Uranyl selenates

Uranyl Selenate, UO2Se04.xH2O, is present in the glassy mass obtained by evaporating a solution of uranyl hydroxide in selenic acid. It has not been obtained in a pure state. Such solutions also yield acid salts, of which the following have been isolated ... 

Selenates, selenites, and tellurates. Uranyl selenates and selenites have been prepared and can be expected to have the same coordination features as the sulfates and sulfites. In addition to the selenate, U02Se04, the ternary selenite (U02)2(OH)2(Se03), has been reported to form in... 

The remarkable class of ID stmctural units with comer-sharing actinyl polyhedra and TO4 tetrahedra are uranyl selenate nanotubules that have recently been described by Krivovichev et al. [173, 174]. They will be considered in detail in Chapter 11. 

Fig. 50. Uranyl selenate-selenite sheet in the structure of [C5Hi4N]4[(U02)3(Se04)4(HSe03)(H20)](H2Se03)(HSe04).

From all graphs shown in Fig. 43, the 15/8d graph is remarkable in the presence of 16-membered rings. The uranyl selenate sheet having this topology is shown in Fig. 52a. It has been observed in the structure of (H30)6[(U02)5(Se04)8(H20)5](H20)5 [215]. This uranyl selenate sheet can be simply obtained by connection of the cl/2a chains by additional 4-connected black vertex. 

The only known example of uranyl selenate framework straeture, (H30)2[(U02)(Se04)3(H20)](H20)4, based upon 3D framework of U and Se polyhedra is shown in Fig. 57a.A description based upon polyhedra fails to capture the details of the framework. By application of the blaek-and-white graph theory technique, the arehitectural principles become transparent (Fig. 57b). The uranyl selenate framework is built up by linkage of the U Se = 2 3 chains of uranyl bipyramids and selenate tetrahedra whose graph is depicted in Fig. 57c. The graph consists solely of 4-membered rings of blaek and white vertices. 

Fig. 3. The crystal structure of 1 projected along the a axis (a), tubular graph describing topology of the uranyl selenate nanotubule in 1 (b) and its unfolded version (c)...

The most intriguing feature of the stmctures of compounds 1 and 2 is that they contain isolated porous nanotubules formed by comer sharing of pentagonal bipyramids and Se04 tetrahedra (Fig. 3a). Such nanometer-scale tubules formed by two types of coordination polyhedra are new in the realm of inorganic oxosalts probably, the most closely related but yet distinct are elliptic porous nanorods in the stmcture of yuksporite, a natural material from the Kola peninsula, Russia [17], The uranyl selenate nanotubules in the stmctures of 1 and 2 have circular cross-sections with outer diameters of 17 and 25 A (= 1.7 and 2.5 nm), respectively. The crystallographic free diameters of the tubules are 4.7 and 12.7 A for 1 and 2, respectively. 

The mechanism of formation of uranyl selenate tubules in aqueous media is probably controlled for compound 2 by the presence of protonated butylamine (C4Hi2N) cations. In this regard, it can be similar to the process that leads to the formation of highly undulated uranyl selenate sheets in the structure of (H30)2[Ci2H3oN2]3[(U02)4(Se04)s](H20)5 (see below). The flexible uranyl selenate complexes crystallize around cylindrical micelles to produce a tubular inorganic substructure. 

Table 2. Crystallographic data for nano structured uranyl selenates...

Fig. 6. Uranyl selenate units observed in the structures of organic-inorganic nanocomposite compounds (a)LUS-l (b)LUS-2 andLUS-4 (c)LUS-3 andLUS-5. Below polyhedral diagrams black-and-white graphs describing topology of structural units are given.

Fig. 8. Structure of LUS-5 projected along two mutually perpendicular directions along the uranyl selenate sheet (a, b) and schematic representation of its structural principle.

Actinide peroxide nanospheres [3] and uranyl selenate nanotubules [4,5] represent the first examples of nanoscale stmctures in actinide compounds. It is noteworthy that, very frequently, formation of nanostmctures corresponds to organic-inorganic composites with clearly defined interfacial interactions. To describe the character of these interactions, one can use the charge-density matching concept with some modifications related to the basic chemical properties of actinides. 

A totally new group of minerals, all of which have been described since Frondel, comprises the uranyl selenates and tellurates. Although other selenate and tellurate minerals have been reported as occurring in many types of uranium deposits, the uranyl compounds have only been recognized at two rather different localities. A Au-Te deposit near Moctezuma, Mexico, has yielded moctezumite, cliffordite and schmit-terite and the Musonoi Cu-Co deposit at Katanga, Zaire, has produced the other minerals. Schmitterite has also been found at the Shinkolobwe uranium deposit in Katanga. All the minerals are secondary. 

---