Masaki Sakurai

Magic numbers in transition metal (Fe, Ti, Zr, Nb, and Ta) clusters observed by time-of-flight mass spectrometry

We have measured time-of-flight (TOF) mass spectra of transition metal free clusters, TMn (TM=Fe, Ti, Zr, Nb, and Ta and n is the number of atoms per cluster), produced by a laser vaporization source. The size resolved TOF intensities at n=7, 13, 15 are much higher than those at the neighboring n values for all TMn. Such specific n values are assigned to the magic numbers of these transition metal clusters and can be related to pentagonal bipyramid, icosahedron, and bcc structure units. The other magic numbers are observed for larger TMn: n=19 and 23 for Fe, n=19 and 25 for Ti, being attributable to the polyicosahedron. The TOF spectra of Nb and Ta clusters are similar to each other and display the common magic number of n=22.

Local Structure Changes around Cu Atoms in the Early Stage of Nanocrystalline Formation of Amorphous Fe73.5Cu1Nb3Si13.5B9

Direct evidences that Cu atoms are directly associated with nano-crystalline formation in Fe73.5Cu1Nb3Si13.5B9 (FINEMET) were obtained by fluorescence XAFS measurements. The XAFS results at the Cu K-edge have shown that the local structure around Cu atoms in FINEMET changes from amorphous to fee Cu prior to the bcc-FeSi precipitation.

Structural and electrical properties of AgI dispersed As-chalcogenide glasses

Abstract X-ray diffraction, EXAFS and Raman spectroscopic studies, together with transport phenomena measurements, have been carried out for (AgI) x (As 2 Se 3 ) 1− x glasses in order to investigate the ionic conduction mechanism in AgI doped non-oxide glasses. The addition of AgI into As 2 Se 3 glass is responsible for a pronounced increase in the electrical conductivity. Particularly, the ionic component of the electrical conductivity is dominant in highly doped glasses. Results of Raman spectra, EXAFS measurements and least squares fitting analysis for observed X-ray structural functions suggest that the local structure of the present glasses can be described as a pseudo-binary mixture of As–Se networks and fourfold coordinated AgI clusters.

Structural study in amorphous Zr–noble metal (Pd, Pt and Au) alloys

Abstract The atomic structures around Zr and the noble metals M (M=Pd, Pt and Au) were studied by the anomalous X-ray scattering method in amorphous and quasicrystal Zr-based alloys. The prepeak in an energy differential intensity profile at the absorption edge of the M atoms in the amorphous state indicates the presence of a strong chemical short-range order around the M atoms. Coordination numbers and atomic distances were determined in all the samples. The atomic distances of the Zr–M pairs, which are shorter than those calculated from their Gold Schmidt radii, also suggest the existence of strong chemical bonds between them.

Partial structure analysis of amorphous Ge15Te80M5 (M=Cu, Ag and In)

Abstract All partial structures of Ge15Te80M5 (M=Cu, Ag and In) have been studied using extended X-ray absorption fine structure. The coordination numbers (CN) around Cu and Ag atoms are smaller than four, which is expected from a rigid percolation model. The CN around In atoms is 3.4. This different CN for the metal atoms is attributed to the difference of the chemical bonds between M and Te. Two models in which the local order of Ge is 4-fold and Te atoms bridge two GeTe4 tetrahedra are supported.

A fluorescence X-ray absorption fine structure study of the effect of small amounts of additives on the formation of the nanocrystalline phase from metallic glass

Abstract Direct evidence that Nb and Cu atoms are associated with the nanocrystalline formation of Fe 73.5 Si 13.5 B 9 Cu 1 Nb 3 was obtained using X-ray absorption fine structure (XAFS) measurements. XAFS results at the Cu K edge showed that the local structure around Cu atoms in Fe 73.5 Si 13.5 B 9 Cu 1 Nb 3 changes from amorphous to f.c.c. prior to the precipitation of b.c.c. Fe(Si).The local structure around Nb atoms retains its amorphous state up to the second crystallization temperature and subsequently changes to the Fe 2 B crystalline state after being annealed at the second crystallization temperature.

