Random alloys

Some semiconductors with compositions close to ABq Gq are known to become ordered. This results in changes in the gap, and electrical and optical properties, compared to random alloys of the same composition. 

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. 

In summary, we have demonstrated the possibility of calculating the phase stability of a magnetic random alloy from first principles by means of LMTO-CPA theory. Our calculated phase diagram is in good agreement with experiment and shows a transition from the partially ordered a phase to an hep random alloy at 85% Co concentration. 

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... 

For the description of the random Hamiltonian we employ TB-LMTO formalism in the most tight binding representation . The Hamiltonian for the binary random alloy takes the form ... 

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. 

LA. Abrikosov, A.V. Ruban, B. Johansson, and H.L. Skriver, Total energy calculations of random alloys Connolly-Wiliams and CPA methods, in Stability of Materials , Series E Applied Sciences, A. Gonis, P.E.A. Turchi, and J. Kudrnovsky, ed., Kluwer Academic Publishers, the Netherlands (1996). 

EFFECTIVE PAIR INTERACTIONS IN RANDOM ALLOYS BY DIRECT CONFIGURATIONAL AVERAGING... 

I.A. Abrikosov and H.L.Skriver, Self-consistent linear-muffin-tin-orbitals coherent-potential technique for bulk and surfaces calculations Cu-Ni, Ag-Pd, and Au-Pt random alloys, Phys. Rev. B 47, 16 532 (1993). 

We have calculated the Bloch Spectral Functlonii (BSF) at the Fermi energy, AB(k, F), for fee CucPdi.c and CUcPti.c, random alloys for various value of c. Die site potentials used have been obtained ab initio via the relativistic LDA-KKR-CPA method at the lattice parameters corresponding to the total energy minimum. 

Figure 1. Calculated Bloch Spectral Funcdon at the Fermi energy along high symmetry directions for CuPt and CuPd random alloys at the concentradons displayed in the figure.

In eq. (1) the ETT order parameter z = s(p-Pc) measures, in a convenient direction, the chemical potential from that corresponding to the ETT. From the values given in Table I for the above s and q, we readily see that the occurrence of the ETTs discussed in this paper always implies an increase of the alloy free energy. Thus, CuPt random alloys, that just below and above the equiatomic concentration present both the relevant ETT s, are less stable than CuPd or AgPd and, thus more likely to be destabilised. Moreover, the proximity to both the critical concentrations implies large contributions to the BSE from the X and L points. Now, the concentration wave susceptibility, Xcc(q). as observed by Gyorffy and Stocks, is proportional to... 

Table L The ETT s discussed in this p r far CuPt, CuPd and AgPd3 random alloys. The meanings of the various quandties repotted are explained in the text.

Figure 2. Calculated length of the Fenni wavevector along the line, kp, for AgPd, CuPt and CnPd random alloys versns the noble metal atomic concentration, c. Dashed lines are drawn as a gnide for the eyes. The solid line indicates the value, V2/2, at which kp is exactly commensurate with LIq or LI2 orderings.

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]. 

M1M2 - All kinds of bimetallic nanoparticles including random alloy nanoparticles. In this case Mi and M2 should be arranged in an alphabetical order. 

Random Alloy 2) Ctuster-in-Cluster 3) Core/Shell 4) Inverted Core/Shell... 

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. 

Both L coefficients and / factors can, in principle, be calculated from microscopic models. For the evaluation of L,j, the random-alloy model [J. R. Manning (1968) A. R. Allnatt, A. B. Lidiard (1987)] is sometimes used. For the evaluation of thermodynamic factors, one takes advantage of the empirical rule that in extended solid solutions AO-BO, the cation vacancy concentration and the oxygen potential are related to each other as... 

Let us present D explicitly for the condition d//0 = 0, omitting all details of the lengthy derivation. By application of Manning s random-alloy model [A. R. Allnatt, A.B. Lidiard (1987)], and by inserting Eqns. (5.126) and (5.131) into Eqn. (5.132), for a constant oxygen potential across the diffusion zone, a Darken type equation is obtained... 

In the last decade an abundant literature has focused more and more on the properties of low-symmetry systems having large unit cells which render unwieldy the traditional description in terms of the Bloch theorem. Low-symmetry systems include compUcated ternary or quaternary compounds, man-made superlattices, intercalated materials, etc. The k-space picture becomes totally useless for higher degrees of disorder as exhibited by amorphous materials, microcrystallites, random alloys, phonon-induced disorder, surfaces, adsorbed atoms, chemisorption effects, and so on. 

Fig. 5. Surface energy curves for a monolayer of a random alloy on surfaces of pure metals.

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