Studies of valence of selected rare earth silicides determined using Si K and Pd/Rh L2,3 XANES and LAPW numerical studies

Abstract We report on the investigation of Si and Pd/Rh chemical environments using X-ray Absorption Near Edge Spectroscopy in two different families of rare earth silicides R 2 PdSi 3 (R = Ce, Nd, Tb, Dy, Ho, Er) and HoRh 2− x Pd x Si 2 ( x  = 0, 0.5, 0.75, 1.0, 1.5, 1.8, 2.0). The Si K, Pd L 3 and Rh L 3 absorption edges were recorded in order to follow their changes upon the variation of 4f and 4d5s electron numbers. In both cases it was found that the Si K edge was shifted ≈0.5 eV toward lower energies, relative to pure silicon. In the first family, the shift decreases with increasing number of f-electrons, while the Si K edge remains constant upon rhodium–palladium substitution. In all cases the Pd L 3 edge was shifted to higher energies relative to metallic Pd. No visible change in the Pd L 3 position was observed either with a varying 4f electron count or upon Pd/Rh substitution. Also, the Rh L 3 edge did not change. For two selected members, Ho 2 PdSi 3 and HoPd 2 Si 2 , the Wien2K’09 (LDA +  U ) package was used to calculate the electronic structure and the absorption edges. Si K edges were reproduced well for both compounds, while Pd L 3 only exhibited a fair agreement for the second compound. This discrepancy between the Pd L 3 theory and experiment for the Ho 2 PdSi 3 sample can be attributed to the specific ordered superstructure used in the numerical calculations. The observed changes indicate that despite possessing a formal inter-metallic character, the chemical bond between the R–Si and R–Pd interactions are different. The variation and the direction of the chemical shift of the Si K edge suggests a weak ionic character of the R–Si bonds, in agreement with the localized character of the 4f electrons. In turn, the changes of the Pd/Rh edge are consistent with a metallic band that is affected by its long range chemical environment.