Binary oxides

One of the most studied uranium oxide is UO3, which probably stems from the large

polymorphic variety. There are seven known structural isomers that have been synthesised with varying levels of success, amorphous (A), α, β, γ, δ, ε, and ζ. The synthesis routes are varied (Figure 2.10), though are achieved mostly via calcination of hydrated uranyl(VI) salts of nitrate or ammonia. The phase selectivity appears both temperature and seldom atmosphere dependent.

Washed uranium peroxide (UO4.2H2O) undergoes amorphisation during calcination

up to 200 °C [166] to form a U2O7 intermediate[167]. Calcination of amorphous-U2O7

(UO4.2H2O), Schoepite (UO3.2H2O), uranyl(VI) oxalate (UO2C2O4.3H2O), and

ammonium uranyl(VI) carbonate ((NH4)4UO2(CO3)3) at 400 °C forms amorphous-

UO3 (Figure 2.10, UO3(A)).

Figure 2.10 Summary of calcination-mediated synthesis routes of the structural isomers of UO3, showing temperature, atmosphere, and starting products.

Adapted from [1], 3D structural representations generated from crystallographic information files from the ICSD.

Upon further calcination at 470 – 500 °C, anhydrous α-UO3 crystallises. This may be

achieved directly using unwashed uranium peroxide. The α-UO3 structure comprises

infinite layers of buckled-UO8 polyhedra (Figure 2.10, blue), that are linked through

the c-axis [168-170]. Heating of α-UO3 at 500 – 550 °C or (rapidly heating)

ammonium polyuranate ((NH4)2U7O22) to 500 °C in air, results in formation of β-UO3

along equatorial vertices. Along the c-axis, chains of UO-polyhedra run alternately parallel and perpendicular, leaving large interstitial voids [171, 172].

The most thermodynamically most stable γ-UO3 phase forms during heating of α-, β-

, δ-, or ε-UO3 at 650 °C, or during thermal degradation of uranyl(VI) nitrate hydrate

(UO2(NO3)2.6H2O) between 400 – 600 °C [173, 174]. The complex γ-UO3 structure

(Figure 2.10, orange) comprises infinite edge-linked UO8 polyhedra arranged parallel

in alternating layers, interspersed by perpendicular chains and isolated polyhedra. One striking feature are the tunnel-like interstices running parallel to the c-axis with a flattened 6-side projected geometry, measuring ~4.8 – 5.5 Å across.

Within the formal U(IV)-oxidation state, lies uranium dioxide (UO2), a synthetic

analogue of naturally occurring Uraninite (Figure 2.11). UO2 may be synthesised via

hydrogen reduction from UO3, or U3O8 at 800 – 1100 °C, and crystallises in the

Fluorite face-centred-cubic (FCC) structure (a = b = c, α = β = γ = 90°). Uranium atoms occupy the positions (0, 0, 0), (½, ½, 0), (½, 0, ½), (0, ½, ½), whilst oxygens occupy all equivalent (¼, ¼, ¼) positions, resulting in a series of alternating cubic UO8-polyhedra, that are edge-linked, with each layer stacked via the sequence

ABCABC. Increasing calcination temperature towards 1700 °C improves density towards crystallographic predictions, and is often utilised in nuclear fuel fabrication processes. Industrial applications usually begin from ammonium diuranate [175], peroxides, or fluorides (see section 1), involving several cold-press and sinter steps [1]. Some novel recent studies have successfully synthesised colloidal UO2 and U3O8

nanoparticles via thermal degradation in non-aqueous solvents [176-178]. UO2+x

tends to form via oxygen diffusion during cooling below 300 °C, or if O2 impurities

are present in the H2-gas flow [179-183], where hyper-stoichiometric oxygen atoms

occupy positions displaced ~1 Å from [110] and [111] planes [184].

