Investigation of the (100) Surface of the Ce₃Pd₂₀Si₆ Intermetallic Cage Compound

Abstract

Intermetallic cage compounds have crystal structures characterized by three-dimensional frameworks of host atoms forming cages that may be filled with guest atoms. They can be considered as a class of complex metallic alloys (CMA), a broad family of crystalline materials having large or giant unit cells in which a cluster substructure exists.Their bulk physical properties have attracted interest in recent years, particularly as several of them are considered as promising thermoelectric materials. As a different type of CMAs, they may exhibit interesting surface properties which have not been explored until now. Here, we report a detailed investigation of the (100) surface of the Ce 3 Pd 20 Si 6 compound and describe its relationship with the three-dimensional cage framework. The study has been carried out using both experimental surface science methods under ultrahigh vacuum conditions and theoretical calculations based on the Density Functional Theory (DFT). After annealing at moderate temperatures, the surface exhibits two different types of terminations, one of them being a metastable (2x2) reconstruction. Annealing at higher temperatures leads to a single termination with a (1x1) structure. Twelve different surface models have been considered for DFT calculations. The surface phase diagram deduced from first principles calculations with the chemical potentials as variables provides insight into the relative stability of the different surface models. Combining experimental and theoretical results leads to the conclusion that the surface forms at corrugated planes of the bulk structure that preserve intact the Pd 12 Si 6 cages, with additional Pd surface atoms. It is concluded that despite this Pd-based cage compound being clearly metallic, covalent-like interactions between Pd and Si atoms on the host cage structure significantly influences its surface structure.

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