Lanthanides crystal structures, lattice parameters

The YbCo.gj carbide reported by Haschke and Eick (1970a) was not verified to exist in the ytterbium-carbon system by other workers. Its crystal structure was not determined by these authors. In contrast, the two forms of the YCq.s+z compound, the face-centered cubic Fe4N-type form and the rhombohedral CdCl-type form, have been well determined. Their lattice parameters are in good agreement with the systematic variation between those of the other heavy lanthanide hypocarbides, although the composition range for this diphase mixture was not determined. The only information about this material was provided by Spedding et al. (1958). They reported that a carbon-rich YbsC compound exists. 

Although the crystal structures of about half of the rare earth metals were well established by the mid- to late 1930 s the work by Klemm and Bommer (1937) stands as a landmark paper because they determined the crystal structures and the lattice parameters of all the lanthanides, except holmium, samarium and radioactive promethium. In so doing they... 

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 this chapter, we present some latest analysis results of lanthanides (Lns Eu and Gd)-M6ssbauer structure and powder X-ray diffraction (XRD) lattice parameter (oq) data of defect-fluorite (DF) oxides with the new defect crystal chemistry (DCC) Oq model [ 1,2] as an upgrade of the former random oxygen coordination number (CN) Oq model [3,4]. This is, thus, the first report of our ongoing efforts to further elaborate the model and extend its applicability to more various systems, especially to pyrochlore (P)-type stabilized zirconias (SZs) and stabilized hafnias (SHs). 

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. 

Several studies of the effect of pressure on the radial Bkq crystal field parameters have been reported. Lanthanide oxyhalides (REOX RE = La, Gd, Y X = Cl, Br) doped with Eu + have been extensively studied. The REOX lattices are iso-structural and incorporate Eu + in bonding sites with 4 symmetry. In the oxybromides, Eu " is coordinated to four oxygens and four bromides. The RE-0 bond lengths ( 2.3 - 2.4 A) are significantly shorter than the RE-Br bond lengths ( 3.2-3.3 A). A fifth, more distant bromide ( 3.5-3.9 A) is located on the C4 axis [191]. Eu + in the oxychlorides is coordinated similarly (RE-0 2.3-2.4 A, RE-Cl 3.0-3.2 A), but differs with respect to the position of the fifth chloride. The smaller size of the chlorides allows the fifth, axial chloride to approach more closely Eu " and enter the coordination sphere at a distance of 3.0 - 3.2 A [ 192]. 

Since only information on the energy spectrum and wave functions of the ground multiplets is necessary for interpretation and prediction of magnetic properties of lanthanide compounds, and bearing in mind an essential role of electron-phonon interaction, we shall confine ourselves in this case to semiphenomenological models of the crystal field which allow one to represent parameters Bpq as definite functions of structural parameters of the crystal lattice. All the models developed until recently are... 

---