Interchange Of Atoms Research Articles

Potential function for the ν7 vibration of phosphorus pentafluoride

The gas phase Raman spectrum of the ν7 fundamental of PF5 has been observed under spectral resolutions approaching 1 cm−1. With the use of a two dimensional harmonic oscillator basis set and a potential function of the form V (ρ,φ) = (1/2) aρ2 − b cos(3φ) ρ3 + cρ4, an adequate fit to the ν7 region was obtained. The barrier to axial–equatorial fluorine atom interchange was estimated to be 1371 cm−1 (3.92 kcal/mole).

Fe<SUB>3</SUB>Al合金の規則化の速度論

The kinetics of ordering in Fe-Al alloys with about 25 at%Al is investigated both theoretically and experimentally. The kinetic equations which express the time variation of the degrees of order in B2 and DO3 structures are obtained by taking into account the direct interchange of atoms between the first and the second nearest neighbors. The experimental results of X-ray diffraction on isothermal annealing or slow cooling of these alloys are reproduced satisfactorily by the theoretical rate equations.The ordering in DO3 type is influenced substantially by the atomic interchange between the second nearest neighbors. The B2 type ordering takes place so rapidly that the quenching from a disordered phase can not prevent the ordering. An oscillatory change of the degree of order in the B2 structure is expected from the calculation in the isothermal annealing.

A theory of surface enrichment in ordered alloys

A very simple theory has been developed to explain experimental data on surface enrichment in Pt3Sn. The computed surface enrichment is in accord with experimental findings. The theory predicts that in the Pt3Sn system enrichment occurs by interchange of atoms of the element with the lower heat of sublimation from the layer just below the surface with atoms of the other element in the surface. Arguments are presented why an experiment performed on AuCu alloys should be capable of verifying an assumption basic to the theory.

Kinetics of Ordering in Fe–Al Alloys

The kinetics of ordering in Fe–Al alloys with about 25 at%Al is investigated both theoretically and experimentally. The kinetic equations which express the time variation of the degrees of order in B2 and DO3 structures are obtained by taking into account the direct interchange of atoms between the first and the second nearest neighbors. The experimental results of X-ray diffraction for these alloys on isothermal annealing or slow cooling are reproduced satisfactorily by theoretical rate equations, and the characteristics of the ordering in the Fe–Al system is investigated. The ordering in DO3 type is influenced substantially by the atomic interchange between the second nearest neighbors. The B2 type ordering takes place so rapidly that the quenching from a disordered phase does not prevent the ordering. An oscillatory change of the degree of order in the B2 structure is expected from the calculation in the isothermal annealing.

On the bainitic transformations in uranium carbides

In view of the high-temperature solid solution that exists from the approximate composition UC to the approximate composition UC 2, a new nomenclature is suggested for the carbide phases of uranium. As far as the uranium atoms are concerned, there are very close relationships between these phases and models are examined for the ‘military transformations’ that are believed to occur in them. It is assumed that the carbon diffusion also required by these transformations, including the interchange of single carbon atoms for double carbon atoms, is easy. The bainitic transformations thus deduced are used to interpret the microstructures revealed by transmission electron microscopy of three uranium-carbon alloys.

Low-temperature dielectric relaxation in polyethylene

The technique due to Vincett has been developed to extend previous measurements of the dielectric loss tangent of polyethylene at cryogenic temperatures upwards in frequency to 100 MHz. A second relaxation peak, centred at 4 MHz, found in oxidized polyethylene is both broader and larger than the well-documented peak at 3 kHz. The dielectric loss is changed by steaming in D 2 O and H 2 O vapour at elevated tempera­tures. In all cases the 3 kHz relaxation is removed. Steaming in D 2 O moves the 4 MHz peak to 30 kHz and subsequent steaming in H 2 O restores it, indicating that there is an interchange of labile hydrogen and deuterium atoms at the site of the dipoles. Subsequent drying in vacuo of polyethylene treated in D 2 O also partially restores the 4 MHz peak, suggesting a further secondary diffusion process. The magnitude of the loss in Rigidex 50 (high-density polyethylene) shows little temperature variation between 1.5 and 4.2 K, while the frequency of maximum loss varies linearly with temperature for the 30 kHz peak; there is, however, a small deviation from linearity at 1.5 K for the 4 MHz peak. The results are analysed in terms of phonon-assisted tunnelling of a particle between two nearly-equivalent potential wells, using exact solutions of Pauling’s cosine model. Tentative estimates of the parameters of the model indicate that the results can be explained by 180° rotation of the proton in a dipolar group of bond length 0.11 nm.

Electron Transfer and Ion—Atom Interchange in C+–CO Collisions

Kinetic-energy distributions of CO+ and C2+ ions resulting from C+–CO collisions were measured over the incident ion kinetic-energy range 13–30 eV. These distributions were used for calculation of the reaction energetics. The CO+ kinetic-energy distributions provide strong evidence that this product is formed by both electron transfer and ion—atom interchange. In addition to the major (forward scattered) peak in the C2+ distribution small yields of C2+ were detected at 180° in the center-of-mass system.

