Actinide binary oxides

In actinide binary compounds an equation of state can also be developed on the same lines. The difference in electronegativity of the actinide and the non-actinide element plays an important role, determining the degree of mixing between the actinide orbitals (5 f and 6 d) and the orbitals of the ligand. A mixture of metallic, ionic and covalent bond is then encountered. In the chapter, two classes of actinide compounds are reviewed NaCl-structure pnictides or chalcogenides, and oxides. 

Americium oxides. Keller [K2, K3] reports three stoichiometric binary oxides of americium AmO, Amj O3, Am02. The dioxide Am02 is the most stable of the americium oxides. It crystallizes with the cubic fluorite structure of aU the actinide dioxides. It can be formed as a dark brown powder, stable up to 1000°C, by heating trivalent americium nitrate, hydroxide, or oxalate in oxygen to 700 to 800 C. Americium dioxide is readily soluble in mineral acids. Hydrogen reduction of the dioxide yields Am2 O3. 

Comparable recent detailed reviews of the actinide halides could not be found. The structures of actinide fluorides, both binary fluorides and combinations of these with main-group elements with emphasis on lattice parameters and coordination poly-hedra, were reviewed by Penneman et al. (1973). The chemical thermodynamics of actinide binary halides, oxide halides, and alkali-metal mixed salts were reviewed by Fuger et al. (1983), and while the preparation of high-purity actinide metals and compounds was discussed by Muller and Spirlet (1985), actinide-halide compounds were hardly mentioned. Raman and absorption spectroscopy of actinide tri- and tetrahalides are discussed in a review by Wilmarth and Peterson (1991). Actinide halides, reviewed by element, are considered in detail in the two volume treatise by Katzet al. (1986). The thermochemical and oxidation-reduction properties of lanthanides and actinides are discussed elsewhere in this volume [in the chapter by Morss (ch. 122)]. 

Comparison of the binary oxides of the lanthanide and actinide elements... 

As stated in the opening section of this chapter, the objective is to discuss and compare the solid-state chemistry and physics of the lanthanide and actinide element oxides. The topics of discussion have been limited to binary oxides of these elements. Therefore, a discussion of the many complex (ternary, mixed, etc.) oxide systems for these f elements, and oxides of actinides representing oxidation states above four that do not have lanthanide counterparts, are not present. 

In section 3, a one-on-one comparison of behavior is given for the 4f- and the 5f-element binary oxides. Although there are some significant differences between the chemistry and the properties of the lanthanide and actinide series, most notably being in the chemistry of the earlier actinides, Pa-Pu, these f-element oxides also have many similarities. In some aspects these f-block elements have many unique features with respect to the other elements in the periodic table. Thus, a comparison of the trends and differences in the chemistry of the oxides of these two f series provides valuable insights into their position in the periodic table. 

All of the actinides from Th through Cf form dioxides but several of these have not been studied thermodynamically, due in part to their instability and to limited availability (e.g., it is very difficult to prepare multi-milligrams of Cf02 even though such quantities of the isotope are available). Plots of enthalpy of solution for the f elements have been established (Morss 1986) which permit estimating values for the other actinide dioxides. Although binary oxides above the dioxide stoichiometry are known for some of the actinides (Pa, U, Np), little thermodynamic data are available for these oxides. 

The objective of this section of the chapter is to compare the properties and behaviors of the binary oxides of the lanthanide and actinide elements. The trends and the differences between the binary oxides of each series of elements are reviewed but a discussion of the more complex (e.g., ternary or larger) oxides that these elements are known to form is excluded. Essentially this section offers a comparison of the monoxides, sesquioxides, dioxides and binary oxides with 0/M ratios intermediate to those found in these three oxides. Since the lanthanide elements do not form oxides with higher O/M ratios than 2.0, actinide oxides with higher oxygen stoichiometries are not discussed in this section. 

