Levente Vitos

Elastic properties of Fe–Mn random alloys studied by ab initio calculations

We have studied the influence of the Mn content on the elastic properties of Fe-Mn random alloys (space group of Fm (3) over barm) using ab initio calculations. The magnetic effects in Fe-Mn alloys ...

Phase stability and elastic modulus of Ti alloys containing Nb, Zr, and/or Sn from first-principles calculations

The alloying effects of Nb, Zr, and/or Sn on the phase stability and elastic properties of Ti are investigated by using a first-principles method. Our calculation results indicate that a carefully designed Ti-Nb-Zr-Sn system can be a good candidate for low modulus biomedical materials. We find that the well-known correlation between the e/a ratio and both elastic and phase stabilities for Ti alloyed with transition metal elements breaks down for the Ti-Sn alloy.

Fermi surface nesting and pre-martensitic softening in V and Nb at high pressures

First-principles total-energy calculations were performed for the trigonal shear elastic constant (C44) of body-centred cubic (bcc) V and Nb. A mechanical instability in C44 is found for V at pressures of ~2 Mbar which also shows a softening in Nb at pressures of ~0.5 Mbar. We argue that the pressure-induced shear instability (softening) of V (Nb) is due to the intra-band nesting of the Fermi surface.

Surface properties of 3d transition metals

Using the projector augmented wave method within density functional theory, we present a systematic study of the layer relaxation, surface energy and surface stress of 3d transition metals. Comparing the calculated trends for the surface energy and stress with those obtained for 4d and 5d metals we find that magnetism has a significant effect on the surface properties. Enhanced surface magnetic moments decrease the size of the surface relaxation, lower the surface energy and surface stress, leading to compressive stress in Cr and Mn.

Parametrization of perovskite structures : an ab initio study

An overview of the pressure, temperature and chemical composition dependence of the lattice distortion in orthorhombic ABO(3) perovskite structures is presented. Within the framework of the so-called global parametrization method (GPM) [Thomas (1998). Acta Cryst. B54, 585--599] an improved description for the position of the A cation in terms of the AO(12) and BO(6) polyhedral volume ratio is proposed. The relationship is derived from an extensive ab initio study based on the density functional theory. The applicability of the improved GPM in combination with ab initio total energy calculations in the prediction of changes in the structural distortion under increasing hydrostatic pressure is investigated. Test calculations are performed for the geophysically important magnesium silicate perovskite and the results are compared with the available theoretical and experimental data.

Strongly enhanced magnetic moments in ferromagnetic FeMnP0.5Si0.5

The compound FeMnP(0.5)Si(0.5) has been studied by magnetic measurements, Mossbauer spectroscopy, and electronic structure and total energy calculations. An unexpectedly high magnetic hyperfine fie ...

Phase stability of Li(Mn100−xCox)O2 oxides: an ab initio study

The full charge density exact muffin-tin orbitals method has been used to study the stability of lithium manganese oxides exhibiting different crystallographic forms. Calculations have been performed in various crystallographic phases of LiMnO2 for the ferromagnetic and antiferromagnetic spin configurations on Mn sublattice, as well as for the state with local moment disorder. For the ordered LiMnO2 compound, the experimentally observed antiferromagnetic orthorhombic ground state structure was reproduced. The effect of doping of the Mn sites by Co was considered with the primary aim to predict the stabilization of the layered structure.

Ab initio calculations of elastic properties of Ru1-xNixAl superalloys

Ab initio total energy calculations based on the exact muffin-tin orbitals method, combined with the coherent potential approximation, have been used to study the thermodynamical and elastic proper ...

Generalized stacking fault energy of gamma-Fe

We investigate the generalized stacking fault energy (-surface) of paramagnetic -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 -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 ( K) towards maximum twinning ( K) and finally to a dominating full-slip regime ( K). Our predictions are discussed in connection with the available experimental observations.

Ab initio electronic structure calculations of correlated systems: An EMTO-DMFT approach

We propose a self-consistent method for electronic structure calculations of correlated systems, which combines the local spin-density approximation (LSDA) and the dynamical mean field theory (DMFT). The LSDA part is based on the exact muffin-tin orbital approach, meanwhile the DMFT uses a perturbation scheme that includes the T matrix with fluctuation exchange approximation. The current LSDA+DMFT implementation fulfills both self-energy and charge self-consistency requirements. We present results on the electronic structure calculations for bulk 3d transition metals (Cr, Fe, and Ni) and for Fe/Cr magnetic multilayers. The latter demonstrates the importance of the correlation effects for the properties of magnetic heterostructures.