Showing papers on "Inorganic compound published in 1989"

Growth and equilibrium structures in the epitaxy of Si on Si(001).

Abstract: Scanning-tunneling-microscopy measurements of submonolayer growth of Si on Si(001) show anisotropic island shapes. From coarsening experiments it is determined that these shapes are a consequence of the growth kinetics. They can be explained by a lateral accommodation coefficient for atoms arriving at the edge of islands that differs by an order of magnitude at the ends and at the sides of dimer rows.

Carbon diffusion in undoped, n‐type, and p‐type GaAs

Abstract: The effects of background doping, surface encapsulation, and As4 overpressure on carbon diffusion have been studied by annealing samples with 1000 A p‐type carbon doping spikes grown within 1 μm layers of undoped (n−), Se‐doped (n+), and Mg‐doped (p+) GaAs. The layers were grown by low‐pressure metalorganic chemical vapor deposition using CCl4 as the carbon doping source. Two different As4 overpressure conditions were investigated: (1) the equilibrium pAs4 over GaAs (no excess As), and (2) pAs4 ∼2.5 atm. For each As4 overpressure condition, both capless and Si3N4‐capped samples of the n−‐, n+‐, and p+‐GaAs crystals were annealed simultaneously (825 °C, 24 h). Secondary‐ion mass spectroscopy was used to measure the atomic carbon depth profiles. The carbon diffusion coefficient is always low, but depends on the background doping, being highest in Mg‐doped (p+) GaAs and lowest in Se‐doped (n+) GaAs. The influence of surface encapsulation (Si3N4) and pAs4 on carbon diffusion is minimal.

Structure determination of the Si(111): B(3×3)R30°surface: Subsurface substitutional doping

Abstract: Synchrotron x-ray diffraction has been used to analyze the ({radical}3{times}{radical}3){ital R}30{degree} reconstruction of B/Si(111). Excellent agreement is obtained with in-plane data for a model in which boron sits in every third site of threefold symmetry. Out-of-plane diffraction, however, is only consistent with boron {ital below} the surface in the fivefold-coordinated substitutional site under a silicon {ital T}{sub 4} adatom. The structure is confirmed by the growth behavior under room-temperature Si deposition in which the silicon adatom is displaced from its ordered site leaving boron in a two-dimensional ordered substitutional array.

Anomalous absence of pressure effect on transition temperature in the electron-doped superconductor Nd1.85Ce0.15CuO4−δ

Abstract: THE recent discovery of the electron-doped superconductors1 added fresh fuel to controversies about the mechanism of high-temperature superconductivity in copper oxide compounds. The superconducting transition temperature (Tc) of almost all the hole-doped copper oxide compounds increases with increasing pressure2. The pressure coefficient has a large positive value compared with a typical conventional (BCS) superconductor. A systematic survey of the TcS of Y–Ba–Cu–O and La–Sr–Cu–O compounds has shown that Tc is strongly correlated with hole concentration3–5, but the Hall coefficient is only weakly dependent on pressure6,7. These results indicate that the large change in Tc with pressure is caused, not by a change in hole concentration, but by other factors related to changes in the interatomic distances. Here we report that the newly discovered electron-doped superconductor Nd,1.85Ce0.15CuO4–δ exhibits almost no pressure effect on Tc up to 2.5 GPa, in remarkable contrast to the hole-doped superconductor Nd1.3Ce0.3Sr0.5CuO4–δ. The Cu–O pyramids characteristic of the hole-doped compounds lose their apical oxygens to become square planes in the electron-doped materials. If the pressure effect on Tc does arise from changes in bond lengths, the difference in behaviour of the two compounds points to the involvement of the bond between copper and apical oxygen, which is missing in the electron-doped material.

