Sesquioxide melting point

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... 

The melting-point of tungsten is higher than that of any other known metal. The metals are stable in air at ordinary temperatures, but when heated they exhibit a remarkable difference in behaviour. Uranium burns briskly at 170° C., producing uranous oxide UO2 molybdenum at a red heat yields the trioxide M0O3 chromium only burns at 2000° C. and forms the sesquioxide CrjOg tungsten is not oxidised at any temperature, except in the vapour form. 

The actinide sesquioxides have many similarities with the lanthanide sesquioxides, such as crystal structures (A, B and C forms), lattice parameters, etc., but there are also some significant differences. One notable difference is their melting points it appears that the transcurium sesquioxides have significantly lower (several hundreds of degrees C) melting points and display a different melting point trend with Z than for comparable lanthanide sesquioxides. The similarities and differences of the two f-series oxides are discussed in more detail in the following section. 

The hexagonal (A-type) of the sesquioxide (Baybarz 1973, Baybarz and Haire 1976) is very difficult to prepare, as it only exists in a narrow phase region between the monoclinic form and the liquid (melting point reported to be 1750°C). This crystal form of CfjOj has also been obtained from old hexagonal forms of BkjOj after about five half-lives (97% transformation) following the beta decay of Bk-249 oxide. The transformation of the cubic form to the monoclinic form of sesquioxide occurs between 1100- 1400°C and the transformation of the monoclinic form to the hexagonal form occurs at about 1700°C (Baybarz and Haire 1976). 

The sesquioxides of the lanthanide and actinides are multiphasic. Figure 19 is a plot of phase formation as a function of temperature and radius of the lanthanide sesquioxides, and is based on a published plot (Chikalla et al. 1973, Schulz 1976). Included in a section of the plot are the radii for the first six actinide sesquioxides (Pu forms the first sesquioxide in the series if Ac is excluded) placed above comparable radii of the lanthanides, as opposed to their positions in the periodic table. If the very high temperature phases of the lanthanides (e.g., X, H phases) and the melting-point behaviors are excluded, there is a reasonable agreement between the expected and observed actinide sesquioxide behaviors based on radii. The X, H phases as such have not been reported for the actinide sesquioxides and there is a discrepancy with the melting points the latter is discussed below. 

The actinide oxides have received intensive scrutiny because their refractory nature makes them suitable for use as ceramic fuel elements in nuclear reactors. UO2 melts at 3150 K, and Th02 has the highest melting point of any oxide, about 3465 K. The actinide oxides are complicated by deviations from stoichiometry, polymorphism, and intermediate phases. The sesquioxides are basic, the dioxides are much less basic, and UO3 is an acid in solid state reactions. The reactivity of these oxides depends greatly on their thermal history. If ignited, they are much more inert. Table XI contains some representative data on actinide oxides. 

The melting points of the sesquioxides are indicated in fig. 27.2. The temperature of solidification has been found by Coutures et al. (1975) to be anomalous for La203, Gd203 and LU2O3 defining clearly the ceric and yttric groups. 

Of the sesquioxides perhaps least is known about Ce203. Unless it is completely stoichiometric (and A-form) it can be pyrophoric hence preparation and protection from oxygen during study requires serious attention. Sata and Yoshimura (1968a, b) have prepared this material and have studied its chemical and physical properties including its melting point, thermal expansion, and electrical and magnetic properties. 

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. 

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