Smaller Atomic Radius Research Articles

Sign of covalency in AlPdRe icosahedral quasicrystals obtained from atomic density and quasilattice constant

Atomic density and quasilattice constant of AlPdRe icosahedral quasicrystals decreases and slightly increases, respectively, with increasing concentration of transition metals, which have smaller atomic radii. Average atomic packing could not be described simply by rigid sphere packing, that is, the covalent nature of icosahedral clusters may form in metallic bonds.

Effect of AI 3+ introduction into hydrothermally prepared ZnFe 2O 4

The effect of aluminum introduction into the zinc ferrite ZnFe 2− x Al x O 4 spinel structure was studied in the concentration range of 0.0 ≤ x ≤ 1.0. Spinel ferrites were obtained by hydrothermal method at 140°C in the whole range of Al concentration ( x). The resulting powders were calcined at 550 and 750°C, and their physicochemical and catalytic properties were compared with those prepared by coprecipitation method [J.A. Toledo, P. Bosch, M.A. Valenzuela, A. Montoya, N. Nava, J. Mol. Catal. 125 (1997) 53]. As in coprecipitated samples, isomorphic substitution of Fe 3+ by Al 3+ into the octahedral sites was observed. The aluminum introduction gives rise to a lattice distortion caused by the introduction of a metal with smaller atomic radius. This lattice distortion facilitates a charge transfer from Fe 3+ to O 2−, which increases the basicity of the oxygen atoms in the Fe–O–Al bonds, increasing its proton affinity. Therefore, it favors the acid–base dissociation that takes place in the C–H bond during the abstraction of the hydrogen atom in the oxidative dehydrogenation (OXD) process. After calcination at 550°C, a maximum in the intrinsic activity and butadiene yield was obtained for an Al concentration of 0.2 ≤ x ≤ 0.5, whereas in coprecipitated catalysts, a higher aluminum content was necessary (0.75 ≤ x ≤ 1.0). In the hydrothermally treated samples calcined at 750°C, the promoter effect of aluminum was not evident. Indeed, the pure ZnFe 2O 4 showed the maximum activity to butadiene. However, the activity of the structural surface sites increased by a factor about 2 as the calcining temperature increased from 550 to 750°C.

Corrections to the wave function and the hyperfine structure in exotic atoms

We present an analytical evaluation of radiative corrections in exotic atoms induced by the one-loop electronic vacuum polarization. We evaluate corrections to the energy levels, to the wave function (at the origin) and to the hyperfine structure. We treat all corrections analytically within a non-relativistic approximation. Agreement is found with a few available numerical results. The analytical treatment allows to determine the asymptotic forms of the corrections in the limit of a small atomic radius, which for the atomic systems considered corresponds to a large mass of the constituent particle as compared to the electron mass. The asymptotics can be verified using the effective charge approach.

The essentiality of calcium ion in the enzymatic activity of Taiwan cobra phospholipase A2.

In order to address the mechanism whereby Ca2+ wad crucial for the manifestation of the enzymatic activity of phospholipase A2 (PLA2), four divalent cations were used to assess their influences on the catalytic activity and the fine structures of Naja naja atra PLA2. It was found that substitution of Mg2+ or Sr2+ for Ca2+ in the substrate solution caused a decrease in the PLA2 activity to 77.5% or 54.5%, respectively, of that in the presence of Ca2+. However, no PLA2 activity was observed with the addition of Ba2+. With the exception of Mg2+, the nonpolarity of the 8-anilinonaphthalene-1-sulfonate (ANS)-binding site of PLA2 markedly increased with the binding of cations to PLA2. In the meantime, the accessibilities of Lys-6 (65) and Tyr-3 (63) toward trinitrobenzene sulfonate and p-nitrobenzenesulfonyl fluoride were enhanced by the addition of Ca2+, Sr2+, and Ba2+, but not by Mg2+. The order of the ability of cations to enhance the ANS fluorescence and the reactivity of Lys and Tyr residues toward modified reagents was Ba2+ > Sr2+ > Ca2+ > Mg2+, which was the same order as the increase in their atomic radii. These results, together with the observations that the ANS molecule binds at the active site of PLA2 and that Tyr-3, Lys-6, and Tyr-63 of PLA2 are involved in the binding with the substrate, suggest that the binding of Ca2+ to PLA2 induces conformational changes at the active site and substrate-binding site. However, the smaller atomic radius with Mg2+ or the bigger atomic radii with Sr2+ and Ba2+ might render the conformation improperly rearranged after their binding to PLA2 molecule.

