Sesquioxides lattice parameters

A similar approach can be taken in comparing the sesquioxides of both series, although some approximations must be considered (Templeton and Dauben 1957) in deriving radii from the sesquioxide lattice parameters. Radii based on these sesquioxides have also been compiled by others (Shannon 1976, Haire and Baybarz 1973). Trivalent radii for these f-element oxides are given in table 30. Radii and cubic lattice parameters are also plotted in fig. 18. Based on such radii, it can be seen that the... 

Fig. 18. Sesquioxide lattice parameters and ionic radii for the lanthanides and actinides.

Derived from sesquioxide lattice parameters, usii% an oxygen radius of 1.46 A and adding 0.08 A for covalent M-O bond character six-coordinate metal ion. 

II. 21 or 11.26 A calculated and found respectively for C-type CcjOj. In another study, the stabilisation of nanosize particles of ceria as cubic Ce203 was suggested on the basis of calculation of the lattice parameter of ceria sols having diameter of a few nanometers. The decrease of particle size strongly stabilised ion, and the extrapolation of lattice parameter values suggested that the C-type sesquioxide exists when constituted by nanoparticles of diameter less than 1.5nm, with most of Ce... 

Figure 6 shows the lattice parameters vs. composition of the phases discussed above. For the C-type sesquioxides or a phases the pseudo-... 

The crystal structure of La202C2 determined from three-dimensional X-ray diffraction on a twin crystal is of monoclinic symmetry, space group C/2m (Seiver and Eick 1976). The lattice parameters are a = 7.069(8) A, b = 3.985(4) A, c = 7.310(9) A and p = 95.70(6)° the calculated density is 5.41 gcm". In this structure, the lanthanum atom has four oxygen and four carbon atoms situated in a distorted bicapped trigonal prismatic arrangement. Interatomic La-O distances range from 2.392(8) to 2.823(9) A and La-C distances from 2.86(1) to 3.11(1) A. The carbon atoms are present as C2 units with an interatomic C-C distance of 1.21(3) A. Oxygen atoms are tetrahedrally coordinated, as in the sesquioxide. 

The structural parameters and space group symmetries of the established lanthanide oxides are given in tables 5-12. Figures 9 and 23 contain plots of the lattice parameters of the sesquioxides and dioxides as a function of the atomic number. The lanthanide contraction is exhibited clearly in these plots. 

There have been a few studies of the effect of pressure on the sesquioxides. Sawyer et al. (1965a) found that under extreme pressure some of the C-type sesquioxides transformed to the A- or B-type. Hoekstra (1966) applied pressure to the whole series of sesquioxides and found the transformation of all of the C-type (Sm203 to LU2O3) to the B-type. This transformation is consistent with the decreased molar volume of the B-type compounds. Lattice parameters are listed in table 8. 

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 fig. 23 plots are shown of the lattice parameters for the cubic sesquioxides and mononitrides of the lanthanides and actinides, and the atomic volumes for the metals of both series (Haire and Gibson 1992). The purpose of these plots is to point out that relationships (such as bonding, volumes of electronic homologs, etc.) between these series change considerably with the material being considered i.e., the bonding involved in the oxides differs from that in the nitrides, which also varies from that in the... 

Fig. 23. Lattice parameters of the lanthanide and actinide sesquioxides and mononitiides, and the atomic volumes of the respective metals.

Additional support for this latter structure being a form of metallic californium is that samples exhibiting this structure have been converted by thermal treatment to the fee structure having the parameter Oq = 0.494 nm, and vice versa [72], The second dhep structure [71] listed in Table 11.3 and the other hexagonal structure [70] may represent the same material. If they are metallic californium, they would represent a hexagonal structure for the divalent form of the metal. They are at present not well-established structures for the metal. The fee structure with the parameter Uq = 0.540 nm [68,69] has been observed in earlier work, where very small quantities of californium were prepared. In a later study [70], this structure (oq == 0.540 nm) was also observed in thin films of californium metal that had been heated to 200-300°C in air. It should be noted that poorly crystallized samples of californium sesquioxide (Cf203. c, body-centered cubic, Oo = 1.080-1.083 nm see Section 11.7.2) can be indexed as an fee strueture with Go = 0.540-0.542 nm. If the fee structure with Oo = 0.540 nm was indeed a metallic phase of californium, the lattice parameter would imply that the metal had an intermediate valence between 2 and 3. 

Body-centered cubic C-type sesquioxide is of space group Ia3. The unit cell contains 32 metal atoms and 48 oxygen atoms. The metal atoms are six-coordinate. Table 27.1 contains the lattice parameters of the established crystalline rare earth oxides. 

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