Matthew A. Marcus

Local structure of 1.54‐μm‐luminescence Er3+ implanted in Si

Extended x‐ray absorption fine structure measurements from Er‐implanted Czochralski‐grown Si samples, which exhibit strong luminescence at 1.54 μm, reveal a local sixfold coordination around Er−not of Si−but of oxygen atoms at an average distance of 2.25 A. By contrast, similar concentrations of Er implanted in high purity float‐zone Si samples, which are essentially optically inactive, show that Er is coordinated to 12 Si atoms at a mean distance of 3.00 A.

Local structure around Er in silica and sodium silicate glasses

Abstract Extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) measurements have been performed on three different Er-doped glasses: silica and sodium silicate doped with Er in the molten phase, and silica doped by MeV ion implantation. All samples show a luminescent transition centered around a wavelength of λ = 1.54 μ m, corresponding to an intra-4f transition of Er 3+ . EXAFS data show that the Er ions have about six O first neighbors, at a distance of 2.26 A in all systems. Both silica glasses show a second-neighbor ErSi shell at 3.11 A which is not seen in the sodium silicate glass. This difference is reflected in a difference in the PL spectra. The ErO shell in the silica glass shows 0.01 A 2 more disorder than that in the Na-loaded material. These findings are rationalized in terms of the network-modifying effect of Na. Both EXAFS and PL show no significant differences between silica samples prepared in the molten phase or by implantation. At an Er concentration of ∼1 at.%, no direct ErEr bonds are observed.

Structural study of Rb and (Rb,Ag) germanate glasses by EXAFS and XPS

Abstract The interatomic distance, R , coordination number, CN, and degree of disorder, Δσ 2 , around the mobile (Rb,Ag) as well as the network-forming (Ge) cations are obtained using EXAFS (extended X-ray absorption fine structure) of x Rb 2 O·(1− x )GeO 2 and mixed rubidium-silver germanate 0.2[ y Ag·(1− y )Rb] 2 O·0.8GeO 2 glasses with x = 0.01, 0.02, 0.05, 0.10, 0.15, 0.20, 0.30, 0.40 and y = 0, 0.25, 0.50, 0.75, 1.00. The non-bridging oxygen (NBO) and bridging oxygen (BO) concentrations are determined by X-ray photoelectron spectroscopy. With the addition of Rb 2 O, both GeO 6 and NBOs are produced for all x ≤ 0.2. For x > 0.2, NBOs continue to increase at the expense of GeO 6 units. A comparison of molar volume with various bond lengths indicates the presence of an ‘unoccupied volume’ which may vary with composition without affecting various interatomic distances. Replacement of Rb by Ag in mixed germanate glasses has the same effect on R Rb−O and Δσ Rb −O 2 as the reduction of Rb content in single Rb germanate glasses. The R Ag−O and Δσ Ag−O 2 show a trend opposite to that of the bond around Rb when Ag and Rb replace each other.

Unimportance of siloxene in the luminescence of porous silicon

Near‐edge‐ and extended‐x‐ray absorption fine structure measurements, as well as luminescence excitation and emission spectra, were obtained from samples of porous Si and siloxene. Combined, these data establish that, contrary to a recently proposed explanation, siloxene is not generally responsible for the observed room‐temperature luminescence in porous Si.

Curved‐crystal (LiF) x‐ray focussing array for fluorescence EXAFS in dilute samples

A focussing monochromator array for detection of fluorescence EXAFS (Extended X−ray Absorption Edge Fine Structure) is described. The array is made of LiF crystals, specially prepared for large mosaic spread. Design criteria, performance specifications, and details of fabrication and crystal preparation are given.

T2-AlLiCu: A stable icosahedral phase?

Abstract The system Al-Li-Cu has recently been found to form an icosahedral phase (Ball and Lloyd 1985). We have re-examined the phase diagram and X-ray data of Hardy and Silcock (1955). The powder X-ray pattern for their T2 phase can be indexed to an icosahedral structure with a quasilattice parameter of 5°02 A. Previous work (Cherkashin, Kripyakevich and Oleksiv 1964) shows that the nearby R phase is an isotype of the (Al, Zn) 49Mg32 structure. The T2 phase appears to be a modification of the nearby R phase in close analogy with the relationship observed by Henley and Elser (1986) in Al-Mg-Zn alloys.

Distance distributions about Mn in icosahedral and crystalline Al Mn Si

Abstract We report EXAFS measurements on icosahedral and orthorhombic Al 6 Mn, icosahedral Al 74 Mn 20 Si 6 , and α-Al 100 Mn 24 Si 14 . We find close resemblance between α-phase and i -Al 74 Mn 20 Si 6 , and poorer agreement between α-phase and i -Al 6 Mn. Neither icosahedral phase showed much resemblance to the orthorhombic phase. We conclude that the icosahedral Al 74 Mn 20 Si 6 resembles a distorted α-phase, and so may be built of icosahedral glued together with Al as in the α structure. Icosahedral Al 6 Mn is perhaps a more disordered version of the above.

Cholesteric Pitch and Blue Phases in a Chiral-Racemic Mixture

Abstract The cholesteric and “blue” phases (BP) of a mixture of the chiral and racemic forms of the same compound were studied as a function of composition and temperature. It was found that the clearing point and the cholesteric-SA transition temperature was independent of composition. The pitch of the cholesteric at a given temperature was inversely proportional to the fraction of chiral material, as was the lattice parameter of BPII. The lattice parameter of BPI did not follow the above rule. No second-order BPI-BPIIb transition was seen. It is concluded that this mixture is thermodynamically ideal(6,7) in that the intermolecular interactions between two molecules are independent of their relative chirality. Thus, this system comes close to the theorists ideal of one in which only the chirality is varied.

A procedure for the manufacture of EXAFS samples of concentrated materials

We describe a method of making samples of concentrated materials for use in fluorescence EXAFS (extended x‐ray absorption fine structure) measurements. With this technique, one can easily make samples of a given areal density, with fine particle sizes. Both of these qualities are necessary in order to insure that self‐absorption will not cause distortions in the fluorescence spectrum. These samples are also suitable for transmission measurements.

Structure of the nematic-isotropic interface in Landau theory

Abstract The structure of the nematic-isotropic interface is probed using Landau theory with a tensor order parameter. Strong biaxiality appears near the isotropic side of the interface when the elastic constants are not equal. However, this effect produces only small changes in the surface tension. The dependence of surface tension on the angle between the bulk director and the interface is explored. Also, when the director in the bulk nematic makes a 38° angle with the interface, the director near the interface “bends” by about 5° to become more parallel with the interface.