EXPERIMENTAL AND THEORETICAL SOFT X-RAY EMISSION SPECTRA OF Cu-Pd SUBSTITUTIONAL ALLOYS

The electronic density of states on Cu and Pd sites in Cu-Pd substitutlonal alloys has been investigated by soft x-ray emission spectroscopy and SCF-KKRCPA calculations. The results indicate that Cu-Pd can be described as a common band alloy with no Pd virtual bound state. Possible origins for the larger theoretical Pd bandwidth with respect to the experimental one are discussed. In a recent paper Winter et ai. [ I 1 reported self consistent field calculations of the electronic structure of substitutionally disordered Cu-Pd alloys using the KKR coherent potential approximation (SCF-KKRCPA). They found that off-diagonal disorder (i.e. randomness in the site-to-site hopping) dominates diagonal disorder (i.e. randomness in the d-orbital energies on each site). An important consequence of this is that Cu-Pd is, in the lanpuage of Winter et al.. a common band alloy4 there is one set of d states which occurs on both sites with different local projections. This is to be distinguished from ripid band alloys (in which the local densities of states are the same, or very similar. on both sites, e.9. Nb-No [ZI), and from split band systems (in which there are two non-degenerate sets of d bands, each associated with one of the atomic components, e.g. Cu-Ni C31). In particular, no Pd virtual bound state (UBS) lying above the Cu d bands was found in the Cu-rich alloys. This Pd VBS tends to occur in model tight-binding CPA calculations (which neglect off-diagonal disorder), and, indeed, much of the experimental x-ray photoemission (XPS) data 141 on Cu-Pd has been interpreted as showing a Pd VBS. However, Winter et al. also made first principles calculations of the XPS spectra and obtained excellent agreement with experiment. They concluded that XPS data do not support the UBS hypothesis. In previous work 151 we showed how soft x-ray emission (SXE) spectra can be used to disentangle the local densities of states (LDOS) in binary alloys. A specifically useful aspect of such studies is that VBS systems show a characteristic increase in the emission band width with the concentration of the associated atomic species. If a Pd UBS is formed in Cu-Pd alloys, therefore, the width of the Pd L3 emlssion spectrum should increase with Pd content, similarly to the Ni L3 spectrum in Cu-Ni alloys. In Figure l(a) we show the experimental Cu and Pd L 3 emlssion spectra for several alloys, obtained as described previously t51. Also shown in Figure l(b) are theoretical spectra calculated from the SCF-KKRCPA results of Winter et al. 111 using the theory given by Durham et al. (including core hole lifetime broadening) C51. Note first that the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879192 C9-1060 JOURNAL DE PHYSIQUE width of the Pd spectra shows no significant increase in as the Pd content rises -in fact it slightly decreases. This is the opposite of VBS behaviour. Moreover. all the spectra shift towards the Ferml energy with increasing Pd concentration. This is simply because electrons are being removed from the system and the Fermi energy falls in roughly rigid band-like feshion, again behaviour which does not occur in VBS systems. Both of these trends are apparent in the theoretical spectra, and we therefore confirm that no Pd VBS occurs in Cu-Pd alloys.