Inelastic collisions between atomic ions and diatomic molecules

Measurements of rate coefficients for the charge exchange collision processes Ar+N2, Ar+CO, Ar+O2, Ar+NO, N+O2, have been made in the energy range 0.04-2 eV in the injected ion drift tube. A new solution procedure for the diffusion problem is presented. Data are sensitive to secondary reactions caused by ions produced in the thermalization of the injected ions; systematic studies of suitable buffer gases habe been made to avoid errors arising from these reactions. Calculations habe been made of cross section functions for charge exchange and ion atom interchange processes using the 'nearest resonance' approximation, including allowance for the exchange interaction, and with restrictions placed upon the rotational quantum numbers of the molecular ion product.

規則状態におけるFe-CoおよびβCuZn合金の動的挙動

Kinetic behaviors of ordered Fe-Co and β CuZn alloys have been investigated with basic equations derived on the basis of the Bragg and Williams theory which is extended to a binary alloy with CsCl structure involving vacancies. It is then assumed that atom movements occur in two mechanisms, a direct interchange of atoms and an interchange through vacancies. The dependence of vacancy concentration upon long-range order is also considered to follow the method of Girifalco. Both of the mechanisms assumed here for the interchange of atoms play an important role in the ordering process. The interchange through vacancies is dominant particularly in the ordering process for β CuZn alloy. The numerical calculations have been carried out by using the appropriate values estimated from the data of diffusion and of other experiments. On heating of ordered Fe-Co alloys, the order parameters at room temperature are retained to about 500°C. Beyond 500°C they begin to vary rapidly with the maximum rate at about 550°C and finally reach their equilibrium values. This result is in agreement with the interpretation by Yokoyama, Takezawa and Higashida. On the other hand, the temperature dependence of the order parameter in the ordered β CuZn alloy exhibits only a negligible anomaly near room temperature.

Thermodynamics of the UO 2+ x phase I. Heat capacities of UO 2.017 and UO 2.254 from 300 to 1000 K and electronic contributions

Heat capacities of UO 2.017 and UO 2.254 have been measured in the range 300 to 1000 K by adiabatic calorimetry. The enthalpy and entropy increments { X o(1000 K) − X o(0)} are 66.49 kJ mol −1 and 170.2 J K −1 mol −1 for UO 2.017, and 72.91 kJ mol −1 and 185.7 J K −1 mol −1 for UO 2.254. The increments { X o(298 K) − X o(0)} are based upon literature data. In UO 2.017 the presence of the U 4O 9 phase causes slightly variable heat capacities in the range 300 to 360 K due to a partial suppression of the transition in U 4O 9 in the super-saturated solid solution of U 4O 9 in UO 2. Augmented heat capacity values are observed in the range 500 to 800 K, due to the solution of U 4O 9 in UO 2. In UO 2.254 a λ-type transition is present with maximum at 348 K and an entropy increment of 2.18 J K −1 mol −1, presumably connected with interchange of interstitial and displaced oxygen atoms. A small irregularity in the heat capacity is observed in the region 900 to 950 K with an entropy increment of 0.15 J K −1 mol −1. It is superimposed on a general increase in the heat capacity, presumably due to the onset of a transition. Attempts to resolve the various contributions to the heat capacity of UO 2 show that the electronic contribution is large. It is interpreted in terms of a doubly degenerate level of the 5f 2 electrons about 900 cm −1 above the triply degenerate ground level followed by a triply degenerate level about 1600 cm −1 above, and a singly degenerate level about 2900 cm −1, above the ground level. Further excited levels are also considered, but they are apparently not numerous enough to explain the enhanced heat capacity above 2000 K. In the region 2000 to 3000 K structural order-disorder processes, vacancy formation, and phase separation from the stoichiometric dioxide probably occur. The results for UO 2.254 also indicate a large electronic contribution. It is tentatively analyzed assuming the presence of U(IV) and U(V) in equal amounts, corresponding to the formula 2UO 2·U 2O 5. If the contribution from U(IV) remains unchanged above 300 K, the electronic entropy of U(V) is about 7.3 J K −1 mol −1 for 1 2 UO 2. 1 4 U 2O 5 at 600 K. This indicates that at least six levels of the 5f 1 configuration are of importance in the region up to 600 K. The possibility of a doubly degenerate ground level followed by a quadruply degenerate level about 500 cm −1 above it is discussed in terms of available ligand field data for U(V).

Temperature Dependence of Slow Ion–Atom Interchange Reactions

First Page

Negative ion – molecule reactions

Laboratory reaction rate constant measurements for negative ion – atom interchange reactions, negative ion charge transfer reactions, and negative ion three-body association reactions of aeronomic interest are reviewed and the available data tabulated. The present experimental techniques in use are briefly summarized. Most of the rate constants have been measured only at 300 °K; in a few cases data is available at energies [Formula: see text] as well as at 300 °K, so that an indication of the energy dependence of the rate constants is available.

