Alloys electronic properties

We shall first review the basic principles of VASP and than describe exemplary applications to alloys and compounds (a) the calculation of the elastic and dynamic properties of a metallic compound (CoSi2), (b) the surface reconstruction of a semiconducting compound (SiC), and (c) the calculation of the structural and electronic properties of K Sbi-j, Zintl-phases in the licpiid state. 

Interpretation of this observed correlation between a lowered affinity of the metal surface to oxygen and a higher rate of ORR measured at a Pt shell over a Pt-alloy core has also been at the center of recent theoretical work, based primarily on DFT calculations of electronic properties and surface bond strengths for a variety of expected ORR intermediates at metal and metal alloy catalysts. The second part of this chapter contains a discussion of these valuable contributions and of outstanding issues in tying together this recent theoretical work and ORR experimental data. 

Mukeijee S, Srinivasan S, SoriagaMP, McBreen J. 1995. Role of structural and electronic properties of Pt and Pt alloys on electrocatalysis of oxygen reduction. J Electrochem Soc 142 1409-1422. 

This allows a direct influence of the alloying component on the electronic properties of these unique Pt near-surface formations from subsurface layers, which is the crucial difference in these materials. In addition, the electronic and geometric structures of skin and skeleton were found to be different for example, the skin surface is smoother and the band center position with respect to the metallic Fermi level is downshifted for skin surfaces (Fig. 8.12) [Stamenkovic et al., 2006a] owing to the higher content of non-Pt atoms in the second layer. On both types of surface, the relationship between the specific activity for the oxygen reduction reaction (ORR) and the tf-band center position exhibits a volcano-shape, with the maximum... 

Toda T, Igarashi H, Watanabe M. 1998. Role of electronic property of Pt and Pt alloys on electrocatalytic reduction of oxygen. J Electrochem Soc 145 4185-4188. 

Manninen, S., Honkimaki, V., Hamalainen, K., Laukkanen, J., Blaas. C., Redinger, J., McCarthy, J. and Suortti, P. (1996) Compton-scattering study of the electronic properties of the transition-metal alloys FeAI, CoAI, and NiAl, Phys. Rev., B53,7714-7720. 

These two features exemplify the behavior of interface states, which are states that are not seen for either component of a bimetallic surface alloy but which exist because of the abrupt change in electronic properties at the interface. 

The effective mass of the electrons changes due to lattice strain, alloy additions, radiation damage, phase transformation, and phase content, directly relates to the ability to use electronic property measurements to assess microstructure phase stability. Electronic properties, such as thermoelectric power coefficients, resistivity and induced resistivity measurements, have a demonstrated correlation to solute and phase content, potential phase transformations, as well as residual strain. 

Metastable amorphous materials can be produced by the rapid quenching of melts in the form of metallic alloys with glassy structures [149]. These materials have attracted the attention of metallurgists, physicists, and, recently, chemists because of their exceptional properties (easy magnetisation, superior corrosion resistance, high mechanical toughness, interesting electronic properties) [150]. The use of these materials in catalysis was reported some years ago [151]. 

If we consider Ni as an active site for Ni-based materials, changing the environment in which the ion is immersed is expected to influence its electronic properties. This is in principle the reason for testing a series of alloys or intermetallic compounds of Ni. On the other hand, on changing the environment, bond lengths will also be modified and this will modify the actual concentration of active sites, in turn determining the active surface area. A few examples can better illustrate these concepts. 

Indeed, eobalt and a promoter metal may form an integral metal particle deposited on the support oxide, altering the electronic properties of the surface cobalt metal atoms (Figure 4C). Depending on the promoter element added to the Co cluster, alloying might lead to an increased catalyst activity, selectivity, as well as stability. 

More recently, Stamenkovic et al. [95,107] reported on the formation of Pt skins on Pt alloy electrocatalysts after high-temperature annealing. Pt skins were reported to exhibit strongly enhanced ORR activity. It was argued that the electronic properties of the thin Pt layer on top of the alloy alter its adsorption properties in such a way as to reduce the adsorption of OH from water and therefore to provide more surface sites for the ORR process (see Section 5.2 in Chapter 4 for a detailed discussion of skin catalysts, compare also Section 4.1.5 in the present Chapter). 

In the apparent absence of a correlation between hydrogenation activity and the bulk electronic properties of alloys, Sachtler et al. [321,322,... 

---