Protactinium dioxide

Protactinium(IV) oxydifluoride, PaOF2, is the only tetravalent protactinium oxydihalide not yet characterized. This seems to be due solely to a lack of investigation since there are no negative reports concerning the most likely routes involving interaction of the tetrafluoride and either protactinium dioxide or antimony sesquioxide, both of which methods have been employed for the preparation of the other tetravalent oxydihalides. 

The results of solid state reactions of protactinium dioxide and pentoxide with other metal oxides (89, 93-96) support the view that the oxide systems of protactinium resemble those of other actinide elements rather than those of niobium and tantalum. However, when assessing results of this type one must always bear in mind the relative ionic radii of the respective M " and M + ions since they obviously play a large part in determining the structures of the complex phases. This comment applies equally well, of course, to the structural properties of other types of compound and in particular to the high coordination numbers exhibited by protactinium(V) in its chloro and nitrato complexes. 

The light actinide metals (Th, Pa, and U) have extremely low vapor pressures. Their preparation via the vapor phase of the metal requires temperatures as high as 2375 K for U and 2775 K for Th and Pa. Therefore, uranium is more commonly prepared by calciothermic reduction of the tetrafluoride or dioxide (Section II,A). Thorium and protactinium metals on the gram scale can be prepared and refined by the van Arkel-De Boer process, which is described next. 

Quantitative methods of obtaining protactinium start from the carbonate precipitate from the treatment of the acid extract of certain uranium ores. After this carbonate precipitate is dissolved, the protactinium remains 111 the silica gel residue, from the solution of which it is obtained on a manganese dioxide carrier. An alternate method effects final separation... 

Protactinium exists naturally in the pentavalent state and although it is possible, by employing strongly reducing conditions, to obtain the tetravalent state in solution and in solid compounds this state is, in general, unstable in the presence of oxygen, exceptions being the tetra-fluoride and dioxide, which are stable in the atmosphere. The potential... 

Protactinium tetrachloride was first prepared (125) on the microgram scale by hydrogen reduction of the pentachloride at 800°C, a method since employed at 400°C for the preparation of 50-mg hatches (44 , 46). It has also been prepared (125) by reacting the dioxide with carbon tetrachloride vapor, hut since the pentachloride is much easier to prepare than the dioxide reduction of the former with aluminum at 400°C appears to he the most attractive route to the tetrachloride (44). 

The first reliable information on the protactinium-carbon system was reported by Lorenz and ScherfF who prepared the monocarbide, PaC, by carbothermic reduction of Pa20s. The dioxide, Pa02, is first obtained at approximately 1100°C and is then converted to PaC above 1900°C. In the presence of excess carbon there is some evidence for the formation of the tetragonal dicarbide. Protactinium monocarbide is isostructural with other actinide monocarbides possessing the fee NaCl-type of structure with Oo = 5.0608 A. In contrast to ThC and UC, however, it is stable in the atmosphere and is relatively inert toward acid solutions. By measuring the CO equilibrium pressures for the reaction,... 

Protactinium oxides. Protactinium forms the fee dioxide PaOj, which, like UOj, adds additional oxygen to form the hyperstoichlometric Pa02+ j. TTie most stable oxide is the pentavalent PajOs, which exists in five different crystalline forms. The pentoxide results from heating any... 

The thermodynamic behavior of protactinium oxides is not as well established as for the Th02 system. The free energy of formation of Pa02(s) has been derived from carbothermic reductions. The entropy of formation of the dioxide is comparable to the other actinide dioxides (see table 27 data from Ackermann and Chandrasekharaiah (1974)]. A partial study of the reduction of Pa205 has also provided data for the protactinium oxide system (Kleinschmidt and Ward 1986). 

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