Vacancies uranium oxides

The lattice parameters of nonstoichiometric uranium oxides, quenched from 1100° C., were determined within the composition range U02 to U4Og. Two separate linear relations for the lattice parameter as a function of oxygen content were obtained one characteristic of U02+2. and the other of U409 J/. The two functions are a0 = 5.4705 - 0.094 x (0 < x < 0.125) and a0 = 5.4423 + 0.029 y (0 < y < 0.31). Helium displacement densities were determined for some samples the values obtained are consistent with an oxygen interstitial model for U02+a. and an oxygen vacancy model for U409 y ... 

The ionic defects characteristic of the fluorite lattice are interstitial anions and anion vacancies, and the actinide dioxides provide examples. Thermodynamic data for the uranium oxides show wide ranges of nonstoichiometry at high temperatures and the formation of ordered compounds at low temperatures. Analogous ordered structures are found in the Pa-O system, but not in the Np-O or Pu-O systems. Nonstoichiometric compounds exist between Pu02 and Pu016 at high temperatures, but no intermediate compounds exist at room temperature. The interaction of defects with each other and with metallic ions in the lattice is discussed. 

The ions in the tetrahedral sites have been shown by Mossbauer spectroscopy to have an oxidation state of +3. In a way similar to the uranium oxide structure, the distance between the closest octahedral holes and the tetrahedral hole is too short to allow both sites to be occupied simultaneously. In this case, four vacancies on the octahedral sites are created for every interstitial tetrahedral ion, as shown in Figure 6.8. This type of cluster occurs at low values of x. As x increases, larger clusters form in which there are thirteen vacancies and four interstitial ions. This is called a Koch-Cohen cluster (Figure 6.9). 

To the defect structure of UO2. Aitken et al. (19) applied statistical mechanical calculations based on the relation derived by Anderson (249) and evaluated the energy parameters. They suggested the possibility of an ordering of vacancy pairs in hypostoichiometric uranium oxide, in view of the relatively large interaction energy between vacancy pairs. Similar treatment for the hypostoichiometric U02 j phase also has been reported by Winslow (259). 

It should be remembered that there is still much confusion about the real defect structures occurring over the broad composition range shown by this oxide. For example, there is uncertainty about whether a population of uranium vacancies best... 

It is interesting to note the differences in slopes of the two curves of Figure 2. The slope of the BaU03+x data is approximately twice that of the UO2+X data this fact reflects the enhanced stability of hexavalent compared to that of tetravalent uranium in complex oxides versus simple oxides. Ackermann and Chandrasekharaiah (24) calculated similar data for U02-x, which plot on a steeper curve than the AH (U02+x) because U02-x is a cation-vacancy U(IV) compound rather than a mixed-valence U(IV)-U(VI) oxide. 

While titanium substituted for antimony and this had a dramatic effect on catalytic activity as expected, there is a question as to how much of the uranium was converted from the +5 to the +6 oxidation state. The shifts in the infrared bands indicate a shortening of the bond distance and a lengthening of the Sb-0 bond distance which is consistent with an increase in hexavalent character, but the magnetic measurements show that a substantial portion of the uranium remained in +5 state. If the valence of uranium is not changed, then the replacement of Sb" by Ti must generate oxygen vacancies in the USb Oj Q lattice. It is these sites that may be responsible for the high activity of the promoted catalysts. 

Another propylene ammoxidation catalyst that was used commercially was U-Sb-0. This catalyst system was discovered and patented by SOHIO in the mid-1960s (26,27). Optimum yield of acrylonitrile from propylene required sufficient antimony in the formulation in order to ensure the presence of the USbaOio phase rather than the alternative uranium antimonate compound USbOs (28-30). The need for high antimony content was understood to stem from the necessity to isolate the uranium cations on the surface, which were presumed to be the sites for partial oxidation of propylene. Isolation by the relatively inactive antimony cation prevented complete oxidation of propylene to CO2. Later publications and patents showed that the activity of the U-Sb-0 catalyst is increased by more than an order of magnitude by the substitution of a tetravalent cation, tin, titanium, and zirconium (31). Titanium was found to be especially effective. The promoting effect results in the formation of a solid solution by isomorphous substitution of the tetravalent cation for Sb + within the catalytically active USbaOio- phase. This substitution produces o gen vacancies in the lattice and thus increases the facility for diffusion of lattice o gen in the solid structure. As is discussed below, the enhanced diffusion of o gen is directly linked to increased activity of selective (amm)oxidation catalysts based on mixed metal oxides. 

In the same way, for the fifth step, the stoichiometric numbers must satisfy the equations displayed in Table 19.10. On the other hand, we do not know the density of defects in UsOg (oxygen vacancies and uranium +4 ions on uranium +6 sites), so this oxide is not stable any more. The knowledge of this density would make it possible to fix the number of lattices concerned and conseqnently the set of stoichiometric numbers. Practically, this knowledge is required only if we are asked to write the speed of this step and only when it is the rate-determining step. To simplify, we will take the simplest integers. 

The interstitials have a complex behavior from a volume point of view, the fluorite crystalline stracture of oxide UO2 is sufficiently open for allowing the steric insertion of these ions in non-occupied octahedral uranium sites. However, the electrostatic repulsions limit the possibilities of indifferently placing oxygen or uranium interstitials in the link. Also, the interstitials of uranium created by irradiation are hardly stable and recombine extremely fast, because of the great mobility of 0 ions, in order to be wholly transformed in an oxygen vacancy. 

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