Sesquioxides phase diagrams

As Indicated previously, the tentative phase diagram of the Pu-0 system shows plutonium sesquioxide as a line compound from room temperature up to its melting point. During the course of the preparation of for low temperature heat capacity... 

In accordance with the well known phase diagram for the rare earth sesquioxides [6], as much as five different structural varieties have been identified for them. They are referred to as A, B, C, H, and X types. A theoretical analysis of the equilibrium crystal lattice dimensions for A, B, and C structures in Ln203 has also been recently reported [29]. Three of the polymorphs above, the hexagonal, A-type, monoclinic, B-type, and cubic, C-type, are known to occur at room temperature, and atmospheric pressure, whereas H and X forms have only been observed at temperatures above 2273 K [6]. For the lighter members of the series. La through Nd, though not exclusively [6,30], the hexagonal, A-type, form is the most usually found, Fig. 2-1. By contrast, the heaviest lanthanoid sesquioxides, from... 

The phase diagram for all the lanthanoid sesquioxides is given in Fig. 3-17 and shows the different transformation temperatures. From the diagram it is obvious that promethium, Pm, was the first metal for which the existence of all three polymorphs and the transition temperatures could be established ]39]. For praseodymium, Pr, under atmospheric pressure the C-type structm-e is formed above 500 °C and transforms to A-type above 700 °G. Depending on the ionic radii only for the rare earth metals from Pm to Gd all five polymorphs will be formed. With increasing temperature the order of transition is C B—>A. 

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... 

Fig. 19. Phase diagram of the lanthanide and actinide sesquioxides as a function of ionic radius.

Figure 8.23A shows a simplified Eh-pH diagram for the Mn-O-H system. Within the stability field of water, manganese occurs in three valence states (2+, 3 +, and 4+). Figure 8.23A shows the condensed phases relative to the three valence states as the hydroxide pyrochroite Mn(OH)2 (2+), multiple oxide haus-mannite Mu304 (2+, 3 + ), sesquioxide Mu203 (3 + ), and oxide pyrolusite Mn02 (4+). 

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