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Surface structures of In-Pd intermetallic compounds. I. Experimental study of In thin films on Pd(111) and alloy formation

Abstract : A combination of experimental methods was used to study the structure of In thin films deposited on the Pd(111) surface and the alloying behavior. X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy results indicate that surface alloying takes place at room temperature. Below 2 monolayer equivalents (MLEs), the LEED patterns show the formation of three rotational domains of InPd(110) of poor structural quality on top of the Pd(111) substrate. Both core-levels and valence band XPS spectra show that the surface alloy does not yet exhibit the electronic structure characteristic of the 1:1 intermetallic compound under these conditions. Annealing the 1 MLE thin film up to 690 K yields to a transition from a multilayer InPd near-surface intermetallic phase to a monolayer-like surface alloy exhibiting a well ordered ( 3‾‾√×3‾‾√ ) R30∘ superstructure and an estimated composition close to In2Pd3. Annealing above 690 K leads to further In depletion and a (1 × 1) pattern is recovered. The ( 3‾‾√×3‾‾√ ) R30∘ superstructure is not observed for thicker films. Successive annealing of the 2 MLE thin film leads the progressive disappearance of the InPd diffraction spots till a sharp (1 × 1) pattern is recovered above 690 K. In the high coverage regime (from 4 to 35 MLE), the formation of three rotational domains of a bcc-In7Pd3 compound with (110) orientation is observed. This In-rich phase probably grows on top of interfacial InPd(110) domains and is metastable. It transforms into a pure InPd(110) near-surface intermetallic phase in a temperature range between 500 and 600 K depending on the initial coverage. At this stage, the surface alloy exhibits core-level chemical shifts and valence band (VB) spectra identical to those of the 1:1 InPd intermetallic compound and resembling Cu-like density of states. Annealing at higher temperatures yields to a decrease of the In concentration in the near-surface region to about 20 at.% and a (1 × 1) LEED pattern is recovered.
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G. M. Mcguirk, J. Ledieu, É. Gaudry, M.-C. De Weerd, V. Fournée. Surface structures of In-Pd intermetallic compounds. I. Experimental study of In thin films on Pd(111) and alloy formation. Journal of Chemical Physics, American Institute of Physics, 2014, 141, 084702 (10 p.). ⟨10.1063/1.4892408⟩. ⟨hal-01075808⟩



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