Anomalous lattice dynamics in EuSi2 nanoislands
We have performed a systematic lattice dynamics study of EuSi2 films and nanoislands by in situ nuclear inelastic scattering on Eu151 and ab initio theory. Comparison of the calculated Eu-partial phonon density of states (DOS) (convoluted with a Gaussian function, FWHM=1.2 meV) and the DOS of the bulk crystal (sample A), reveals a remarkable agreement between the experiment and theory (see the figure). Complete understanding of the DOS of the thin film (sample B) is achieved by combining the calculated DOS of the surface and bulk EuSi2. The Eu-partial phonon density of states of the nanoislands (samples C and D) exhibits anomalous excess of phonon states at low and high energies, not present in the bulk and at the EuSi2(001) surface. We have demonstrated that atomic vibrations along the island-substrate interface give rise to phonon states both at low and high energies, while atomic vibrations across the island-island interface result in localized high-energy phonon modes.
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.
This research has been supported by the COST Action MP0903: "Nanoalloys as advanced materials: from structure to properties and applications" and the article is part of themed collection: Recent advances in the chemical physics of nanoalloys.
Electronic and dielectric properties of FeO
Wustite (FeO) has challenged experimental and theoretical condensed matter physics for over sixty years. In particular, the extent of its nonstoichiometry and its defect structure have been the subject of numerous experimental studies and are still a matter of discussion and a source of controversy. Generally, these studies indicate that the crystal lattice of FeO is modified by the presence of Fe(3+) cations and vacant Fe(2+) sites. The crystal structure and charge distribution in FeO are very complex as the defects are likely to coalesce and arrange into clusters, which in turn can aggregate into larger defect structures. This work investigates the role of strong electron correlations on Fe atoms and the high concentration of cation vacancies in modifying the electronic and dielectric properties of wustite. As we have shown, both of them influence substantially the electronic properties of this compound and have opposite effect on its band structure. While the local electron interactions in the Fe 3d states are responsible for the opening of the insulating gap, the Fe vacancies reduce its magnitude.
The mechanism of gap reduction remains the same, irrespectively of the type of incorporated defects. Either monovacancies or vacancy clusters induce the band of empty Fe(3+) states inside the gap of the perfect FeO crystal. The width of this band increases monotonically with the increased concentration of cation vacancies reducing the distance from the top of valence states, so effectively diminishing the band gap. We discuss the effects observed in the optical properties of wustite and explained them by the presence of defects. Our studies also explain the anomalous broadening of infrared spectra, demonstrating a strong effect of cation vacancies on phonons in wustite.
Structures of Late Transition Metal Monoxides
Transition metal monoxides i.e. chemical compounds of the formula MO are often found in nature and they have been used as early as in Neolithic period for pottery - and later glassware - colouring. These important materials exhibit a great variety of physical properties such as diverse electronic and thermal conductivity, magnetic or optical features, and they find numerous daily life applications. Nearly twenty MOs have been prepared to this day and even more wait to be synthesized. It has long been known that most of them crystallize in the prototypic rock salt (NaCl) structure – either undistorted or only slightly distorted due to magnetic interactions operating at low temperatures. This important cubic structure is frequently found for inorganic compounds, especially ionic ones. But five oxides: those of copper, silver, palladium, platinum and mercury, do not fit the family since they adopt much more complex lower - symmetry structures. Still, do they have anything in common with the cubic ones?
We have open positions for PhD students in the field of computational material science. The PhD work will be guided by the professors of the Department and will be focused on computational investigation of crystals, nanostructures and surfaces.
Our reasearch activity focuses on solid state properties of a broad range of materials including bulk crystals, nanostructures, surfaces, and disordered systems. The main fields of our research include:
- the basic structural, electronic, and phonon properties of crystals
- phase transitions
- electron-phonon interaction
- phonons in multilayes, thin films and surfaces
- thermodynamic and elastic properties of minerals
- lattice dynamics in strongly correlated systems, superconductors, molecular crystals
- shape memory and disordered alloys
- Magnetostructural phase transition in MnAs
- Disordered hydrogen dynamics in Mg(OH)2
- Lattice dynamics in actinide compounds (URu2Si2)
- Verwey transition in magnetite (IXS, NIS)
- Structural and electronic properties of defected SiC crystals
- Phonon properties of FeO crystal and thin films