Jan Łażewski

Ab initio elasticity of chalcopyrites

A critical review of the experimental data of elastic stiffness constants of AgGaX2 and CuInX2 (X=S,Se) chalcopyrites is given. Using the ab initio pseudopotential method, their elastic constants have been calculated from strain–energy and from strain–stress relationships. On the basis of the linear elasticity approximation, axial and total compressibilities as well as elastic anisotropy factors have been derived. The calculated results are compared with available experimental data and previous calculations based on phenomenological models.

Structure and elastic properties of Mg(OH)2 from density functional theory

The structure, lattice dynamics and mechanical properties of magnesium hydroxide have been investigated by static density functional theory calculations as well as ab initio molecular dynamics. The hypothesis of a superstructure existing in the lattice formed by the hydrogen atoms has been tested. The elastic constants of the material have been calculated with a static deformations approach and are in fair agreement with the experimental data. The hydrogen subsystem structure exhibits signs of disordered behaviour while maintaining correlations between the angular positions of neighbouring atoms. We establish that the essential angular correlations between hydrogen positions are maintained to a temperature of at least 150 K and that they are well described by a physically motivated probabilistic model. The rotational degree of freedom appears to be decoupled from the lattice directions above 30 K.

Comparative ab initio study of lattice dynamics and thermodynamics of Fe2SiO4- and Mg2SiO4-spinels.

Lattice dynamics and thermodynamic properties of antiferromagnetic Fe(2)SiO(4)-spinel have been studied using density functional theory. Phonon dispersions are obtained for several hydrostatic pressures up to 20 GPa. They are used to calculate thermodynamic properties within the quasiharmonic approximation. Comparison with ab initio results obtained for Mg(2)SiO(4)-spinel is made in order to study the effect of the cation exchange on the dynamic and thermodynamic properties of (Mg, Fe)(2)SiO(4)-spinel. The obtained results have been compared with the available experimental data.

Dynamics and stability of icosahedral Fe–Pt nanoparticles

The structure, dynamics and stability of Fe-Pt nanoparticles have been investigated using DFT-based techniques: total energy calculations and molecular dynamics. The investigated systems included multi-shell and disordered nanoparticles of iron and platinum. The study concerns icosahedral particles with the magic number of atoms (55): iron-terminated Fe43Pt12, platinum-terminated Fe12Pt43, and disordered Fe27Pt28. Additionally, the Fe6Pt7 cluster has been investigated to probe the behaviour of extremely small Fe-Pt particles. Molecular dynamics simulations have been performed for a few temperatures between T = 150-1000 K. The calculations revealed high structural instability of the Fe-terminated nanoparticles and a strong stabilising effect of the Pt-termination in the shell-type icosahedral particles. The platinum termination prevented disordering of the particle even at T = 1000 K indicating very high melting temperatures of these Fe-Pt icosahedral structures. The analysis of evolution of the radial distribution function has shown a significant tendency of Pt atoms to move to the outside layer of the particles - even in the platinum deficient cases.

Vibrational properties and stability of FePt nanoalloys

The structural and dynamical properties of FePt nanoparticles were studied within the density functional theory. The effect of size and chemical composition on the dynamical stability of nanoparticles was investigated for the cuboctahedral and icosahedral symmetries. In cuboctahedra, a structural distortion is observed, which for systems with odd number of Pt layers leads to lowering of the tetragonal symmetry. Significant differences between the vibrational properties of FePt particles and bulk crystals are observed, but similarly to the FePt crystal, cuboctahedral particles exhibit a strong anisotropy of atomic vibrations. The icosahedral particles with perfect shell geometry are unstable due to enlarged distances between Fe atoms. They can be stabilized by removing a central atom or replacing it by a smaller one. The heat capacity and entropy of nanoparticles show typical enhancement due to low-energy vibrations at the surface layers.