NaCl-type compounds

A first group of hydrides (ionic hydrides) is formed with the more electropositive elements of the 5-block of the Periodic Table. This group of hydrides includes the salt-like MeH (Me+H ) NaCl-type compounds of the alkali metals and the di-hydrides (Co2Si-type) formed by the divalent metals Ca, Sr, Ba and also by Eu and Yb. The thermal stability of these hydrides decreases from Li to Cs and from Ca to Ba the chemical reactivity on the contrary increases from Li to Cs and from Ca to Ba. While the reaction of NaH with water is very violent, the reaction of LiH or CaH2 can be used for a portable source of hydrogen. 

Within other classes of actinide compounds, as, e.g. intermetalhcs or NaCl-type compounds, no data on highly radioactive actinides have been reported so far (with the exception of PuSb measured by Baptist et al. ). 

The phenomenon of superconductivity is common in several particular types of compounds. Thus more than two dozen binary compounds with the fee sodium chloride (NaCl) stracture are superconducting. The carbides AC and nitrides AN, such as NbN with Tc = 17 K, have the highest transition temperatures of this group, and the metallic A atoms with values above 10 K were Nb, Mo, Ta, W, and Zr. The NaCl-type superconductors are compositionally stoichiometric but not structurally so. hi other words, these compounds have a small to moderate concentration of vacancies in the lattice. For example, YS has 10% vacancies, which means that its chemical formula should properly be written 0,980.9. Nonstoichiometric NaCl-type compounds such as Tai.oCo.ye also exist. Ordinarily the vacancies are random, but sometimes they are ordered. 

Fig. 45. ZF and LF spectra of different uranium NaCl-type compounds (UN, UP, UAs, USb) below their Neel temperatures. The fits to the data are Lorentzian Kubo-Toyabe functions (static for UN, UP and UAs, dynamic for USb (see discussion in text). Extended from Asch (1990).

Fig. 46. Crystal structure (unit cell) and muon location for NaCl-type compounds MX. Solid circles, M atoms open circles, X atoms.

In this section we treat some physical properties of the NaCl-type compounds that reflect the influence of the crystal electric field on the energy of the J ground state of the cationic 4f" levels. Due to the small radius of the 4f orbitals the crystal field of the anion neighbors acts as a small perturbation only. At room temperature no particular f orbitals are favored so that the crystal structure, for instance, is determined by the cation size only. Below say 100 K the influence of the crystal field on the physical properties becomes evident. In contrast to certain rare earth phosphates, arsenates and vanadates, however, crystal-structure distortions never occur due to the Jahn-Teller effect alone but are always coupled with a magnetic transition, that is, the structural changes are due to a magnetostrictive effect. However, the crystal field influences and may even inhibit magnetic transitions as in the case of certain Tm compounds. 

By diagonalizing the crystal-field Hamiltonian the eigenvalues and eigenfunctions are obtained with x as a parameter. Lea et al. (1962) have calculated them for / = 2 through / = 8. For the NaCl-type compounds, where each Ln ion is coordinated by an octahedron of six anions with charges Ze, the point-charge model leads to... 

The electron transfer from a partially filled band is obtained by multiplying Nj.j by the fractional occupation of the T band. The purely ionic model is transformed by the latter two equations into a model containing a bonding charge that reproduces the essential features of the electronic structure of NaCl-type compounds when the hybridization is reasonably weak. Whereas the atom and angular-momentum projected densities of states in the unhybridized model arose simply from the T unhybridized bands, they now contain additional parts due to hybridization with the T bands. Dashed lines have been added to complete the hybridized density of states in fig. 58. The number of electrons transferred between the atoms by this interaction is estimated from eq. (68) to be, with UN as example,... 

In all NaCl-type compounds of the lanthanides and actinides, we should expect (with the exception of S states) anisotropic exchange forces, which are sometimes hidden by the anisotropy which originates from the crystal-field splitting. A more... 

Specific-heat experiments by Stewart et al. (1991) have been analyzed and give an electronic specific heat of 130 + 10 mJ mol K and no clear evidence for the 30 K transition was found. This is the highest value of y yet found for a NaCl-type compound and speculation that it may be a medium-weight fermion system cannot as yet be excluded. 

The role of the crystal-field interaction remain obscure in all these systems, both for actinides and cerium. Whereas in the other lanthanide NaCl-type compounds, LnX and LnZ, the easy directions are given by straightforward crystal-field considerations and the value of the crystal-field potential varies in a systematic way across the lanthanide series, this is not the case in the materials discussed here. The crystal-field energy levels can be measured in the cerium compounds with neutron inelastic scattering, but have not been observed in the uranium (or higher) actinides. It is assumed that this inability to observe directly the crystal-field levels is because they are strongly broadened by the interaction between the 5f and conduction electrons. This has been the subject of much work by Cooper and his collaborators. 

Moving away from the NaCl-type compounds it becomes difficult to perceive common themes. We have already noted the complexities in structures such as those found in UNiSn and UPdSn. The latter is unusual in that the moments rotate as a function of temperature, something that has not been observed previously in actinide... 

It is well known that the majority of refractory carbides and nitrides exhibit superconductivity. For NaCl-type compounds the critical temperature of transition to the superconducting state rises with the increase of transition element group number. This spurred a number of researchers to study the band structure of a series of nitrides (VN, NbN, TaN, CrN, MoN, WN,...) and to calculate the temperature with the objective of finding the compounds with the maximum - see Chapters 2 and 3. 

A large number of metaffic compounds with the NaCl-type structure are found at the AnX stoichiometry. These form when X is a chalcogenide, a pnictide or carbon. An3X4 compounds having the cubic Th3P4-type structure frequently coexist with the NaCl-type compounds in systems of the actinides with the chalcogenides and pnictides. 

Fig. 28.28. The structural units used to describe the ordered NaCl-type compounds. The 0M4 octahedron in a) is designated as type 1, that in b) as type 2. Small circles represent R ions, large circles M ions.

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