Triuranium octoxide

Triuranium octoxide (U₃O₈)^[4] is a compound of uranium. It is present as an olive green to black, odorless solid. It is one of the more popular forms of yellowcake and is shipped between mills and refineries in this form.

Triuranium octoxide

Names
Other names Uranium(V,VI) oxide Pitchblende C.I. 77919
Identifiers
CAS Number	1344-59-8 Y
3D model (JSmol)	Interactive image
ChemSpider	10142128
ECHA InfoCard	100.014.275
EC Number	215-702-4
PubChem CID	11968241
CompTox Dashboard (EPA)	DTXSID60893930
InChI InChI=1S/8O.3U Key: IQWPWKFTJFECBS-UHFFFAOYSA-N
SMILES [O].[O].[O].[O].[O].[O].[O].[O].[U].[U].[U]
Properties
Chemical formula	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)
Solubility in water	Insoluble[2]
Solubility	Soluble in nitric acid and sulfuric 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:
Pictograms
Signal word	Danger
Hazard statements	H300, H330, H373, H411
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

U₃O₈ has potential long-term stability in a geologic environment.^[5] In the presence of oxygen (O₂), uranium dioxide (UO₂) is oxidized to U₃O₈, whereas uranium trioxide (UO₃) loses oxygen at temperatures above 500 °C and is reduced to U₃O₈.^[6][7][8] The compound can be produced by the calcination of ammonium diuranate or ammonium uranyl carbonate.^[9] Due to its high stability, it can be used for the disposal of depleted uranium.^[10] Its particle density is 8.38 g cm⁻³.

Triuranium octoxide is converted to uranium hexafluoride for the purpose of uranium enrichment.

Formation

Triuranium octoxide can be formed by the multi-step oxidation of uranium dioxide by oxygen gas at around 250°C:^[7]

${\displaystyle {\ce {3 UO2 ->[ O2 ] 3/4 U4O9 ->[ O2 ] U3O7 ->[ O2 ] U3O8}}}$

It can also be formed from the reduction 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][11][12]}

${\displaystyle {\ce {3 UO3 -> U3O8 + 1/2 O2}}}$

Calcination of ammonium uranyl carbonate and ammonium diuranate is the main method for the production of U₃O₈.^[9]

Uranium trioxide can be reduced by other methods, such as reaction with reducing agents like hydrogen gas at around 500°C−700°C:^[11][12]

${\displaystyle {\ce {3 UO3 ->[ H2 ] U3O8}}}$

This process can produce other uranium oxides, such as U₄O₉ and UO₂.^[12]

Chemical properties

Oxidation state

While many studies have shown contradicting results on the oxidation state of uranium in U₃O₈, a study on its absorption spectrum determined that each formula unit of U₃O₈ contains 2 U^V atoms and 1 U^VI atom, without any atoms of U^IV. The study used the compounds uranium dioxide and uranyl acetylacetonate as references for the spectra of U^IV and U^VI, respectively.^[13]

The analysis that U₃O₈ contains 2 U^V and 1 U^VI is supported by other studies.^[14]

Reactions

Triuranium octoxide can be reduced to uranium dioxide through reduction with hydrogen:^[11][12]

${\displaystyle {\ce {U3O8 + 2 H2 -> UO2 + 2 H2O}}}$

Triuranium octoxide also loses oxygen to form a non-stoichiometric compound (U₃O_8-z) at high temperatures (>800°C), but recovers it when reverted to normal temperatures.^[15]

Triuranium octoxide is slowly oxidized to uranium trioxide under high pressures of oxygen:^[15]

${\displaystyle {\ce {U3O8 + 1/2 O2 -> 3 UO3}}}$

Triuranium octoxide is attacked by hydrofluoric acid at 250 °C to form uranyl fluoride:^[16]

${\displaystyle {\ce {U3O8 + 6 HF + 1/2 O2 -> 3 UO2F2 + 3 H2O}}}$

Triuranium octoxide can also be attacked by a solution of hydrochloric acid and hydrogen peroxide to form uranyl chloride.^[17]

Structure

Triuranium octoxide has multiple polymorphs, including α-U₃O₈, β-U₃O₈, γ-U₃O₈, and a non-stoichiometric high-pressure phase with the fluorite structure.^[6][15][18]

Alpha

The crystal structure of α-U₃O₈.

α-U₃O₈ is the most commonly encountered polymorph of triuranium octoxide, being the most stable under standard conditions. At room temperature, it has an orthorhombic pseudo-hexagonal structure, with lattice constants a=6.72Å, b=11.97Å, c=4.15Å and space group Amm2. At higher temperatures (~350 °C), it transitions into a true hexagonal structure, with space group P62m.^[6][15][18]

α-U₃O₈ is made up of layers of uranium and oxygen atoms. Each layer has the same U-O structure, and oxygen bridges 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 a molecular geometry of pentagonal bipyramidal.^[6]

Beta

The crystal structure of β-U₃O₈.

β-U₃O₈ can be formed by heating α-U₃O₈ to 1350 °C and slowly cooling. The structure of β-U₃O₈ is similar to that of α-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 group Cmcm.^[6]

Like α-U₃O₈, β-U₃O₈ has a layered structure containing uranium and oxygen atoms, but unlike α-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 molecular geometries, and the other U atom has a molecular geometry of tetragonal bipyramidal.^[6]

Gamma

γ-U₃O₈ is formed at around 200-300 °C and at 16,000 atmospheres of pressure.^[15] Very little information on it is available.

Fluorite-type

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 than α-U₃O₈.^[18]

This phase has a cubic structure with a high amount of defects. Its formula is UO_2+x, where x ≈ 0.8.^[18]

Natural occurrence

Triuranium octoxide can be found in small quantities (~0.01-0.05%) in the mineral pitchblende.^[19]

Uses

Production of uranium hexafluoride

Triuranium octoxide can be used to produce uranium hexafluoride, which is used for the enrichment of uranium in the nuclear 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 in nitric acid to form uranyl nitrate, followed by calcining to uranium trioxide in a fluidized bed reactor.^[20][21]

${\displaystyle {\ce {U3O8 ->[ HNO3 ] UO2(NO3)2 ->[ heat ] UO3}}}$

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 form uranium tetrafluoride and then with fluorine gas to produce uranium hexafluoride. This can then be separated into uranium-235 and uranium-238 hexafluoride.^[20][21]

${\displaystyle {\ce {U3O8 + 2 H2 -> 3 UO2 + 2 H2O}}}$

${\displaystyle {\ce {UO3 + H2 -> UO2 + H2O}}}$

${\displaystyle {\ce {UO2 + 4 HF -> UF4 + 2 H2O}}}$

${\displaystyle {\ce {UF4 + F2 -> UF6}}}$

As a reference material

Triuranium octoxide is a certified reference material and can be used to determine the impurity of a sample of uranium.^[2][22]

Hazards

Triuranium octoxide is a carcinogen 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 also radioactive, being an alpha emitter.^[2]