogel primarily concentrated on cerium systems - the first studied was the Ce-Sn system (Vogel 1911) - and... 

Among the LaAlOa RAIO3 systems, the most extensively studied are the systems with R = cerium and praseodymium, for which a full or partial phase diagrams were reported. Since the phase behaviour in those systems is for the most part determined by peculiarities of the CeAlOs and PrAlOs structures, they will be analysed in the corresponding sections devoted to CeAlOs- and PrAlOs-based solid solutions. 

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