Stark effect spectra of Ru(diimine)32+ complexes

Abstract: in zeolite-Y.I6 [Note pentamethylbenzene with u = 7.15 A passes readily, whereas hexamethylbenzene with u = 7.95 A does not-despite its enhanced donor strength.] Once the arene has penetrated this window, a cavity dimension of-1 3 8, is large enough to accommodate both MV2+ and anthracene (but not tetracene), as depicted in Figure 3. A similar shape selectivity is also observed with zeolite-)< (of related dimensions to zeolite-Y)\" but not with the more constricted 4.2 8, aperture in zeolite-A'* or with completely amorphous silica alumina. Owing to the wide structural variations that are allowed by different organic cations, we anticipate that shape selectivity can also be finely tuned in the formation of CT complexes with other types of hydrocarbon donors. W e hope that further tests in progress will provide the requisite information for the detailed mapping of substrate access to various zeolite structures,19 especially with regard to both their kinetic and thermodynamic properties in the liquid phase.

Proton-coupled electron transfer in [(bpy)2Mn(O)2Mn(bpy)2]3+

Abstract: Etude des reactions electrochimiques du complexe de manganese sur une electrode de carbone a l'etat vitreux dans de l'acetonitrile

A model for the nonlinear photoconductivity of BaTiO3

Abstract: Essai d'explication de l'origine de la photoconductivite non lineaire dans BaTiO 3 dope. On utilise le modele de transport de charge introduit par Brost et al. pour decrire l'absorption en fonction de l'intensite lumineuse

Neon and argon bonding in first-row cations NeX+ and ArX+ (X = Li-Ne)

Abstract: schungsgemeinschaft, the Fonds der Chemischen Industrie, and the Rechenzentrum der Universitat Koln for support. Registry No. He, 7440-59-7; Li', 17341-24-1; Be', 14701-08-7; B', 14594-80-0; c+, 14067-05-1; N+, 14158-23-7; 0+, 14581-93-2; F+, 14701-13-4; Ne', 14782-23-1; BHe2+, 74891-40-0; CHe2+, 53262-54-7; HeN2+. 80896-02-2: He02+. 12269-22-6: HeF2+. 119455-36-6: HeNe2+. Acknowledgment. This research has been Supported by a grant from the San Diego Supercomputer Center, which provided computer time at the CRAY-XMP/48. We thank Dr. Bowen Liu -for helpful discussions. D.C. 'thanks the Deutsche For57143-65-4.

The structure of C4 as studied by the Coulomb explosion method

Abstract: Application de la methode du titre a l'etude de la structure de C 4 . Observation pour la premiere fois d'un C 4 rhombique

Reactions of hydrated electron clusters (H2O)n-: scavenging the excess electron

Abstract: Observation de la formation d'agregats anioniques dans la reaction de (H 2 O) n − avec O 2 , CO 2 , NO, N 2 O, Br 2 et CH 3 Br

The magnetic susceptibility and EPR of the organic conductors α'-(BEDT-TTF)2X, X=AuBr2, CuCl2 and Ag(CN)2

Abstract: The authors have investigated the magnetic properties of some linear chain anion salts of BEDT-TTF which adopt the alpha '-structure. These are (BEDT-TTF)2X, where X=AuBr2-, CuCl2- and Ag(CN)2-. In the alpha '-morphology the donor stacks are formed from twisted BEDT-TTF dimers, with the BEDT-TTF molecules within the dimeric unit rotated with respect to each other about the stacking axis by about 32 degrees . Contact between the dimer pairs along the stacking axis is poor, and this has a strong influence on the electronic properties. They find that this series of compounds are highly localised, low-dimensional spin systems with S=1/2 per BEDT-TTF dimer. They describe this behaviour as that of a Mott-Hubbard insulator, and consider that it is due here to the narrow band width along the stacking direction. The behaviour of the susceptibility at low temperatures indicates weak antiferromagnetic coupling, with an exchange energy of about 50 K. The Ag(CN)2- salts show a sharp fall in susceptibility at 7 K, and they consider that this is probably associated with a spin-Peierls transition.