Kazuki Nozawa

Catalytic Properties Dominated by Electronic Structures in PdZn, NiZn, and PtZn Intermetallic Compounds

The catalytic functions of Pd are completely modified by alloying with Zn, and PdZn exhibits comparable catalytic selectivity to Cu in the steam reforming of methanol (SRM). We perform theoretical and experimental studies to confirm our previous argument that the position of the d -band is a significant factor determining catalytic properties. First-principles slab calculations for M –Zn ( M = Pd, Ni, Pt) reveal that the bond breaking on the surface leads to some reduction in the d -bandwidth, but that the position of the d -band for stable surfaces remains essentially unchanged from that of the bulk. The origin of the dramatic change in the electronic structure caused by alloying is theoretically demonstrated. Our previous argument is experimentally examined not only in SRM, but also in elemental reactions such as CO and H 2 adsorptions. Magnetic measurements also indicate the importance of the d -band position in SRM.

First-principles studies for structural transitions in ordered phase of cubic approximant Cd6Ca

Recently a low-temperature structural transition has been reported for complex cubic compounds Cd6M (M = Ca, Yb, Y, rare earth) and it is believed that the transition is due to orientational ordering of an atomic shell in the icosahedral cluster in Cd6M. The first-principles electronic structure calculations and structural relaxations are carried out to investigate structures and orientational ordering of the innermost tetrahedral shell of the icosahedral cluster in Cd6Ca. The very short interatomic distances in the experimental average structures are relaxed and the innermost tetrahedral shell of an almost regular shape is obtained. Three types of orientation for the tetrahedral shell and eight different combinations of them for the clusters at a vertex and body-centre of a cubic cell are obtained. A possible model describing the orientational ordering at low temperatures or high pressures is discussed.

Templated three-dimensional growth of quasicrystalline lead

Quasicrystals, materials with aperiodic long-range order, have been found in intermetallics, soft materials such as colloids and supermolecules, and also in two-dimensional monolayer films. Here we present the first example of three-dimensional growth of a single-element quasicrystalline film. Using a hitherto unexplored template, the icosahedral Ag-In-Yb quasicrystal, and various experimental techniques combined with theoretical calculations of adsorption energies, we find that lead atoms deposited on the surface occupy the positions of atoms in the rhombic triacontahedral cluster, the building block of the substrate, and thus grow in layers with different heights and adsorption energies. We show that the adlayer-adlayer interaction is crucial for stabilizing this epitaxial quasicrystalline structure. The finding opens an avenue for further investigation of the impact of the aperiodic atomic order over periodic order on the physical and chemical properties of materials.

Theoretical Study of Structures of Solid Oxygen under High Pressure

The structure of solid oxygen under high pressure is studied up to 6 GPa with no empirical parameters. The potential of the crystal is described by superposition of O 2 pair potentials calculated by the ab initio method. Structural optimization is achieved with this potential energy surface. The monoclinic structure is stable up to 6 GPa and a monoclinic-orthorhombic structural transition is not found. The pressure dependence of lattice constants agrees with the result of recent X-ray diffraction experiments.

Ab initio studies on orientational ordering in cubic Zn-Sc

Abstract Complex cubic compounds Zn6Sc and Cd6M (M = Yb, Ca, Y, rare earth) are approximant crystals of Zn- and Cd-based quasicrystals. A low-temperature structural transition has been observed for these compounds universally and is considered to be due to orientational ordering of an atomic shell in the icosahedral cluster. To get microscopic information on structure and orientational ordering of the atomic shell in the icosahedral cluster, the first-principles structural relaxation for Zn6Sc is carried out. We obtain variety of favorable structures for different lattice constants. The results are compared with those for isostructural Cd6Ca obtained in the previous work. We stress differences between Zn6Sc and Cd6Ca and discuss their possible consequences on phase diagrams.

First-principles studies on orientational ordering of atomic clusters in Cd6Ca

First-principles structural optimizations are carried out for 1/1 cubic approximant Cd6Ca. Newly proposed structure by Gómez and Lidin is used as the starting structure for the structural optimizations. The optimized structure is more stable than the non-optimized one by several eV. Two structures, which are almost degenerate before the structural optimizations, are split by about 100 meV per unit cell after the structural optimizations. Clusters are deformed to relax interatomic distances and most of the neighbouring Cd–Cd distances close to that of hcp-Cd. Relaxed structures of clusters depend on the structure of the innermost shells of adjacent clusters.

Theoretical calculations for solid oxygen under high pressure

The crystal structure of solid oxygen at low temperatures and at pressures up to 7 GPa is studied by theoretical calculations. In the calculations, the adiabatic potential of the crystal is approximated by a superposition of pair-potentials between oxygen molecules calculated by an ab initio method. The monoclinic α structure is stable up to 6 GPa and calculated lattice parameters agree well with experiments. The origin of a distortion and that of an anisotropic lattice compressibility of the basal plane of α-O2 are clearly demonstrated. In the pressure range from 6 to 7 GPa, two kinds of structures are proposed by x-ray diffraction experiments: the α and orthorhombic δ structures. It is found that the energy difference between these structures becomes very small in this pressure range. The relation between this trend and the incompatible results of x-ray diffraction experiments is discussed.

Electronic structures and stability of Ag-In-Ca surfaces

Surface properties and stability of quasicrystal-related complex metallic compounds Ag-In-Ca has been studied theoretically. Optimal structures of surface is obtained by relaxing the atoms in a slab to their force-free positions by the first-principles method. It is found that, after relaxation, In is at topmost position near the vacuum, Ag intrudes into the bulk and Ca is located at intermediate layers between In and Ag. To simulate STM images for the slab model, we calculate the charge distribution constructed from the electronic states in an appropriate energy range. We find that bright contrast is observed on different subsets of the In sites on surface for the charge distributions calculated from occupied and unoccupied states. Roles of the low-lying unoccupied Ca-3d states and the sp-d hybridization on surface electronic structures are discussed.

Theoretical studies on clean and adsorbed surfaces of Ag-In-Yb

The adsorption structure of Pb atoms on the five-fold surface of a AgInYb quasicrystal is studied using a first-principles calculation. The aperiodic five-fold surface is replaced by a cluster model, following the quasicrystal model by Takakura et al. The potential energy surface (PES) for a single Pb adsorption reflects the five-fold symmetry of the substrate. The most preferred adsorbed site is a five-fold hollow site surrounded by three In and two Yb atoms, and the most preferred sites form a pentagon with edge length 9.9 Å. The PES for the second adsorbed atom is calculated to examine the effects of interadsorbate interaction. It turns out that the interadsorbate interaction is limited, and it does not change the PES essentially. Thus, at low coverage, the Pb atoms presumably adsorb onto the five-fold hollow sites, forming a Pb pentagon with edge length 9.9 Å around the cluster center. Possible adsorption structures at higher coverage are also discussed.

First-principles study for effect of lattice defects on the crystal structure of the Zn-Sc cubic crystalline approximant

Effects of impurity and vacancy introductions are investigated by the first-principles structural relaxation. Studied concentration of the lattice defects is 0.6 at%. As the impurity, a Cu atom is substituted for a Zn atom in Zn6Sc. For this concentration, the plausible candidate site of Cu substitution is the glue site distributing in the interstitial region between icosahedral clusters. The most stable vacancy site is the core of the icosahedral cluster. Some of the introduced vacancies migrate several angstrom and reach the core. A special site being outside the core plays an important role for the vacancy migration. The vacancy introduced near this site migrates to the core. Significant Zn-Zn bonds are found in the charge density map. The stability of the defect-introduced structure is discussed in terms of a breaking of this bond.