Energetics and hydrogen passivation of carbon-related defects in InAs and In0.5Ga0.5As.

We perform ab initio pseudopotential calculations for studying the stability of carbon-related defects and the atomic model for the hydrogen passivation of substitutional carbons in InAs. Among various C-related defects, the most stable one is found to be a substitutional C acceptor occupying an As site. As compared to GaAs, substitutional C impurities are found to have higher formation energies, due to large lattice distortions surrounding the C atom, thus, C incorporation into bulk InAs is more difficult. Because of the small atomic radius and deep atomic energy levels of C, when C occupies an In site, its defect energy level lies below the valence band maximum (VBM), and it behaves as an acceptor, however, the formation energy is much higher than for ${\mathrm{C}}_{\mathrm{As}}$. We note that an inversion between the VBM and the ${\mathrm{C}}_{\mathrm{In}}$ energy level takes place as pressure increases. For both the substitutional ${\mathrm{C}}_{\mathrm{As}}$ and ${\mathrm{C}}_{\mathrm{In}}$, we find that hydrogen neutralizes the electrical activity of acceptors by occupying a bond-centered site between the C atom and one of its neighbors. In an ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As alloy, the C acceptor is found to favor an As site with In neighbors, with the formation energy lying between InAs and GaAs. Thus, the calculated acceptor concentration of ${10}^{17}$ -- ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ is much lower than the maximum carrier density achievable in GaAs. \textcopyright{} 1996 The American Physical Society.

Kinetics and mechanisms of activation and inhibition of porcine liver fructose-1,6-bisphosphatase by monovalent cations.

K+ and Li+ were used to study the kinetic effects of monovalent cations on porcine liver fructose-1,6-bisphosphatase (FBPase). At saturating fructose 1,6-bisphosphate (FBP) concentrations, Li+ was found to be a linear noncompetitive inhibitor with respect to Mg2+. K+ was found to activate the wild-type enzyme at low concentrations (K(m) = 17 mM) and to inhibit the enzyme at high concentrations (K(IK+) = 68mM). A steady-state random ter mechanism was proposed, and a mathematical equation was derived to account for the Mg2+ and K+ kinetics and activation of FBPase. Interestingly, when Glu280 was mutated to glutamine by site-directed mutagenesis, K+ lost the ability to activate the enzyme and became a noncompetitive inhibitor with respect to Mg2+. These kinetic data suggest that K+ has two distinct sites. One is a high-affinity activation site and the other a low-affinity inhibition site. Glu280 is essential for allowing K+ to bind at the activation site. Due to the geometric constraints and its small atomic radius, Li+ can bind only at the inhibitory site. It is postulated that monovalent cations activate FBPase by helping the Arg276 residue "deshield" the partial negative charge on the 1-phosphoryl group of the substrate so that nucleophilic attack on the 1-phosphorus atom is facilitated. In addition, the monovalent cations may, along with Mg2+ ions and surrounding residues of the protein, help orient the 1-phosphoryl group so as to achieve the optimal position required for catalysis. Monovalent cations inhibit FBPase either by distorting the geometry of the active site or by retarding turnover or product release.

Effect of aluminium substitution on the diffusion coefficient and jump length of zinc ferrite

Abstract Polycrystalline samples of mixed ferrite ZnAlxFe2—xO4, where ′ = 0, 0.3, 0.6, 0.9 and 1.2 have been prepared. The jump length of electrons in the octahedral site and conductivity were studied as a function of aluminium substitution. The decrease of the jump length and decrease of bulk density with aluminium concentration is attributed to the substitution of Fe3+ for Al3+ which has a smaller atomic radius and lower atomic weight than Fe3+. The nonmagnetic Al3+ ions substitute Fe3+ ions and reduce the exchange electrons between ferric and ferrous ions and thereby reduce the conductivity and mobility. The increase of aluminium concentration decreases the diffusion coefficient of vacancies which is the predominant role for densification during sintering.

