We have systematically studied band bowing, band edges, and electronic properties of both zinc-blende and wurtzite Cd1–x Zn x S solid solutions by using a special quasirandom structures approach combined with hybrid DFT calculations. Hybrid DFT gives a more accurate description of the lattice constants, formation enthalpies, and electronic structures of the parent semiconductors than standard DFT. Alloying CdS with ZnS causes a downward band bowing that is dominated by volume deformation. The conduction- and valence- band edges straddle the redox potentials of (O2/H2O) and (H+/H2) over the whole Zn concentration range. The high photocatalytic activity of Cd1–x Zn x S is due to the elevation of the conduction band minimum (CBM). The optimal Zn content is around 0.5, determined as a result of balancing the elevation of the CBM and the widening of the band gap. The valence bandwidth increases with Zn content and thus raises the mobility of photogenerated holes, which may be related to photocorrosion and lead to the leaching of Zn in Cd1–x Zn x S photocatalyst during water splitting.

Due to their high strength and advantageous high-temperature properties, tungsten-based alloys are being considered as plasma-facing candidate materials in fusion devices. Under neutron irradiation, rhenium, which is produced by nuclear transmutation, has been found to precipitate in elongated precipitates forming thermodynamic intermetallic phases at concentrations well below the solubili-ty limit. Recent measurements have shown that Re precipitation can lead to substantial hardening, which may have a detrimental effect on the fracture toughness of W alloys. This puzzle of sub-solubility precipitation points to the role played by irradiation induced defects, specifically mixed solute-W interstitials. Here, using first-principles calculations based on density functional theory, we study the energetics of mixed interstitial defects in W-Re, W-V, and W-Ti alloys, as well as the heat of mixing for each substitutional solute. We find that mixed interstitials in all systems are strongly attracted to each other with binding energies of À2.4 to À3:2 eV and form interstitial pairs that are aligned along parallel first-neighbor h111i strings. Low barriers for defect translation and rotation enable defect agglomeration and alignment even at moderate temperatures. We propose that these elongated agglomerates of mixed-interstitials may act as precursors for the formation of needle-shaped intermetallic precipitates. This interstitial-based mechanism is not limited to radiation induced segregation and precipitation in W-Re alloys but is also applicable to other body-centered cubic alloys. Published by AIP Publishing. [http://dx.

A systematic study of stacking fault energy (γ(SF)) resulting from induced alias shear deformation has been performed by means of first-principles calculations for dilute Ni-base superalloys (Ni(23)X and Ni(71)X) for various alloying elements (X) as a function of temperature. Twenty-six alloying elements are considered, i.e., Al, Co, Cr, Cu, Fe, Hf, Ir, Mn, Mo, Nb, Os, Pd, Pt, Re, Rh, Ru, Sc, Si, Ta, Tc, Ti, V, W, Y, Zn, and Zr. The temperature dependence of γ(SF) is computed using the proposed quasistatic approach based on a predicted γ(SF)-volume-temperature relationship. Besides γ(SF), equilibrium volume and the normalized stacking fault energy (Γ(SF) = γ(SF)/Gb, with G the shear modulus and b the Burgers vector) are also studied as a function of temperature for the 26 alloying elements. The following conclusions are obtained: all alloying elements X studied herein decrease the γ(SF) of fcc Ni, approximately the further the alloying element X is from Ni on the periodic table, the larger the decrease of γ(SF) for the dilute Ni-X alloy, and roughly the γ(SF) of Ni-X decreases with increasing equilibrium volume. In addition, the values of γ(SF) for all Ni-X systems decrease with increasing temperature (except for Ni-Cr at higher Cr content), and the largest decrease is observed for pure Ni. Similar to the case of the shear modulus, the variation of γ(SF) for Ni-X systems due to various alloying elements is traceable from the distribution of (magnetization) charge density: the spherical distribution of charge density around a Ni atom, especially a smaller sphere, results in a lower value of γ(SF) due to the facility of redistribution of charges. Computed stacking fault energies and the related properties are in favorable accord with available experimental and theoretical data.

In terms of first-principles phonon calculations and the quasiharmonic approach, the structural, vibrational, and thermodynamic properties have been investigated for the ordered and disordered Ni 1−x Pt x alloys, with the main focus being on disordered Ni 0.5 Pt 0.5 . To gain insight into the disordered alloys, we use special quasirandom structures (SQSs) and demonstrate their capabilities in predicting (i) the bond-length distributions, (ii) the phonon spectra, and (iii) the elastic stiffness constants of the disordered alloys. It is found that the Pt-Pt atomic pairs possess the longest bond lengths relative to the Ni-Pt and Ni-Ni ones in the disordered alloys, the predicted force constants indicate that the Pt-Pt bond is stiffer when compared to the Ni-Pt and the Ni-Ni ones for both the ordered and disordered alloys, and the phonon density of states of the disordered alloys are similar to the broadened versions of the ordered cases. Based on the results of the ordered and disordered alloys, a slightly positive deviation from Vegard's law is found for the volume variation of Ni 1−x Pt x , and correspondingly, a negative deviation is predicted for the change of bulk modulus. With increasing Pt content, the bulk modulus derivative relative to pressure increases approximately linearly, whereas the magnetic moment decreases. In addition, the SQS-predicted relative energies (enthalpies of formation) for the disordered Ni 1−x Pt x are also compared to cluster expansion predictions. As an application of the finite temperature thermodynamic properties, the phase transition between the ordered L1 0 and the disordered Ni 0.5 Pt 0.5 is predicted to be 755 ± 128 K, which agrees reasonably well with the measurement ∼900 K, demonstrating that the driving force of the phase transition stems mainly from the configurational entropy rather than the vibrational entropy.

In an attempt to obtain reliable first-principles phonon dispersions of random alloys, we have developed a method to calculate the dynamical matrix, with respect to the wavevector space of the ideal lattice, by averaging over the force constants of a special quasi-random structure. Without additional approximations beyond standard density functional theory, the present scheme takes into account the local atomic position relaxations, the composition disorder, and the force constant disorder in a random alloy. Numerical results are presented for disordered Cu(3)Au, FePd, and NiPd and good agreement between the calculations and the inelastic neutron scattering data is observed.