The actinide sesquioxides can crystallize with three different forms: a hexagonal close-packed (a), a mono­clinic (b), or a cubic (c) structure. The hexagonal form is in most of the cases the stable phase at room
temperature. The cubic phase may be considered as a fluorite structure from which 1/4 of the oxygen ions have been removed. The crystal data on the actinide sesquioxides are listed in Table 6.

Few experimental data are available concerning the thermal expansion coefficients of actinide sesqui — oxides. The thermal expansion can be fitted with the following equation (in percentage):

DL

= a0 + a1 x T + a2 x T2 [9]

L0

Konings43 has extracted from experiments the ther­mal expansion of monoclinic B-Cm2O3. We have done the same for Pu2O3 (from Taylor76). In the case of Am2O3, we have used the data obtained by Uchida et a/.65 by MD calculations. A summary is available in Table 7.

2.02.3.2

Other Actinide Oxides

As mentioned in Section 2.02.3.1, the fluorite struc­ture of UO2 has empty octahedral sites that can be occupied by O2_ ions to form UO2 + x The phase diagram data show that the maximum oxygen content corresponds to x = 0.25 (or U4O9) (Figure 1). From his interpretation of neutron diffraction data on UO213, Willis100 found that the interstitials tend to
aggregate to form clusters made of oxygen intersti­tials interacting with normal oxygen anions.101,102

The so-called cluster 2:2:2 is composed of two oxygen vacancies and four interstitials. Below 1400 K, these clustered excess oxygens tend to form an ordered phase with the composition U2O9 _y.

U4O9 is a narrowly hypostoichiometric phase (U4O9 _y) and exists with three different forms:

a-U4O9 _^ (at T< 353 K), p-U4O9 _^ (at 353 K < T< 823 K), and g-U4O9 _(at 823 K < T< 1400 K). The structure of the p-U4O9 phase was studied by Bevan et at}03 who showed that this phase is a superlattice structure based on the fluorite structure of UO2 with a unit cell 64 times the volume of the UO2 cell. The additional O atoms are arranged in cuboctahedral clusters. According to the later analy­sis by Cooper and Willis,104 the centers of the clusters are unoccupied, whereas they are occupied by single O-ions according to Bevan eta/.103 U4O9 decomposes at 1400 K into UO2 + x (disordered with x ~ 0.25) and U3O8 (see Figure 1).

U3O8 is a mixed valence compound with U(V) and U(VI) cations. U3O8 exists in several forms as a function of temperature. At room temperature, a-U3O8 is orthorhombic and transforms to a pseudo­hexagonal structure p-U3O8 at 483 K. Heat capacity measurements by Inaba et a/.105 showed other phase transitions at 568 and 850 K.

UO3 can crystallize in six forms. The stable form at room temperature a-UO3 is orthorhombic.

Partial information on crystal data of plutonium and curium intermediate oxides with O/metal ratio below 2 is given in Table 8.