Hydrogen-induced transformation to an amorphous state in the Laves phases Ce(Ru, M) 2 (M  Fe, Co, Ni)

The structural changes of the Laves phases Ce(Ru 1− x M x ) 2, where x = 0.05–0.25 and M is the transition elements, such as Fe, Co and Ni, during hydrogen absorption are investigated through X-ray diffraction (XRD), transmission electron microscopy and differential scanning calorimetry (DSC). All the examined ternary alloys, regardless of the composition and the species of the third elements, can be amorphized by hydrogen absorption at room temperature, even though the binary CeRu 2 alloy absorbs hydrogen in the crystalline hydride state. The amorphization mechanism is suggested to be identical to that of the binary CeM 2 Laves phases, indicating that the elastic strain caused by hydrogen plays a key role in the amorphization. The critical strain applied on the Ce(Ru 0.75Ni 0.25) 2 compound prior to amorphization is calculated through the DSC and XRD analysis, and it is about 1.9 ± 0.5 vol.%. It is proposed that the distinct change in the hydrogenation properties of the CeRu 2 system by this substitution is caused by the build-up of the lattice distortion owing to the presence of the third element, which has a smaller atomic radius.

Crystallization behaviors of the hydrogenated Zr-rich amorphous Zr xNi 1−x:H alloys and the effects of titanium substitution

In order to investigate the effect of hydrogen on the thermal stability of the amorphous Zr xNi 1−x alloys, the crystallization of the hydrogenated alloys is examined using differential scanning calorimetry. By hydrogenation, the crystallization reaction is accelerated and the cubic ZrH 2 phase is formed at first. At the next step, cubic ZrH 2 is further transformed to the tetragonal structure. The corresponding mechanism is different from that of the as-received state and it is proposed to be related to the motion of nickel atoms, these having smaller atomic radius. When partially substituting titanium, the crystallization temperature and activation energy are greatly increased for both the as-received and the hydrogenated systems. This is thought to be caused by the retardation of the nickel diffusion and by hardening of the amorphous structure elastically, due to the presence of titanium atoms.

Tracer impurity diffusion in amorphous NixZr1−x alloys

Impurity tracer diffusion coefficients for Co and Hf were measured in a series of amorphous NixZr1−x alloys at various temperatures for 0.3<x<0.7. The diffusion coefficients for Co, an impurity with small atomic radius were, depending on alloy composition and temperature, two to four orders of magnitude larger than those for Hf, an impurity with large atomic radius. Whereas the Hf tracer diffusion coefficients are independent of alloy composition, those of Co decrease strongly with increasing Ni concentration. These results suggest that the diffusion mechanisms for atoms with small or large atomic radii are different in amorphous Ni-Zr alloys.

Heat resistance of hydrogenated amorphous silicon films prepared by compressed magnetic field-magnetron sputtering with He and Ar gases

Usually, an Ar gas adding hydrogen gas is used for the fabrication of hydrogenated amorphous silicon (a-Si:H) films by reactive sputtering. The sputtered films contain Ar atoms of more than 5 at. %. These implanted Ar+ ions degrade the film quality, and H atoms, which terminate dangling bonds, leave the film easily by heating to about 300 °C. A He atom has a lighter atomic mass and smaller atomic radius than an Ar atom. Therefore, the reactive sputtered films using the He gas will be less affected by the implanted He+ ions than they will by the implanted Ar+ ions of those sputtered by Ar gas. Infrared absorption (IR) and electron spin resonance (ESR) measurements of the films sputtered by He gas are made and compared with those sputtered by Ar gas.

