Triuranium octoxide

Key takeaways

Triuranium octoxide belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Triuranium octoxide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Triuranium octoxide from memory before moving on to harder problems.

Reference excerpt

Triuranium octoxide (U3O8) is a compound of uranium. It is present as an olive green solid, and is one of the forms of yellowcake. U3O8 has potential long-term stability in a geologic environment. 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. The compound can be produced by the calcination of ammonium diuranate or ammonium uranyl carbonate. Due to its high stability, it can be used for the disposal of depleted uranium. Its particle density is 8.38 g cm-3. Triuranium octoxide is converted to uranium hexafluoride for the purpose of uranium enrichment.

Production Triuranium octoxide is produced industrially by the calcination of ammonium uranyl carbonate or ammonium diuranate. The ammonium uranyl carbonate (AUC) method is as follows: Uranium hexafluoride is hydrolyzed in water to form uranyl fluoride...

UF6(g) + 2 H2O(l) → UO2F2(aq) + 4 HF(aq) ... which is then precipitated with ammonium carbonate:

UO2F2(aq) + 3 (NH4)2CO3 → (NH4)4UO2(CO3)3 + 2 NH4F The resulting ammonium uranyl carbonate is left to dry and then heated in air:

3 (NH4)4UO2(CO3)3 → U3O8 + 4 NH3 + 5 CO2 + 2 H2O + ½ O2

Formation Triuranium octoxide is formed by the multi-step oxidation of uranium dioxide by oxygen gas at around 250 °C:

8 UO2 + O2 → 2 U4O9 6 U4O9 + O2 → 8 U3O7 2 U3O7 + O2 → 2 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):

3 UO3 → U3O8 + 1/2 O2 Uranium trioxide can be reduced by other methods, such as reaction with reducing agents like hydrogen gas at around 500 °C−700 °C:

3 UO3 + H2 → U3O8 + H2O This process can produce other uranium oxides, such as U4O9 and UO2.

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 UV atoms and 1 UVI atom, without any atoms of UIV. The study used the compounds uranium dioxide and uranyl acetylacetonate as references for the spectra of UIV and UVI, respectively. The analysis that U3O8 contains 2 UV and 1 UVI is supported by other studies.

Reactions Triuranium octoxide can be reduced to uranium dioxide through reduction with hydrogen:

U3O8 + 2 H2 → UO2 + 2 H2O Triuranium octoxide also loses oxygen to form a non-stoichiometric compound (U3O8-z) at high temperatures (>800 °C), but recovers it when reverted to normal temperatures. Triuranium octoxide is slowly oxidized to uranium trioxide under high pressures of oxygen:

U3O8 + 1/2 O2 → 3 UO3 Triuranium octoxide is attacked by hydrofluoric acid at 250 °C to form uranyl fluoride:

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.

Structure Triuranium octoxide has multiple polymorphs, including α-U3O8, β-U3O8, γ-U3O8, and a non-stoichiometric high-pressure phase with the fluorite structure.

Alpha

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. 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 coordination geometry of pentagonal bipyramidal.

Beta

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. 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 coordination geometries, and the other U atom has a coordination geometry of tetragonal bipyramidal.

Gamma γ-U3O8 is formed at around 200-300 °C and at 16,000 atmospheres of pressure. 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. This phase has a cubic structure with a high amount of defects. Its formula is UO2+x, where x ≈ 0.8.

Natural occurrence Triuranium octoxide can be found in small quantities (~0.01-0.05%) in the mineral pitchblende.

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 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. 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.

U3O8 + 2 H2 → 3 UO2 + 2 H2O UO3 + H2 → UO2 + H2O UO2 + 4 HF → UF4 + 2 H2O UF4 + F2 → UF6

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Triuranium octoxide

Start with the simplest possible case. Write down what Triuranium octoxide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Triuranium octoxide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Triuranium octoxide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Triuranium octoxide

In research: Triuranium octoxide appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Triuranium octoxide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Triuranium octoxide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Mixed valence compounds, Nuclear materials, Oxides, so understanding it makes those chapters shorter.
In everyday life: Look for Triuranium octoxide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Triuranium octoxide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Triuranium octoxide means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Triuranium octoxide out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Triuranium octoxide in simple terms?

Triuranium octoxide (U3O8) is a compound of uranium. It is present as an olive green solid, and is one of the forms of yellowcake.

Why does Triuranium octoxide matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Triuranium octoxide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Triuranium octoxide.