T.I. Morrison

The structure of silicon-selenium glasses: I. Short-range order☆

Abstract The structures of SixSe1−x glasses with 0.0 ⩽ x ⩽ 0.40 have been investigated with neutron diffraction and Se K-edge extended X-ray absorption fine-structure measurements. Nearest-neighbor bonding is characterized by coordination numbers for Si and Se of 4 and 2, independent of concentration. Heteropolar bonding is highly favored. The second-neighbor coordination shell undergoes a systematic increase in area with increasing x and indicates, near the stoichiometric SiSe2 composition, a structure that is consistent with the presence of edge-sharing of mSi ( Se 1 2 ) 4 tetrahedra. “First sharp diffraction peaks” are well defined for the x ⩽ 0.17 glasses and exhibit remarkable systematics.

The coordination environment of Eu(III) ions in hydrated A and Y zeolites as determined by luminescence lifetime and EXAFS measurements

Luminescence lifetimes and EXAFS show that the average coordination of Eu3+ in the large‐pore zeolite Y is identical to that in aqueous solution, while the coordination number in the small‐pore zeolite A is lower, probably because of steric constraints.

EXAFS as a probe of local coordination environments of transition metal ions in polycrystalline zeolite catalysts. Co(ii) in hydrated zeolite –A and –Y

Synthetic zeolites exchanged with transition metal or rare earth ions are of immense importance in catalysis, both commercially and in basic research; however, difficulty in growing single crystals and problems of long range disorder make crystallographic studies formidable tasks. In this study, extended x‐ray absorption fine structure (EXAFS) spectroscopy has been successfully applied to characterize the local coordination of Co(ii) ions in polycrystalline zeolite –A and –Y, thereby establishing the utility of this technique for structural study of intrazeolitic complexes. The Co(ii) ions in Co18.2Na20.7–Y⋅xH2O are found to be ’’solutionlike’’, six‐coordinate with Co–O bond distances of 2.06(1) A, but in Co4.3Na3.4–A⋅xH2O and Co5.5Na1.0–A⋅xH2O, they are five coordinate with Co–O distances of 2.07(1) A. The latter result is compatible with several published spectroscopic measurements, but is in conflict with a previous single crystal x‐ray diffraction study; this discrepancy has been critically analyzed. ...

Anomalous L3L2 white line ratios and spin pairing in 3d transition metals and alloys: Cr metal and Cr20Au80

Abstract It is shown by transmission electron energy loss spectroscopy that a large difference exists between the L 3 L 2 ratio obtain for chromium metal and that obtained for chromium in Cr20Au80 alloy for which chromium exists in a highly spin-aligned state. An argument is given that the anomalous L 3 L 2 white line ratios observed for 3d transition metals may be correlated strongly with the degree of spin pairing in the 3d band.

X-ray absorption fine structure study of small metal clusters isolated in rare-gas solids

Abstract The use of extended X-ray absorption fine structure (EXAFS) to determine the interatomic distances in small metal clusters is of considerable scientific as well as technical importance, we have used synchrotron radiation to study the near edge structure as well as the EXAFS spectra of metal clusters isolated in rare-gas matrices, by studying such small metal clusters in inert-gas matrices, it is possible to investigate metal-metal interactions without the interference of strongly interacting supports. Considerable contraction in small metal clusters of iron and chromium consisting of a few atoms has been observed. Associated with this contraction is a large variation in the electronic properties as detected by the isomer shift and magnetic hyperfine field at the 57 Fe nucleus. The chromium results are compared with recent gas-phase measurements reported in the literature. EXAFS was also used to determine the interatomic distance for small metal particles of silver isolated in solid argon.

EXAFS study of dehydrated cobalt exchanged A zeolite

The authors study the structure of the transition metal exchanged zeolites by the Extended X‐ray Absorption Fine Structure of the exchanged metal ion. (AIP)

Framework chemistry and structure involving electron transfer and europium in zeolites

Abstract The relationship between the zeolite framework and the coordination number and electron-transfer properties of Eu2+ and Eu3+ have been investigated by time-resolved luminescence (TRL) EXAFS, EPR and Mossbauer spectroscopy. TRL-determined lifetimes are sensitive to non-framework oxygen atoms and can be used to help distinguish between coordination involving OH groups and the framework oxygen atoms. The suitability and some of the present limitations of this application of TRL, which is new to the study of zeolites, are described. The average coordination of hydrated Eu3+ in the large-pore zeolite Y is identical to that in aqueous solution, while the coordination number in zeolites A and ZSM-5 is smaller, with framework oxygen atoms displacing water from the Eu3+ coordination sites. Europium metal dissolved in liquid ammonia can be used to generate Eu(NH3)62+ and solvated electrons. With zeolite X, this solution results in the formation of Na43+ and two kinds of defects. These are assigned to an electron trapped in an oxygen vacancy arising from a Lewis acid site and a V-type hole center in which the unpaired electron resides on a non-bridging oxygen atom bonded to silicon.

Materials science EXAFS line at the NSLS: characterization and initial operations

The characterization and initial operation of a materials science EXAFS beam line at the NSLS is described. This line has a unique combination of components including a 4-crystal focusing monochromator and SiC collimating mirror. Discussion of the monochromator will include a comparison with conventional two crystal designs and techniques for maintaining crystal alignment during EXAFS scans. The dispersive nature of the monochromator also allows a measurement of the NSLS emittance which appears to be near design values. This means significant intensity gains can be realized with a collimating mirror which is planned as a future addition to the beam line optics. Some experimental results will be used to demonstrate the capabilities of the beam line. 5 refs.

X‐ray absorption studies of Cu(en)2+2 exchanged Y zeolite

X‐ray absorption studies have been performed on Cu(en)2+2 exchanged Y zeolite. The copper EXAFS shows that there is little change in the equatorial copper‐ligand bond lengths, but the XANES is indicative of axial distortions. Both the EXAFS and XANES can be interpreted to show that the complex remains intact upon exchange and that it is structurally and electronically similar to the bis(ethylenediamine) copper (II) complex in solutions.

EELS as a probe of d-band occupancy in magnetic alloys

Abstract It is shown that L-edge electron energy loss spectroscopy (EELS) can be used as a direct probe of the number of unoccupied d-states in amorphous transition metal alloys. Since L III and L II “white lines” arise predominantly from p-d transitions, these spectra can be used to measure changes in d-band occupancy with composition. This information is necessary in order to understand magnetic behavior of amorphous alloy systems.