Electrical resistivity and lattice constant of (Ce1−xLax)Ni2

Measurements of the lattice constant and the magnetic susceptibility on (Ce 1− x La x )Ni 2 show that the electron state of Ce atoms having small atomic radii in CeNi 2 recovers the normal Ce 3+ state with the increase of La concentration. The Kondo effect was observed for the resistivity of samples having a wide range of Ce concentration.

Dopant states ina-Si: H. I. Tight-binding-model results

The gap states of substitutional dopants in $a$-Si: H are studied with the use of tight-binding models. Within its 1.8-eV-wide gap, N is found to be the only truly shallow donor, and P is borderline deep, with As, Sb, and Bi deeper still. For acceptors, only B is found to be shallow because of its small atomic radius. It is argued that bond-length disorder at substitutional impurity sites is small, as elsewhere in $a$-Si. The remaining forms of disorder are calculated to cause only a small \ifmmode\pm\else\textpm\fi{}0.05-eV broadening of donor levels but a dramatic \ifmmode\pm\else\textpm\fi{}0.2-eV broadening of any deep acceptor levels. Pairing of dopant sites is found to cause a small broadening of levels, but donor-acceptor pairing is shown to give shallower levels.

Unusual isotope effects for diffusion in VH0.50 and VT0.50

The diffusion of hydrogen in the monohydride phase (β phase) of the V1−xCrx–H (x ≤ 0.1) system was studied by means of 1H nuclear magnetic resonance (NMR). Hydrogen atoms in the hydrides occupied the octahedral (O) sites in a body centered tetragonal (BCT) structure. The BCT lattice was contracted by addition of Cr because Cr has a smaller atomic radius than V. The activation energy for hydrogen diffusion, EH, and the Korringa constant, T1eT, were estimated from the temperature and frequency dependence of 1H spin–lattice relaxation times, T1. The value of EH increased with increase in the Cr content. The contraction of the BCT lattice by addition of Cr suppressed the diffusion of hydrogen which occupied the interstitial sites. Addition of Cr also increased the T1eT, which was accompanied by reducing in the stability of the monohydride phase of the V–H system.

Factors controlling enrichment of vanadium and nickel in the bitumen of organic sedimentary rocks

Enriched concentrations of vanadium and nickel have been noted in a variety of naturally occurring organic substances including crude oils, asphalts, and organic matter in some sedimentary rocks. Vanadium and nickel concentrations in bitumens extracted from a variety of organic sedimentary rock types of different geological ages and geographical areas range from less than 0.2 to 4760 ppm and less than 7 to 1240 ppm, respectively. Vanadium concentrations showed a polymodal frequency distribution, while nickel concentrations showed a near-normal frequency distribution. The concentrations of these two metals showed no significant correlations with bitumen content, organic carbon content, or proportionality between bitumen and organic contents. Enriched vanadium and nickel concentrations greater than 100 ppm are only observed in bitumens that are associated with Type II and Type I kerogens. Conversely, bitumens associated with Type III kerogens contained vanadium and nickel concentrations less than 100 ppm. The high stability of vanadium and nickel in crude oils, asphalts, and bitumens suggest that they occur in tetrapyrrole complexes. These complexes may occur as free molecules or assimulated subunits in macromolecules because of their availability in anaerobic systems, small atomic radii, and favorable electron configurations. The potential for an organic sediment to be enriched in these two metals depends upon the amount of tetrapyrroles preserved in its organic matter. Tetrapyrrole preservation preferentially decreases in organic matter as exposure time to aerobic conditions increases. The potential for vanadium and nickel enrichment is therefore the highest in organic matter derived from algae that encountered anaerobic conditions early in their depositional history. Metallation of tetrapyrrole complexes appears to occur within sediments prior to their lithification, and interstitial waters are the most likely source for enriched concentrations of vanadium and nickel. The amount of metal enrichment in a sediment depends upon the diffusion of metal cations from its overlying water body, and this source is only effective as long as the sediment system remains open. The complete metallation of tetrapyrroles is most favorable under conditions of slow sedimentation rates, which would allow open sediment systems to be maintained for long durations.