X-ray absorption fine-structure study on the fine structure of lutetium segregated at grain boundaries in fine-grained polycrystalline alumina

Local atomic structures around the Lu3+ ion in a 0.1 mol% LuO1.5-doped fine-grained Al2O3, in which the doped Lu3+ ions segregate to the grain boundaries, was characterized by Lu L3-edge X-ray absorption fine structure. Structural parameters in LuO1.5-doped Al2O3 were determined by a curve-fitting method, and the results showed that six O2− ions coordinate with the Lu ion in LuO1.5-doped Al2O3. In addition, it was also revealed that the Lu–O interatomic distance in Lu-doped Al2O3 was close to that in Lu2O3, which was about 19% longer than the Al–O interatomic distance in undoped Al2O3. The present results indicated that the local atomic structures around Lu in Al2O3 are close to that in Lu2O3. It is thus supposed that atomic distances between Al3+ and O2− ions in the vicinity of Lu-segregated grain boundaries are shortened in comparison with that in undoped Al2O3. A first-principles molecular orbital calculation was performed for the [Al2O9]−12 model cluster, and the shortening of the Al–O interatomic distance was found to have an effect of increasing the ionicity in Al3+ ions.

EXAFS study on amorphous and nanocrystalline M–W (M=Fe,Ni) alloys produced by electrodeposition

Abstract Ductile amorphous and nanocrystalline M–W (M=Fe,Ni) alloys were produced by electrodeposition. Structural studies of these alloys were made by using EXAFS and SAXS methods. The crystal structure of the electrodeposited Ni–W alloy was similar to that of the Ni4W crystal phase. The nearest Ni–Ni distance was determined to be 2.49 A; the Ni–W distance was determined to be 2.50 A. The atomic distances in the nanocrystalline Ni–W alloys increases with the tungsten concentration. The average crystal grain size of the Ni–W alloy was estimated at about 2.5 nm from a Guinier plot of the SAXS spectra. The electrodeposited amorphous Fe–21.6 at.% W alloy has only short range order, and no medium range order.

Structural and physical properties of Ag-As-Te glasses

Abstract Differential scanning calorimetric (DSC), dc electrical conductivity, X-ray diffraction and EXAFS measurements for (Ag 2 Te) x (AsTe) 1− x glasses with x =0 to 0.3 have been carried out to investigate physical properties and the co-ordination environment of constituting atoms in ternary Ag–As–Te glasses. Results of the DSC measurement suggest that the glass structure is more thermally unstable with increasing Ag content. The incorporation of Ag 2 Te into AsTe glass is responsible for a pronounced increase in the electrical conductivity and corresponding decrease in the electrical activation energy. Least-squares fit analyses for the observed X-ray structure functions have been carried out under the assumption that the first co-ordination shell is composed of As–Te, Ag–Te and As–As correlations. The results indicate that the interatomic distances of three atomic correlations are all composition-independent. The co-ordination number of As atoms is determined to be about three and remains practically unchanged. Ag atoms are roughly fourfold co-ordinated with Te atoms, as suggested by the formal valence shell (FVS) model proposed for the chalcogenide glasses. The co-ordination number of Te atoms increases from 1.93 to 3.60 with increasing Ag content. The thermal stability and electrical properties of the present glasses may be associated with the local structure and bonding nature of Ag–Te bonds.

Magic Numbers in Fe Clusters Produced by Laser Vaporization Source

We have tried to produce free iron clusters using a laser vaporization source. The cluster beam intensities of Fe n with n =13, 15, 19 and 23 measured using a time-of-flight mass spectrometer are higher than those of neighboring size clusters for both autoionized and postionized conditions, i.e., there are magic numbers in the iron clusters. These magic numbers can be related to the icosahedral and bcc structure units and to the existence of iron cluster isomers.