Actinide lattice parameters

Thus the rather easily obtained atomic sizes are the best indicator of what the f-electrons are doing. It has been noted that for all metallic compounds in the literature where an f-band is believed not to occur, that the lanthanide and actinide lattice parameters appear to be identical within experimental error (12). This actually raises the question as to why the lanthanide and actinide contractions (no f-bands) for the pure elements are different. Analogies to the compounds and to the identical sizes of the 4d- and 5d- electron metals would suggest otherwise. The useful point here is that since the 4f- and 5f-compounds have the same lattice parameters when f-bands are not present, it simplifies following the systematics and clearly demonstrates that actinides are worthy of that name. 

Figure 3. The lattice parameter for the family of rock-salt structure actinide-antimonide compounds is shown where the line is for the corresponding lanthanide compounds. The metallic radii for the light actinide elements are plotted. The smooth line simply connects Ac to the heavy actinides. In both cases the smooth line represents the ideal tri-valent behavior.

Antimonide-actinide compounds, lattice parameter for rock-salt... 

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

Table 16. Lattice parameters and Molecular volumes (unit cell vol. jZ) of some lanthanide and actinide triiodides (781)...

The impact on actinide solid state physics of Zachariasen s ideas has been great. It has triggered a very fruitful effort in structure determination. In the meantime, it has underlined the importance, in the theory of actinides, of knowing exactly atomic volumes as an indicator of the complexities of the metallic bond. The same picture of the variation of atomic radii appears for compounds having partial metallic character. In Fig. 5, for instance, the variation vs. Z of the lattice parameter ao of actinide pnictides is compared with the atomic radii of metals. Actinide pnictides have an NaCl type structure. Their... 

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. 

Fig. 13. Lattice parameters of the actinide and rare earth nitrides and arsenides. Full circles and triangles represent experimental values open circles and triangles, the theoretical values, for actinides and rare earths (from )...

Table 1 b. Potential parameters for selected rare earths (hep structure assumed) and actinides (fee structure assumed) at fixed lattice parameters equivalent to an fee lattice parameters of 5.0 A and 4.8 A, respectively... 

Self-consistent energy band calculations have now been made through the LMTO method for all of the NaCl-type actinide pnictides and chalcogenides . The equation of state is derived quite naturally from these calculations through the pressure formula extended to the case of compounds . The theoretical lattice parameter is then given by the condition of zero pressure. 

Another way to approach the lattice parameter trend is to consider the actinide series of a given pnictide or chalcogenide. The f-p bond should then draw the minimum in lattice parameter towards the beginning of the series. A series of calculations for the... 

Fig. 13. Lattice parameters of the actinide nitrides from LMTO (labelled Pauli pramagnetic), RLMTO (labelled Dirac) and LMTO spin polarized (labelled Pauli spin polarized) calculations. The black filled circles are the experimental lattice parameters...

The trend in lattice parameter across the actinide nitride series is reproduced by an energy band theory in which it is assumed that the f-electrons are itinerant. The results with and without spin polarization do not differ greatly until AmN is reached but in this... 

Figure 6. Lattice parameters of fluorite-related oxides of some lanthanide and actinide elements as a function of composition...

Phenomenological, theoretical and practical aspects of the double-double effect are presented on the basis of the review of original papers concerning the variation with Z of thermodynamic functions of complex formation, lattice parameters, and several other properties of lanthanide and actinide compounds. 

The Double-Double Effect in Lattice Parameters and Unit Cell Volumes of Actinides. .. 37... 

Fig. 11. Plot of lattice parameter ao vs L-quantum number for isostructural monoclinic actinide tetrafluorides. Data reported by Keenan and Asprey28 ...

Thorium oxide dichloride, like other actinide oxide dichlorides, possesses orthorhombic symmetry, space group Pbam. Its lattice parameters have been obtained by Bagnall et al. as a = (15.494 + 0.008) A, b = (18.095 + 0.008) A, c = (4.078 + 0.002) A [1968BAG/BRO]. 

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 monocarbide has the NaCl face-centred cnbic stmeture, space group Fm 3 m, isomorphous with most of the actinide MX componnds. The limits of composition are still not well-defined, but up to ca. 1300 K probably extend from ThCo67(cr) to ThCo97(cr), the lattice parameter at room temperatnre varying from (5.303 + 0.002) to (5.346 + 0.002) A, see [1984HOL/RAN]. As noted earher, at higher temperatures the... 

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). 

Promethium (Pm) is the last element in the lanthanides to assume dhcp structure at ambient conditions. It is also the only radioactive element besides technetium with stable neighboring elements. The work of Haire et al. (1990), where they obtained Pm-147 as a reactor byproduct is summarized here. An extended discussion on the comparison of Pm with that of Sm, Nd, and actinide materials is given in this work. Pm has been compressed to 60 GPa in a diamond anvil cell and EDXD was employed in observing the structural changes in Pm. The lattice parameters of this Pm sample at ambient conditions are given as a = 0.364nm, c= 1.180nm. 

Among complex halides of R " and An, the elpasolites CsjNaMClg represent the one class of compounds for which thermochemical measurements have been made on both lanthanide and actinide examples (Morss 1971, Schoebrechts et al. 1989). Although CsjNaNdClg and Cs2NaPuClg have essentially identical lattice parameters, as do NdClj and PuClj, the enthalpy of complexation of the lanthanide chloride system... 

Bearing this correlation in mind, one can immediately say that the hydrides of Np, Pu and Am are rare-earth-like with wide cubic solid solution ranges up to H/M = 2.7-2.S and somewhat smaller lattice parameters. The hexagonal phase appears with regularity, unlike the behavior encountered for Ce, Pr and Nd. P-C-T data are not available for the Cm system and beyond, but relatively normal behavior is expected. As pointed out above, the large and simple lattices of the early rare earth and late-actinide hydride systems preclude f-f overlap. Since the actinide contraction is delayed as compared to the rare-earth contraction, phase relationships such as those seen for Gd hydride and beyond are not expected for the actinides. 

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