Substituent Effects in Heterogeneous Catalysis. III. Competitive Hydrogenation of Cyclohexanone and 2-Alkylcyclo-hexanones over Platinum Group Metals

Abstract Cyclohexanone (1) and one of its 2-alkyl (methyl, ethyl, or propyl) derivatives (2) were hydrogenated competitively at 30 °C in cyclohexane over platinum group metals. The alkyl derivatives 2 were all less reactive than 1 on all the catalysts used. The relative reactivity R2⁄R1 was given by log10(R2⁄R1)=σ**+k, with σ** depending only upon the substituent and k upon the catalyst. The greater the substituent size, the more negative was the substituent constant σ**. The catalyst-dependent constant K was in general more negative for a catalyst metal having a smaller atomic radius. The above empirical rate expression has been interpreted based on the absolute reaction rate theory. The σ** values calculated on this theoretical ground were in excellent agreement with the observed values.

Si-Stabilised C14 laves phases in the transition metal systems

A systematic exploration of the ABSi systems, where A and B are two transition elements of large and small atomic radii, respectively, has been made in order to detect Si-stabilised Laves phases. In all, 27 Si-stabilised Laves phases have been found. The stabilised Laves phases are of the hexagonal C14 type, irrespective of whether the A-B binary system had no Laves phase or had a C15-type cubic Laves phase.

A simple interpolation formula for the Debye temperatures of disordered alloys

A simple formula is derived which expresses the Debye temperature of a disordered multicomponent alloy with small atomic radius disparity in terms of the Debye temperatures of the pure components, the mole fractions of the components, and a set of undetermined parameters, each of which can be found from the Debye temperature for one composition of each possible binary alloy made from the components. The formula is an improvement on a similar formula for the binary case due to Mitra and Chattopadhyay because the physical significance of the undetermined parameters is more readily apparent. The influence of short-range order in the binary case is considered. Application is made to the Ag-Au-Pd, Cr-Fe, and Cu-Ni systems in order to test the formula.

Kristallstruktur von Pt 3Ga(r) und einigen phasen der mischung PtAl

The structure of Pt 3Ga(r) is a tetragonal variant of the Cu 3Au structure, which has a displacive superstructure stabilized by the small atomic core radius of the minority component. Pt 3Al(r) is isotypic or at least closely related to Pt 3Ga(r). The new phases Pt 2Al(h) (Ni 2Si type) and Pt 2Al(r) (Pt 2Ga(r) type) are reported. Pt 2Al 3 crystallizes in the Ni 2Al 3 type of structure. The results are-interpreted by means of the two-correlations model.

Mo中のFeの不純物拡散

The residual activity technique was employed to determine the impurity diffusion coefficients of Fe59 in Mo in the temperature range of 1000 to 1350°C. The diffusion coefficient at 1350°C was 1.2×10−12 cm2/sec, which was larger than the value for the impurity diffusion coefficient of Co in Mo obtained by Peart et al., but smaller than any value for the impurity diffusion coefficient of Fe in refractory metals as Nb, Ta and W at the corresponding temperature. It was found that the temperature dependence of Fe59 diffusivity in Mo showed the Arrhenius relationship, DFe→Mo=(0.15+0.09−0.06)exp(−82.7±1.9⁄RT) cm2/sec, and no anomaly as in the impurity diffusion of Fe in β-Ti was observed. The activation energy for the diffusion of Fe in Mo was found to be smaller than that for the self-diffusion in Mo by about 10 kcal/mol. This trend holds in case of the impurity diffusion of Fe in the refractory metals such as Nb, Ta and W. The size effect appears to be important for the impurity diffusion of Fe in those metals, and the above trend may be tentatively explained by the small atomic radius of Fe with respect to those of such other elements. Using Swalin’s model, the activation energy for the impurity diffusion of Fe in Mo was calculated to be 84.4 kcal/mol, which was in good agreement with the experimental value. The empirical equation, lnD0=0.12Q−11.6, was obtained by the data on the self-diffusion of Mo, Nb, Ta and W the impurity diffusion of Fe and the other elements in these metals.