Soft X-ray Emission and Electronic Structure of Alloys

Soft X-ray band-emission data for solid binary solutions are reviewed, with particular reference to aluminium alloys, and their interpretation is discussed in relation to the rigid-band and virtual-bound-state models. The emission spectra only rarely support the rigid-band model. Measurements on alloys of the noble and transition metals with aluminium are interpreted in terms of virtual-bound or resonance-bound states, and appear to support the concept of local electron densities. The electronic structure of alloys has been the subject of much research commencing at about the time of the well-known text by N. F. Mott and H. Jones (( The theory of the properties of metals and alloys )) which appeared in 1936. It is interesting to weigh the advances since then : they have been many, and have derived from both experimental and theoretical work. However, the theory of the solid state is still not well understood and even for pure metals there are considerable difficulties in the interpretation of experimental data ; for alloys in particular a satisfactory theory has continually eluded physicists and metallurgists. The questions that arise, when two different kinds of metal atom are alloyed to form a disordered array, are : how much is the band structure affected, and can we still speak of a Brillouin-zone structure once we have destroyed the translational symmetry of the lattice ? In 1936 Sir Nevi11 Mott wrote <( The question has not been cleared up yet in a satisfactory way, but it seems fairly certain from the evidence (: evidence associated with the Hume-Rothery rules and their explanation in terms of a rigid-band model) that the break in periodicity may be considered not to affect the zone structure very much n. In 1970 the question has still not been cleared up, nor in fact is it much nearer to solution. (*) This paper was prepared while the author was on leave of absence at the Laboratorium fiir Festkorperphysik, Eidg. Technische Hochschule. Ziirich, Suisse. Soft X-Ray Emission. Soft X-ray band emission from metals provided the first experimental proof of the fundamental band theory of crystalline solids, and a considerable impetus was given to the early development of the subject, in connection with the electronic structure of metals and alloys, by the work of Farineau [I], Skinner [2] and Cauchois [3] ; lasting from about 1932 to 1948. As early as 1933 Jones, Mott and Skinner successfully interpreted the intensity distributions observed [4] for the K and L spectra of simple metals (generally meaning metals whose electronic structures do not involve d-bands). The theoretical simplicity of that interpretation, which showed for the low-energy region of the emission band that for K-emission I(E) cc E3I2 (1)