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We scrutinise the muffin-tin approximation and the screening within the framework of the Exact Muffin-Tin Orbitals method in the case of cubic and tetragonal crystal symmetries. Systematic total en ...

Ab initio total-energy calculations, based on the exact muffin-tin orbitals method, are used to determine the equilibrium chemical configuration of alloy as a function of disorder and temperature. The transition from a substitutionally disordered face-centred-cubic crystallographic phase to an ordered phase is monitored using the coherent potential approximation which allows us to continuously scan the order parameter and thermodynamics of the alloy. We find signs of a first-order phase transition from a substitutionally disordered state to a partially ordered state around 210 K temperature and the completely ordered state is predicted around 90 K.

Titanium is a strong, corrosion resistant metal with low mass density, making it ideal for various purposes, including aviation and medical applications. In the present work, the elastic properties ...

We investigate the generalized stacking fault energy ( [GRAPHICS] -surface) of paramagnetic [GRAPHICS] -Fe as a function of temperature. At static condition, the face-centred cubic (fcc) lattice is thermodynamically unstable with respect to the hexagonal close-packed lattice, resulting in a negative intrinsic stacking fault energy (ISF). However, the unstable stacking fault energy (USF), representing the energy barrier along the [GRAPHICS] -surface connecting the ideal fcc and the intrinsic stacking fault positions, is large and positive. The ISF is calculated to have a strong positive temperature coefficient, while the USF decreases monotonously with temperature. According to the recent plasticity theory, the overall effect of temperature is to move paramagnetic fcc Fe from the stacking fault formation regime ( [GRAPHICS] K) towards maximum twinning ( [GRAPHICS] K) and finally to a dominating full-slip regime ( [GRAPHICS] K). Our predictions are discussed in connection with the available experimental observations.

The thermodynamic stability of S-substituting Si into a bcc Fe0.80Ni0.05S x Si(0.15−x) random alloy, with x=0.025 n (n=0−− 4), was investigated up to 350 GPa by using an ab initio density functional method based on the full charge density–exact muffin-tin orbital–coherent potential approximation scheme. From the analysis of the calculated Δ H values, we suggest that the FeNiSSi stoichiometry with zero sulphur content always behaves as the most stable system along the whole pressure range of 0–350 GPa. As a general tendency, we found that the alloys with 6.8–8.1 wt.% Si are energetically more stable than those with lower silicon content, thus supporting the dissolution mechanism for the Earth's inner core compositional model.

First principles band structure calculations are presented for the layer relaxation of the close-packed surfaces of 5d transition metals. Anomalously large relaxations are found for group IVA and VIIA hcp metals. For these elements, the size of the layer relaxation exhibits an unusually slow decay with the distance from the surface. We argue that this phenomenon can be attributed to the peculiar flat and degenerate d-bands located close to the Fermi level in the L-A-H top-plane of the hcp Brillouin zone, and which are also responsible for the anomalously low [001] longitudinal optical phonon frequencies observed in these hcp metals.

We have applied our recently established database of surface free energies and surface free energy anisotropies of low-index surfaces in a study of the energetics of surface steps for cubic transition metals. The first-principles results from the database are used for a cluster expansion of the surface free energy which is subsequently used for high-index surfaces. The accuracy of the expansion is established by a comparison with first-principles results for a few high-index surfaces. Using the cluster expansion for the surface energy of vicinal surfaces we derive the monoatomic step-formation energies for different close-packed bcc and fcc surface facets. Trends in the anisotropy of the step energy are also discussed.