Density functional theory study of Au-fcc/Ge and Au-hcp/Ge interfaces

In recent years, nanostructures with hexagonal polytypes of gold have been synthesised, opening new possibilities in nanoscience and nanotechnology. As bulk gold crystallizes in the fcc phase, surface effects can play an important role in stabilizing hexagonal gold nanostructures. Here, we investigate several heterostructures with Ge substrates, including the fcc and hcp phases of gold that have been observed experimentally. We determine and discuss their interfacial energies and optimized atomic arrangements, comparing the theory results with available experimental data. Our DFT calculations for the Au-fcc(011)/Ge(001) junction show how the presence of defects in the interface layer can help to stabilize the atomic pattern, consistent with microscopic images. Although the Au-hcp/Ge interface is characterized by a similar interface energy, it reveals large atomic displacements due to significant mismatch. Finally, analyzing the electronic properties, we demonstrate that Au/Ge systems have metallic character, but covalent-like bonding states between interfacial Ge and Au atoms are also present

Origin of monoclinic distortion and its impact on the electronic properties in KO2_2

We use the density functional theory and lattice dynamics calculations to investigate the properties of potassium superoxide KO$_2$ in which spin, orbital, and lattice degrees of freedom are interrelated and determine the low-temperature phase. After calculating phonon dispersion relations in the high-temperature tetragonal $I4/mmm$ structure, we identify a soft phonon mode leading to the monoclinic $C2/c$ symmetry and optimize the crystal geometry resulting from this mode. Thus we reveal a displacive character of the structural transition with the group-subgroup relation between the tetragonal and monoclinic phases. We compare the electronic structure of KO$_2$ with antiferromagnetic spin order in the tetragonal and monoclinic phases. We emphasize that realistic treatment of the electronic structure requires including the local Coulomb interaction $U$ in the valence orbitals of the O$^-_2$ ions. The presence of the `Hubbard' $U$ leads to the gap opening at the Fermi energy in the tetragonal structure without orbital order but with weak spin-orbit interaction. We remark that the gap opening in the tetragonal phase could also be obtained when the orbital order is initiated in the calculations with a realistic value of $U$. Finally, we show that the local Coulomb interactions and the finite lattice distortion, which together lead to the orbital order via the Jahn-Teller effect, are responsible for the enhanced insulating gap in the monoclinic structure.Comment: accepted by Physical Review

Phonon confinement and interface lattice dynamics of ultrathin high-rare earth sesquioxide films: the case of Eu₂O₃ on YSZ(001)

The spatial confinement of atoms at surfaces and interfaces significantly alters the lattice dynamics of thin films, heterostructures and multilayers. Ultrathin films with high dielectric constants (high-k) are of paramount interest for applications as gate layers in current and future integrated circuits. Here we report a lattice dynamics study of high-k Eu$_{2}$O$_{3}$ films with thicknesses of 21.3, 2.2, 1.3, and 0.8 nm deposited on YSZ(001). The Eu-partial phonon density of states (PDOS), obtained from nuclear inelastic scattering, exhibits broadening of the phonon peaks accompanied by up to a four-fold enhancement of the number of low-energy states compared to the ab initio calculated PDOS of a perfect Eu$_{2}$O$_{3}$ crystal. Our analysis demonstrates that while the former effect reflects the reduced phonon lifetimes observed in thin films due to scattering from lattice defects, the latter phenomenon arises from an ultrathin EuO layer formed between the thin Eu$_{2}$O$_{3}$ film and the YSZ(001) substrate. Thus, our work uncovers another potential source of vibrational anomalies in thin films and multilayers, which has to be cautiously considered

Phononic drumhead surface state in distorted kagome compound RhPb

RhPb was initially recognized as one of a CoSn-like compounds with $P6/mmm$ symmetry, containing an ideal kagome lattice of $d$-block atoms. However, theoretical calculations predict the realization of the phonon soft mode which leads to the kagome lattice distortion and stabilization of the structure with $P\bar{6}2m$ symmetry [A. Ptok et al., Phys. Rev. B 104, 054305 (2021)]. Here, we present the single crystal x-ray diffraction results supporting this prediction. Furthermore, we discuss the main dynamical properties of RhPb with $P\bar{6}2m$ symmetry. The bulk phononic dispersion curves contain several flattened bands, Dirac nodal lines, and triple degenerate Dirac points. As a consequence, the phononic drumhead surface state is realized for the (100) surface, terminated by the zigzag-like edge of Pb honeycomb sublattice.Comment: 10 pages, 7 figure

Phononic drumhead surface state in the distorted kagome compound RhPb

RhPb was initially recognized as one of CoSn-like compounds with P6/mmm symmetry, containing an ideal kagome lattice of d-block atoms. However, theoretical calculations predict the realization of the phonon soft mode, which leads to the kagome lattice distortion and stabilization of the structure with P$\bar{6}$2m symmetry [A. Ptok et al., Phys. Rev. B 104, 054305 (2021)]. Here, we present the single crystal x-ray diffraction results supporting this prediction. Furthermore, we discuss the main dynamical properties of RhPb with P$\bar{6}$2m symmetry, i.e. phonon dispersions and surface Green's functions using the modern theoretical methods based on density functional theory. The bulk phononic dispersion curves contain several flattened bands, Dirac nodal lines, and triple degenerate Dirac points. As a consequence, the phononic drumhead surface state is realized for the (100) surface, terminated by the zigzaglike edge of Pb honeycomb sublattice

Production of Activated Carbons from Food/Storage Waste

This paper deals with the adsorption of organic and inorganic pollutants on the surface of carbonaceous adsorbents prepared via the chemical activation of expired or broken food products—the solid residue of the “cola-type” drink as well as spoilt grains of white rice and buckwheat groats. The activation process was conducted in the microwave furnace with the use of two activating agents of different chemical nature—potassium carbonate and orthophosphoric acid. The activated carbons were characterized based on the results of elemental analysis, low-temperature nitrogen adsorption/desorption, Boehm titration, thermal analysis, and scanning electron microscopy. Additionally, the suitability of the materials prepared as the adsorbents of methylene blue and iodine from the aqueous solutions was estimated. The materials obtained via chemical activation with H3PO4 turned out to be much more effective in terms of both model pollutant adsorptions. The maximum sorption capacity toward iodine (1180 mg/g) was found for the white-rice-based activated carbon, whereas the most effective in the methylene blue removal (221.3 mg/g) was the sample obtained from the solid residue of the expired “cola-type” drink. For all carbonaceous materials, a better fit for the experimental adsorption data was obtained with the Langmuir isotherm model than the Freundlich one