Fermi intermetallic compounds

Y. Onuki and A. Hasegawa, Fermi surfaces of intermetallic compounds 1... 

The understanding of the near-edge absorption features of intermetallics is far from satisfactory. Band-structure effects are expected to play a role in determining the observed absorption characteristics, but calculations are not available for intermetallic compounds at sufficient energy above the Fermi level. 

The factors determining the particular structure adopted hy an intermetallic compound or, indeed, whether such a compound exists at all as a single-phase material, have been the subject of much discussion for a considerable period of time. The Hume-Rothery rules for electron compound formation will he very familiar and are related physically to the size of the Fermi sphere in the appropriate Brillouin zone. For example, electron compounds are expected for valence electron concentrations of , fj and l for the bcc, y-brass and cph structures, respectively. The interplay of other factors such as the atomic size, solubility and crystal structure of the components on the formation of intermetallic compounds has been considered in considerable detail by many workers, including Yao (1962), who suggested that transition metal binary systems could be classified into groups according to an excess energy dE expressed as... 

Onuki, Y., and A. Ilascgawa, 1995, Fermi surfeces of intermetallic compounds, in Handbook on the Physics and Chemistry of Rare Earths, Vol. 20, eds K.A. Gschneidner Jr and L. Eyring (North-Holland, Amsterdam) ch. 135. 

The recent observations of Coulomb transitions in actinide intermetallic compounds (McEwen et al. 1990, Osborn et al. 1990) are of particular interest because the 5f electrons are on the boundary between localised and itinerant behaviour. The neutron results give direct evidence of the persistence of strong intra-atomic correlations in, e.g., the heavy-fermion compound UPtj. The theoretical challenge is to reconcile this with the substantial evidence that the f electrons also form a coherent Fermi liquid (Fulde et al. 1988, Zwicknagl 1988). 

The study of Coulomb transitions is especially valuable in actinide metals and intermetallic compounds (McEwen et al. 1990, Osborn et al. 1990). Because of the greater radial extent of the 5f charge distribution, the actinide f electrons tend to hybridise more strongly with band electron states than their lanthanide counterparts. In a number of actinide metals, it is evident that the f electrons contribute to the cohesive energy through the formation of 5f bands, either by direct f-f overlap, as in a-uranium, or by hybridisation with conduction bands, as in URUj or URhj (Oguchi and Freeman 1986, Johansson et al. 1987). In these cases, relativistic band theory is successful in predicting lattice constants, photoemission and Fermi surfaces (Arko et al. 1985) provided the f states are included as itinerant. 

Although it is possible to use XPS to distinguish between homogeneous and inhomogeneous mixed valence (Wertheim et al. 1978, 1980), when quoting average valences one has always to take into account that the electronic properties of the surface of both stable and mixed valence intermetallic compounds may differ from the bulk especially whenever the 4f level lies close to the Fermi level. The role of the escape depth, i.e, dependence of the mean-free-path of the photoelectrons from their kinetic energy, must be taken into account. 

---