Actinides crystal structures, lattice parameters

Table 5.16. Actinides crystal structures, lattice parameters of their allotropes, calculated densities.

Figure 5 gives the variation of the atomic volume in the actinide series, for the room temperature crystal structures as well as for the ccp and bcc high temperature allotropes, which exist for a number of actinides. The graph is based on the lattice parameters of Table 1, which includes also recent results. The marked dip in the curve from Th to Am illustrates the shrinkage of interactinide distance which is linked to the itinerancy of the 5f electrons in this part of the actinide series. 

This compound is obtained by reacting thorium oxide with H2S or sulphur vapour above 1273 K. It was reported by Zachariasen [1949ZAC] to be tetragonal (PbFCl structure, - P4/nmm) with a = (3.962 + 0.002) and c = (6.746 + 0.004) A, isomorphous with heavier actinide oxysulphides. From single crystal data, Amoretti et al. [1979AMO/GIO] confirmed the space group attributed by [1949ZAC] and reported the lattice parameters as o = (3.973 + 0.004) and c = (6.773 + 0.008) A. 

Thorium tetrachloride has been shown to possess two polymorphic forms. The crystal structure or the p form, stable above 678 K, has been well established to be tetragonal with space group lAfamd and lattice parameters a = 8.486 and c = 7.465 A. The low-temperature a form, which can only be obtained if extreme care is taken in cooling the sample, has been shown to be tetragonal with space group l /a with a = (6.408 + 0.001) and c = (12.924 + 0.003) A. The p form is isomorphous with the other actinide tetrachlorides [1968BRO]. As this is the form normally obtained at room temperature, the values for this polymorph are considered to be the standard thermodynamic data for ThC cr). 

The actinide sesquioxides have many similarities with the lanthanide sesquioxides, such as crystal structures (A, B and C forms), lattice parameters, etc., but there are also some significant differences. One notable difference is their melting points it appears that the transcurium sesquioxides have significantly lower (several hundreds of degrees C) melting points and display a different melting point trend with Z than for comparable lanthanide sesquioxides. The similarities and differences of the two f-series oxides are discussed in more detail in the following section. 

Assuming that the lanthanide and actinide dioxides are truly stoichiometric, a self-consistent and relative set of quadrivalent radii can be calculated from their lattice parameters. These radii are given in table 30. The actinide dioxides are known to have larger lattice parameters than their lanthanide electronic homologs. Thus, PuOj has a parameter similar to that for Ce02, which is four elements to the left of samarium, the vertical 4f homolog of Pu. This is a greater shift than that observed with the sesquioxides (see below). Structurally, the dioxides of the lanthanides and actinides are isostructural, and crystallize in the fluorite-type, cubic (fee) structure. 

In the third theoretical paper (chapter 112), B. Johansson and M.S.S. Brooks describe the use of first principles calculations to obtain the cohesive properties of lanthanide and actinide materials. As the authors note, in the past 20 years theorists have been able to calculate the lattice parameters of the material, evaluate the correct crystal structure, and accurately calculate its cohesive energy. 

---