Smart interfacial bonding alloys Page: 10 of 30
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compressive stress due to the substrate in these areas is larger than that applied to the Ag
atoms in the nearly atop sites at the maxima regions. This is confirmed by the smaller local
lattice spacing in these areas [12]. Modulations in the local lattice spacing of overlayer
systems has also been predicted theoretically both in the modeling of lattice mismatch
accommodation within the Frenkel-Kontorova model [24, 25] and in embedded atom method
simulations of the coincidence lattice structure of Ag on Cu(111) [26]. Incorporation of Cu
into these highly strained areas of the Ag film results in the largest strain relief.
Strain relief also determines the film structure and local composition as the Cu
concentration increases. Within the alloy domains, the atoms reside in hollow binding sites
and are locally strain relieved. The existence of two types of alloy domains (Figure 4b)
corresponds to the two types of hollow sites available for occupation, the fcc and the hcp.
Energetically, only type of hollow is preferred, (which most likely is the hcp domain since
Cu is known to reside in Ru hcp sites [10]), as is indicated by only one domain existing as the
Cu-Ag mixture reaches 1 to 2 composition. The question remains as to why the minority
alloy domain forms, (region B of Figure 4a), for Xcu/XAg < 0.5. Below this saturation
composition, there remains unaccommodated Ag. The film phase segregates into regions of
the alloy and regions of pure Ag. In order to minimize the total energy of the system, this
phase segregation results in the formation of the two types of alloy domains sparated by Ag
domain walls. The Ag in the walls occupy transition sites between the hcp and fcc hollows.
These sites are of lower coordination and, following the arguments above, allow a greater
degree of strain relaxation as compared to hollow sites. Therefore, the energy gain in
reducing the strain in the Ag phase overcomes the cost of forming the minority alloy domain
and the interfaces between the two phases.
In conclusion, we have demonstrated that the reduction of strain can be the driving
mechanism in the formation of alloys in multi-component thin film systems. This has been
shown for the case of monolayer films of Ag and Cu on Ru(0001). Although Ag and Cu are
immiscible in their bulk phases, their lattice mismatches with the Ru substrate leads to the
formation of a strain relieved Ag-Cu alloy with near 2:1 stoichiometry. We have also
observed that strain relief also plays a dominant role in the nucleation and growth of the Ag-
Cu domains. This phenomena is expected to be important in superlattice systems in which
stresses of opposite signs are applied to the constituents of the film. It also may be a
mechanism by which novel materials can be produced.
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Hwang, R. Q.; Hamilton, J. C. & Houston, J. E. Smart interfacial bonding alloys, report, April 1, 1999; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc703793/m1/10/: accessed April 30, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.