The wide variety of oxidation states known for the actinides is reflected in the stoichiometry of their binary oxides however, the highest attainable oxidation state may not be observed. The largest 0/M ratio for an /-element binary oxide is achieved in UO3. 

The sesquioxide is known for actinium and all the actinides from plutonium through einsteinium and is probably the highest binary oxide that could be formed for the heaviest actinides with nobelium as an exception, which may only form a solid monoxide. Oxides of the heaviest actinides beyond einsteinium have not been prepared or studied experimentally. The sesquioxides of Pu, Am, and Bk are readily oxidized to their dioxides, whereas those of Cm, Cf, and Es are resistant to air oxidation. 

Although a number of binary uranium oxides with 0/U > 2.00 are known, along with many of their thermodynamic properties, there are no comparable properties of heavier actinide oxides with which to compare them. (The only transuranium binary oxide with O/An > 2.00 is NpjOs and none of its thermodynamic properties have been measured.) To compare thermodynamic properties of actinide oxides in high oxidation states, scientists have studied complex oxides. 

In addition to the binary oxides of the actinides there are many compounds that are made by reaction of these with oxides of alkali metals, transition metals, alkaline earths, and some other elements. Comprehensive surveys of these by Keller [38] and Morss [540] provide lists of known compounds and organize the available crystallographic data to show the isostructural series that exist. They... 

Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 8 (13,17,18,22). The binary compounds with carbon, boron, nitrogen, siUcon, and sulfur are not included these are of interest, however, because of their stabiUty at high temperatures. A large number of ternary compounds, including numerous oxyhaUdes, and more compHcated compounds have been synthesized and characterized. These include many intermediate (nonstoichiometric) oxides, and besides the nitrates, sulfates, peroxides, and carbonates, compounds such as phosphates, arsenates, cyanides, cyanates, thiocyanates, selenocyanates, sulfites, selenates, selenites, teUurates, tellurites, selenides, and teUurides. 

Synthesis in liquidAl Al as a reactive solvent Several intermetallic alu-minides have been prepared from liquid aluminium very often the separation of the compounds may be achieved through the dissolution of Al which dissolves readily in several non-oxidizing acids (for instance HC1). For a review on the reactions carried out in liquid aluminium and on several compounds prepared, see Kanatzidis et al. (2005) binary compounds are listed (Re-Al, Co-Al, Ir-Al) as well as ternary phases (lanthanide and actinide-transition metal aluminides). Examples of quaternary compounds (alumino-silicides, alumino-germanides of lanthanides and transition metals) have also been described. As an example, a few preparative details of specific compounds are reported in the following. 

The succeeding actinides (Cm, Bk, Cf, Es, Em, Md, No, Lr) mark the point where the list of isolated compounds tends to involve binary compounds (oxides, halides and halide complexes, chalcogenides, and pnictides) rather than complexes. Those studies of complexes that have been made are usually carried out in solution and, from Em, onwards, have been tracer studies. 

The immense number of chemical compounds formed by the halogens provides chemists with an extraordinary database from which numerous chemical and physical phenomena can be correlated with respect to various periodic trends. From databases like Inorganic Crystal Structure Data (ICSD, http //www.fiz-karlsruhe.de ) and International Centre for Diffraction Data (ICDD, http //www.icdd.com) with 67 000 and 25 000 entries, respectively, one can easily make out that halides are one of the dominant classes of compounds besides oxides. Even within the subset of inorganic solids, there is tremendous diversity of composition, stracture, and properties and to summarize this would create its own encyclopedia. Therefore, the discussion in this article is limited primarily to binary halides, their structures, and some of their properties, except halides of elements which are nonmetals. Binary actinide hahdes are discnssed elsewhere see Actinides Inorganic Coordination Chemistry). Complex hahdes (sohd phases containing two or more kinds of metal ions), ... 

Eyring L., The Binary Lanthanide Oxides Synthesis and Identification, in Synthesis of Lanthanide and Actinide Compounds, eds, Morss L.R. and Meyer G. (Kluwer, Dordrecht 1991), 187-224. 

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