Random alloy structure

The structure of the Cu/Pd bimetallic nanoclusters was investigated by XRD, XPS and EXAFS. The observed data showed that Cu/Pd bimetallic nanoclusters have a random alloy structure.However, recent precise analyses by EXAFS technique have indicated that these nanoclusters have a heterobond-philic structure in which the number of heterobonds (Cu-Pd bond) should be maximized. The concept of the heterobond-philic structure, where the heterobond is favored, is opposite to that of the homobond-philic structure, where the homobond (the bond between atoms of the same element) is favored. The core/shell and cluster-incluster are the examples of homobond-philic structures. PVP-capped Ni/Pd and Ni/Pt bimetallic nanoclusters could also be prepared by the modified alcohol... 

From the above examples, it is clearly evident that supported bimetallic nanoalloy catalysts have much more interesting catalytic properties compared to their monometallic catalysts. However, to exploit the catalytic properties of these nanoalloy catalysts, it is important to tune the structural properties appropriately. Example-1 demonstrates the synthesis strategies utilized to control the size, composition and nanostructure of supported Au-Pd catalysts. This in-tum has a dramatic effect on the catalytic activity and stability of these materials for the direct synthesis of H2O2 from H2 and O2. The catalyst with Au-Pd particles between 2-10 nm and a homogeneous random alloy structure is found to... 

We have studied the fee, bcc, and hep (with ideal eja ratio) phases as completely random alloys, while the a phase for off-stoichiometry compositions has been considered as a partially ordered alloy in the B2 structure with one sub-lattice (Fe for c < 50% and Co for c > 50%) fully occupied by the atoms with largest concentration, and the other sub-lattice randomly occupied by the remaining atoms. 

Second, using the fully relativistic version of the TB-LMTO-CPA method within the atomic sphere approximation (ASA) we have calculated the total energies for random alloys AiBi i at five concentrations, x — 0,0.25,0.5,0.75 and 1, and using the CW method modified for disordered alloys we have determined five interaction parameters Eq, D,V,T, and Q as before (superscript RA). Finally, the electronic structure of random alloys calculated by the TB-LMTO-CPA method served as an input of the GPM from which the pair interactions v(c) (superscript GPM) were determined. In order to eliminate the charge transfer effects in these calculations, the atomic radii were adjusted in such a way that atoms were charge neutral while preserving the total volume of the alloy. The quantity (c) used for comparisons is a sum of properly... 

AUGMENTED SPACE RECURSION METHOD FOR THE CALCULATION OF ELECTRONIC STRUCTURE OF RANDOM ALLOYS... 

In conclusion we propose ASR as an efficient computational scheme to study electronic structure of random alloys which allows us to take into account the coherent scattering from more than one site. Consequently ASR can treat effects such as SRO and essential off-diagonal disorder due to lattice distortion arising out of size mismatch of the constituents. 

Ffom a theoretical point of view, stacking fault energies in metals have been reliably calculated from first-principles with different electronic structure methods [4, 5, 6]. For random alloys, the Layer Korringa Kohn Rostoker method in combination with the coherent potential approximation [7] (LKKR-CPA), was shown to be reliable in the prediction of SFE in fcc-based solid solution [8, 9]. 

In an earlier study, Turkevich and Kim proposed gold-layered palladium nanoparticles (39). Three types of Au/Pd bimetallic nanoparticles, such as Au-core/Pd-shell, Pd-core/Au-shell, and random alloyed particles, are prepared by the application of successive reduction. Two kinds of layered Pd/Pt bimetallic nanoparticles were also reported by successive reduction (43). However, detailed analyses of the structure of these bimetallic nanoparticles were not carried out at that time. Only the difference of UV-Vis spectra between the bimetallic nanoparticles and the physical mixtures of the corresponding monometallic nanoparticles was discussed. 

A full conversion from the c(4x4) to c(2x2) phase should result in 0.5 ML of displaced Cu. Previous STM measurements are consistent with this expectation [35]. However, LEEM measurements taken with the sample at 400 K, indicate that there is only 0.23 ML of added material during the conversion. To account for this difference, Plass and Kellogg [82] propose a new model for the higher temperature c(2x2) phase in which some of the Cu remains randomly alloyed in the c(2x2) overlayer structure. This model is at odds with LEED I-V analysis of Hosier and co-workers [79], who concluded that the c(2x2) structure is a pure overlayer phase. The discrepancy is still not resolved, but may be due to the different manner in which the c(2x2) phase was prepared in the two studies. In the LEED I-V study the overlayer was prepared by depositing excess Pb and flashing the surface repeatedly to 700 K until the c(2x2) pattern was its strongest. In the LEEM study the c(2x2) structure evolved from the c(4x4) structure during Pb deposition at 400 K. 

The trends in Table I are self-evident. At very low coverages, Pb forms random lattice gas structures. The Pb atoms are substituted into the Cu surface on the (111), (100), and (110) crystal planes. Most likely, this is the case for the stepped surfaces as well. As the Pb coverage increases, the lattice gases condense into stable surface alloy structures that can be either random, as for the (111) face, or ordered, as for the (100) and (110) faces. Further increases in the Pb coverage results in de-alloying and various ordered overlayer stmctures. Upon completion of the most dense 2-D overlayer structures, 3-D islands grow. 

By courtesy of Dr. Colliex (1980) we report some peculiar aspects of magnetic domains in amorphous alloys which have been prepared either by e-gun or by thermal evaporation. Fig. 5 shows nearly aligned Bloch walls with cross-ties observed in YFe films after slow annealing before heating it appeared as a randomly rippled structure. Figs 6 to 9 show characteristic domains of YCo, TbCo, CeNi and DyNi films. Unfortunately their compositions were not determined. 

The structure of bimetallic nanoclusters depends on the preparation method and the kind of elements composing the bimetallic nanoclusters. Typical examples of structures are shown in Fig. 3.18. The random alloy indicates a structure in which the atoms of both elements are located in a completely random way, resulting in the formation of a kind of solid solution. The cluster-in-cluster indicates a structure in which the atoms of one element forms clusters and these clusters aggregate to form a larger cluster by including atoms of the other element which may form small clusters as well. In the core/shell structure, atoms of one element form a core and atoms of the other element surround the core to form the shell. The inverted core/shell is similar to the core/shell structure. The difference is the kind of element that forms the core and shell. The element forming a core in the core/shell structure now forms the shell in the inverted core/shell structure and vice versa. Thermodynamically, the core/shell structure is stable, while the inverted core/shell is metastable. 

---