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| Names | |
|---|---|
| Other names Uranium(V,VI) oxide Pitchblende C.I. 77919 | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider | |
| ECHA InfoCard | 100.014.275 |
| EC Number |
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| |
| |
| Properties | |
| U3O8 | |
| Molar mass | 842.08 g/mol |
| Density | 8.38 g/cm3[1] |
| Melting point | 1,150 °C (2,100 °F; 1,420 K) |
| Boiling point | decomposes to UO2 at 1,300 °C (2,370 °F; 1,570 K) |
| Insoluble[2] | |
| Solubility | Soluble innitric acid andsulfuric acid[2] |
| Thermochemistry | |
Std molar entropy(S⦵298) | 282 J·mol−1·K−1[3] |
Std enthalpy of formation(ΔfH⦵298) | −3575 kJ·mol−1[3] |
| Hazards | |
| GHS labelling: | |
   | |
| Danger | |
| H300,H330,H373,H411 | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Triuranium octoxide (U3O8)[4] is a compound ofuranium. It is present as an olive green to black, odorless solid. It is one of the more popular forms ofyellowcake and is shipped between mills and refineries in this form.
U3O8 has potential long-term stability in ageologic environment.[5] In the presence ofoxygen (O2),uranium dioxide (UO2) isoxidized to U3O8, whereasuranium trioxide (UO3) loses oxygen at temperatures above 500 °C and isreduced to U3O8.[6][7][8] The compound can be produced by the calcination ofammonium diuranate orammonium uranyl carbonate.[9] Due to its high stability, it can be used for the disposal ofdepleted uranium.[10] Its particle density is 8.38 g cm−3.
Triuranium octoxide is converted touranium hexafluoride for the purpose ofuranium enrichment.
Triuranium octoxide is produced industrially by thecalcination of ammonium uranyl carbonate or ammonium diuranate.[9] The ammonium uranyl carbonate (AUC) method is as follows:[11]
Uranium hexafluoride ishydrolyzed in water to formuranyl fluoride...
... which is then precipitated withammonium carbonate:
The resulting ammonium uranyl carbonate is left to dry and then heated in air:
Triuranium octoxide is formed by the multi-stepoxidation of uranium dioxide byoxygen gas at around 250°C:[7]
It can also be formed from thereduction of compounds like ammonium uranyl carbonate, ammonium diuranate, and uranium trioxide through calcination at high temperatures (~600°C for (NH4)2U2O7, 700°C for UO3):[8][9][12][13]
Uranium trioxide can be reduced by other methods, such as reaction withreducing agents likehydrogen gas at around 500°C−700°C:[12][13]
This process can produce other uranium oxides, such as U4O9 and UO2.[13]
While many studies have shown contradicting results on theoxidation state of uranium in U3O8, a study on itsabsorption spectrum determined that eachformula unit of U3O8 contains 2 UV atoms and 1 UVI atom, without any atoms ofUIV. The study used the compounds uranium dioxide anduranyl acetylacetonate as references for the spectra of UIV and UVI, respectively.[14]
The analysis that U3O8 contains 2 UV and 1 UVI is supported by other studies.[15]
Triuranium octoxide can be reduced to uranium dioxide through reduction with hydrogen:[12][13]
Triuranium octoxide also loses oxygen to form anon-stoichiometric compound (U3O8-z) at high temperatures (>800°C), but recovers it when reverted to normal temperatures.[16]
Triuranium octoxide is slowly oxidized to uranium trioxide under high pressures of oxygen:[16]
Triuranium octoxide is attacked byhydrofluoric acid at 250 °C to formuranyl fluoride:[17]
Triuranium octoxide can also be attacked by a solution ofhydrochloric acid andhydrogen peroxide to formuranyl chloride.[18]
Triuranium octoxide has multiplepolymorphs, includingα-U3O8,β-U3O8,γ-U3O8, and a non-stoichiometric high-pressure phase with thefluorite structure.[6][16][19]
α-U3O8 is the most commonly encountered polymorph of triuranium octoxide, being the most stable under standard conditions. At room temperature, it has anorthorhombic pseudo-hexagonalstructure, withlattice constantsa=6.72Å,b=11.97Å,c=4.15Å and space groupAmm2. At higher temperatures (~350 °C), it transitions into a true hexagonal structure, withspace groupP62m.[6][16][19]
α-U3O8 is made up of layers of uranium and oxygen atoms. Each layer has the same U-O structure, and oxygenbridges connect corresponding uranium atoms in different layers. Within each layer, the U sites are surrounded by five oxygen atoms. This means that each U atom is bonded to seven oxygen atoms total, giving U acoordination geometry ofpentagonal bipyramidal.[6]
β-U3O8 can be formed by heatingα-U3O8 to 1350 °C and slowly cooling. The structure ofβ-U3O8 is similar to that ofα-U3O8, having a similar sheet-like arrangement and similar lattice constants (a=7.07Å,b=11.45Å,c=8.30Å [c/2=4.15Å]). It also has an orthorhombic cell, with space groupCmcm.[6]
Likeα-U3O8,β-U3O8 has a layered structure containing uranium and oxygen atoms, but unlikeα-U3O8, adjacent layers have a different structure- instead, every other layer has the same arrangement of U and O atoms. It also features oxygen bridges between U and O atoms in adjacent layers, though instead of all U atoms having a geometry of pentagonal bipyramidal, 2 U atoms per formula unit have distinct pentagonal bipyramidal coordination geometries, and the other U atom has a coordination geometry oftetragonal bipyramidal.[6]
γ-U3O8 is formed at around 200-300 °C and at 16,000 atmospheres of pressure.[16] Very little information on it is available.
A high-pressure phase of U3O8 with a hyperstoichiometric fluorite-type structure is formed at pressures greater than 8.1 GPa. During the phase transition, the volume of the solid decreases by more than 20%. The high-pressure phase is stable under ambient conditions, in which it is 28% denser thanα-U3O8.[19]
This phase has acubic structure with a high amount ofdefects. Its formula is UO2+x, wherex ≈ 0.8.[19]
Triuranium octoxide can be found in small quantities (~0.01-0.05%) in the mineralpitchblende.[20]
Triuranium octoxide can be used to produceuranium hexafluoride, which is used for theenrichment of uranium in thenuclear fuel cycle. In the so-called 'dry' process, common in the United States, triuranium octoxide is purified through calcination, then crushed. Another process, called the 'wet' process, common outside the U.S., involves dissolving U3O8 innitric acid to formuranyl nitrate, followed by calcining to uranium trioxide in afluidized bed reactor.[21][22]
No matter which method is used, the uranium oxide is then reduced using hydrogen gas to form uranium dioxide, which is then reacted with hydrofluoric acid to formuranium tetrafluoride and then withfluorine gas to produce uranium hexafluoride. This can then be separated intouranium-235 anduranium-238 hexafluoride.[21][22]
Triuranium octoxide is acertified reference material and can be used to determine the impurity of a sample of uranium.[2][23]
Triuranium octoxide is acarcinogen and is toxic by inhalation and ingestion with repeated exposure. If consumed, it targets the kidney, liver, lungs, and brain, and causes irritation upon contact with the skin and eyes. It should only be handled with adequate ventilation. In addition, it is alsoradioactive, being analpha emitter.[2]
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