Surface structures of In-Pd intermetallic compounds. II. A theoretical study

Abstract

The (110) surface of the InPd intermetallic compound and the In-Pd surface alloy properties are investigated in the framework of the density functional theory, within the projector augmented plane-wave method. Surface segregation is calculated to be energetically unfavorable at stoichiometric InPd(110) surfaces, while indium antisites are shown to segregate to the surface in off-stoichiometric InPd(110) systems. Concerning surface alloys obtained by burying In-doped Pd layers in Pd(111), we demonstrated that the most stable ones are those presenting atomic indium concentrations below 50 at. % (11 at. %, 25 at. %, 33 at. %). According to our calculations, the In-doped Pd layers with concentration above or equal to 50% lead to In-doped Pd multilayers, each presenting an atomic indium concentration below 50 at. %. Alloying and segregation effects in InPd intermetallic compound and In-Pd surface alloys clearly agree with the larger bonding strength of In-Pd (-0.44 eV) compared to In-In (-0.29 eV) and Pd-Pd (-0.31 eV).

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

“…This agrees with the theoretical prediction of a negative mixing enthalpy for the In x Pd 1 − x solid solution in the dilute In limit reported in Ref. 25. In the low coverage regime (θ ≤ 2 MLE), the LEED patterns are consistent with the formation of three rotational domains of InPd(110) on top of the Pd(111) substrate.…”

“…This was interpreted as a consequence of the larger thermodynamic driving force for Ga-Pd compound formation compared to Zn-Pd based on the calculated cohesive energies. The onset alloying temperature was not determined in the present study but is lower or equal to 300 K. This is consistent with the cohesive energy calculated for the InPd compound (−3.57 eV/atom) 25 which is intermediate between that of PdGa (−3.9 eV/atom) 19,40 and ZnPd (−3.0 eV/atom). 41 Concerning the orientation of the surface alloys, we found that the cubic 1:1 InPd NSIP adopts a (110) orientation with respect to the Pd(111) substrate whereas the 1:1 tetragonal ZnPd NSIP adopts a (111) orientation.…”

“…31,35,36 Calculations of segregation energies indicate that an In-doped layer buried in a Pd(111) substrate is preferentially located at the surface, therefore we expect this surface alloy to be confined in the outermost plane at this stage. 25 When the film is further annealed above 690 K, the In content in the near-surface region decreases and a (1 × 1) pattern is recovered in our case.…”

“…This conclusion is consistent with the results of a theoretical study of InPd surface alloys using first-principles calculations demonstrating that In-doped Pd layers on Pd(111) are energetically more stable for In concentrations lower than 50 at.%. 25 The formation of metastable phases with a global 1:1 composition is common to the related Zn/Pd(111) 10,16,18,27,38 and Ga/Pd(111) 19 systems. However, the composition changes are more gradual in the cases of Ga/Pd(111) and In/Pd(111) compared to Zn/Pd(111) for which a more pronounced plateau in the surface composition was observed between 400 and 550 K. For Zn/Pd(111), the 1:1 plateau correlates with the formation of an ordered (2 × 2) surface structure corresponding to 3 rotational domains of ZnPd(111)-(2 × 1) 27,39 whereas for Ga/Pd(111) the surface alloy is disordered in the same temperature range.…”

“…We also discuss our experimental results in the light of a joint theoretical study of InPd surface alloys using first-principle calculations. 25…”