Uranyl vanadates

Uranyl-vanadates, which are probably derived from uranyl-vanadic acid,... 

Uranyl-vanadates.—By adding uranic anhydride to fused potassium or sodium metavanadate, microscopic, rectangular, fluorescent plates of the compositions K(U02)V04 and Na(U02)V04 are obtained. These are probably derivatives of a relatively stable uranyl-... 

Figure 1 The francevillite anion topology (a) and the uranyl vanadate layers in camotite-type compounds (b) layer P in hT-, or 1,3-diaminopropane- containing compounds (c) layer P in ethylenediamine- and 1,4-dimithylpiperazine- containing compounds.

Table 1. Minerals and synthetic uranyl vanadates containing camotite-type layers...

The M2[(U02)2V208]-xH20 compounds with M being a monovalent cation are monoclinic. The uranyl vanadate layer is parallel to (100). Adjacent P layers are related by the a translation, resulting in the simple PPPP sequence (Fig. 2a). For the anhydrous species that have been prepared by solid state reactions of UsOs, V2O5 and M2CO3 at 600°C [22] the inter-layer distance asinP varies linearly with the ionic radii of the 6-coordinate ions (Fig. 3) [27]. The introduction of water molecules into the interlayer space is accompanied by an increase of the interlayer distance (Table 1). 

All divalent uranyl vanadates of the camotite group are orthorhombic, with the exception of the Cu compound. These compounds possess a second layer, labeled b, obtained from P by a (100) mirror plane. P and b layers alternate to yield a PbPb sequence (Fig. 2b). 

Uranyl vanadates with 2-D uranophane-type sheets +... 

Occupation of the triangles of the uranophane anion topology results in the formation of the uranophane-type sheet (Fig. 7b). Layered compounds containing uranyl vanadate anionic sheets of this type have recently been obtained with diamines, piperazine and l,4-diazabicyclo[2,2,2]octane dabco), as counter cations in the interlayer space (Fig. 7c) [31]. 

Table 3. Crystallographic parameters of the 2-D and 3-D uranyl vanadates built from 2-D layers with the uranophane anion topology. The unit ceU parameter corresponding to the J [ U]e chains (U-7.09A) is shown in bold letters, the second parameter for the layer is in itahc.

Figure 7 The uranophane sheet anion topology (a) and the uranophane-type sheet (b) found in some diamine-templated layered uranyl vanadates (c), in structure of (U02)3(V0 )2.5H20 where layers are connected by UO7 pentagonal bipyramids (d), in diamine-templated 3-D uranyl vanadates (e), in uranyl divanadate U2V2O11 (f). The occupation of the sqnares of the uranophane anion topology by the VO5 entities results in 2-D layers (g) that are linked into a 3-D framework inUVOj.

Uranyl vanadates with 3-D framework based on uranophane-type sheet ... 

In diamine-templated compounds such as diaminobutane- (H2dbu) and diaminopropane-(H2dpr) templated uranyl vanadates, the aa/aa isomer allows sharing of two opposite equatorial edges of the interlayer UO7 bipyramids with four VO4 tetrahedra of the adjacent layers, so one interlayer uranyl ion is linked to four vanadate tetrahedra (Fig. 7e). Thus, the stmctural notation of these... 

There are no alkali metal uranyl vanadates built of this type of sheet. Chemically similar compounds, UVOe, adopt the camotite-t5 e stmcture described above which is characterized by a high stability. It is of interest that the stmcture of KUNbOe [50] is built from the uranophane-type sheets interleaved with ions, whereas CsUNbOg adopts the camotite-t5 e stmcture [51]. 

Table 6. Uranyl vanadates based on 2-D network of the VO5 square pyramids and isolated UOg bipyramids.

In the first part of this chapter, we described two hydrated copper uranyl vanadates, Cu[(U02)2V20s]-41120 and Cu2[(U02)2V20s](0H)2-6H20, that have structures based upon camotite-type sheets interleaved with the... 

Cu(H20)4] and [Cu2(0H)2(H20)4] cations, respectively. Using solid-state synthesis methods, an anhydrous copper uranyl vanadate, Cu2(U02)(V04)2, has... 

Figure 19 shows Arrhenius plots of conductivity vs. 10 /T for several uranyl vanadates. As clearly shown, the camotite type compounds with 2-D networks exhibit relatively low conductivity with a high activation energy. In contrast, the 3-D M(U02)4(V04)3 compounds with partial occupancies of the alkali metal M sites in perpendicular channels, exhibit high conductivity with a low activation... 

Figure 19 log o vs. 10 /T Arrhenius plots of some uranyl vanadates with 2-D and 3-D structural arrangements showing the high conductivity observed for the 2-D compound CSUV3O11 and the 3-DA/(U02)4(V04)3 compounds. 

The main structural characteristics of uranyl vanadates reported so far are listed in Table 7. With the exception of Cu2[(U02)2V208](0H)2-6H20, where the role of the I [(CUO2] chains can not be excluded from consideration, all the arrangements of uranium and vanadium polyhedra are either 2-D layers or 3-D frameworks. It appears that, for U V > 1, structural organization is governed by... 

Table 7. Structural characteristics of the uranyl vanadate families.

Carnotite is a potassium uranyl vanadate for which the formula K20 2U03 V206 8H20 is frequently given. The ore, however, varies materially from this composition. It contains 19 or 20 per cent V206 and is valuable for both vanadium and uranium, as well as radium. It occurs extensively in Paradox Valley, Colorado, and in Utah, South Australia, Portugal, etc. 

Camotite is a potassium uranyl vanadate, K2O.2UO3.V2O5.11H2O. Since the uranium is in the fully oxidized condition, leaching can usually be carried out without an oxidant, at a particle size of about 100 mesh. The vanadium dissolves on leaching, to sodium vanadate, and the uranium forms a soluble double carbonate. The reaction may be represented approximately as ... 

From 600°C to 8S0°C sodium vanadates are formed, and these then react with uranium compounds to form sodium uranyl vanadates which are readily soluble. 

Above 8S0°C sodium vanadates and sodium uranyl vanadates decompose, or react with silica to produce insoluble silicates. 

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