Between UVIO3 and UIVO2 oxidation states or O/U-stoichiometry between 3 and 2, lie

several UO-phases (Figure 2.11), each with their own structural isomers [1, 7, 185]. U3O8 or triuranium octoxide [186, 187], is sometimes given the misnomer uranyl(VI)

uranate. However, with an oxidation state lying between U(VI) and U(V) [188, 189] the uranyl(VI) ion is absent, whilst the UO-sheet structure deviates far from traditional MIIUO4 uranates. Between UO3 and U3O8, is UO2.9 (U12O35) [186], a suspected

distinct phase with structural properties lying somewhere between U3O8 and UO3 in

terms of oxygen vacancies.

Several phase transformation routes are apparent between UO3 and U3O8 phases. For

example, U3O8 forms via heating of δ/ε-UO3 at 450 °C in air with moderate heating

rates, otherwise heating to 620 – 700 °C is required due to re-oxidation to γ-UO3.

Alternatively, oxidation of UO2 using air at 800 °C with slow cooling, results in α-

structure (see section 2.4.1), though the layers of P, D-type chains are linked vertically via U-O-U bonds.

Figure 2.11 Temperature – O/U phase diagram for the binary UO-system. Note the transition from cubic Fluorite-like crystal structure towards UIV_{, and} the layered structures towards U(VI). Phase diagram Adapted from [1]. 3D

structural representations generated from crystallographic information files from the ICSD.

Due to the similarity between the α-U3O8 [001] and UO2 [111] planes, and almost no

change in UU-distances nor angles during oxidation, it was proposed that lattice infusion of oxygen causes stepwise distortion of the fluorite structure (UO2) towards

tetragonal (U3O7), monoclinic-distorted fluorite (U2O5). U2O5 undergoes phase

transitions via layered-β and α forms before further oxidation to α-U3O8 [191, 192].

β-U3O8 is synthesised via heating of α-U3O8 at 1350 °C in air/O2 followed by cooling

at 100° day-1 to room temperature [193].

U2O5 (2.5 O/U), U3O7 (2.33 O/U) and U4O9 (2.25 O/U) all have α, β, and γ

polymorphs. U2O5 and U4O9 are both synthesised from stoichiometric mixtures of

UO2 and U3O8 precursors, whereas α/β-U3O7 is synthesised from UO2, and γ-U3O7

from U4O9.

α-U2O5 is synthesised via solid-state reaction between UO2 and U3O8 at 400 °C and 3

mPa pressure for 8 hours, or at half the pressure (1.5 mPa) when temperature was elevated 500 °C. At 40 – 50 mPa and temperature (> 800 °C), hexagonal-β-U2O5

forms. At higher pressure (60 mPa) monoclinic γ-U2O5 [194]. Remarkably, the sheet-

structure for U2O5 exhibits similar features to (Sr/Pb)3U11O36 [195, 196], where

equatorially aligned sheets of UO7 and UO6 polyhedra are interspersed by trimeric

UO-defects, which would otherwise be occupied by (Sr/Pb)O polyhedra.

α-U3O7 forms during oxidation of UO2 at <160 °C [197-201], whereas the β-

polymorph forms above 200 °C [202]. The γ-polymorph forms via oxidation of U4O9

at 160 °C [186]. All three polymorphs of U3O7 are tetragonal, with some minor

alterations in unit cell dimensions (c/a ~1.01 ±0.02) and O/U-stoichiometry (2.3 – 2.33).

Ceramic synthesis of α-U4O9 involves calcination of UO2 with half molar equivalent

of U3O8 at 1000 °C for up to 2 weeks [203, 204], followed by a 2 week cooling period.

Reversible phase transformations occur at ~77 (β-U4O9) and ~577 °C (γ-U4O9) ,

indicating that only the α-form is stable at room temperature. The β-forms of U4O9

and U3O7 (Figure 2.11) exhibit increasingly distorted cubic structures with furthering

deviation of O/U-stoichiometry from UO2 [1, 185], and appear far more distinct from

the layered polymorphs typical of U2O5 or U3O8. Excess oxygens for both phases are

expected to be accommodated in cuboctahedral clusters [202].