Interchange of hydrogen and oxygen atoms in [formula omitted]-nitrotoluene at high temperatures

Interchange of hydrogen and oxygen atoms in [formula omitted]-nitrotoluene at high temperatures

Thermal-Energy Ion—Neutral Reaction Rates. V. Measured Rate Constants for C+ and CO+ Reactions with O2 and CO2

The rate constants for the charge-transfer reactions of CO+ with O2 and CO2, and the ion—atom interchange reactions of C+ with O2 and CO2 have been measured at 300°K in a pulsed-discharge flowing-afterglow system. These reactions are all efficient, having rate constants ∼10−9 cm3/sec except for CO+ charge transfer to O2 which has a rate constant 2×10−10 cm3/sec.

REACTION OF N2(A3∑U+) WITH ATOMIC NITROGEN

Attempts to measure the actual lifetime of N2(A3∑u+) in active nitrogen resulted only in a very small upper limit of ≈5 × 10−4 s. This result is consistent with quenching of the A3∑u+ state by atomic nitrogen in an atom–atom interchange process,[Formula: see text]having a rate coefficient of [Formula: see text] It is suggested that similar processes are involved in determining the distribution of excited states observed in atom association.

The structure of liquid tin

Abstract X-ray diffraction patterns have been obtained from liquid tin under good experimental conditions with particular emphasis on obtaining clean, oxide-free surfaces. The patterns show that a considerable degree of structure exists in the melt in contrast with simple liquids such as molten Na, K, Cu and Au. Analysis by a radial distribution method shows that there are two distances of atomic separation in the melt. The first is clearly defined as 3·27 A. The second is about 2·7 A, so that the near distance of closest approach of grey tin is re-established in the liquid. The picture of liquid tin which emerges is one of tetrahedra of grey tin floating in a matrix of metallic tin, with possibly, interchange of atoms between the two states. It is not possible to put an exact figure to the relative proportions of atoms in the two states, other than those in the grey tin state, represent between 5 and 20% of the whole.

OBSERVATIONS OF TWILIGHT AND SUNLIT AURORA

The 3914-Å band of [Formula: see text] has been observed in the zenith at twilight by a photoelectric spectrometer; simultaneous measurements of the 5577-Å line were made as a check on auroral activity. A small twilight enhancement was probably observed in the absence of aurora in 1960, but it could not be detected in 1961. After subtraction of this background, the auroral measurements showed [Formula: see text] densities of about 1000 ions/cm3 at 160 km, with a sharp decrease at lower altitudes; some displays, presumably located near 100 km, showed no twilight effect at all. It is suggested that [Formula: see text] ions are rapidly removed in the 100-km region by either charge–exchange or ion–atom interchange reactions which convert them into NO+ or some other ion. The ions in the twilight airglow are therefore probably located at 550 to 620 km, being produced there by solar extreme ultraviolet. However, this process requires a much higher N2 concentration at such heights than is normally supposed. This may be produced at times of high solar activity, and especially during magnetic storms.

Crystallographic aspects of transformations in the β, β' and ζ phases of the silver-zinc alloys

The structures and transformations which occur in the silver-zinc alloys of around 50 at. per cent composition are reviewed. The crystallographic relations in the β' and ζ phases are examined. It is shown that the β' →ζ transformation may be brought about by an interchange of nearest neighbour silver and zinc atoms in order to produce certain (112) planes containing only zinc atoms. A movement corresponding to a shear in the [111̄] direction of rows of adjacent zinc atoms in these planes completes the transformation. The mechanism gives a simple explanation of the partial order of the ζ phase found by other workers and also gives an explanation for the structures of the anti-phase domains which occur in the β → ζ transformation previously described on the basis of experimental work by the author. Applications of these results to the transformations reviewed are given.

Formation of modulated structures in copper-nickel-iron alloys

By means of X-ray diffraction, the formation of modulated structures with periodic variations in composition due to a simple interchange of atoms has been studied in copper-nickel-iron alloys as a function of composition, temperature and time. The results are compared with two models suggested for this type of “exchange transformation”. The reaction characteristics seem to favor a kinetic model rather than the Guinier model. The activation energies for the transformation, evaluated at the first detectable stage and for a later stage, show that both these stages are diffusion-controlled inside as well as outside the spinodal. This indicates that the spinodal is of no real significance with regard to the nucleation process. The variation with temperature and composition of the first wavelength to form in the modulated structure is in agreement with the kinetic model.

Statistical dynamics of crystalline diffusion: Part I. Exchange mechanism

The path probability method for irreversible cooperative phenomena proposed by the author is applied to atomic diffusion in crystals. A binary alloy of body-centered cubic structure with the concentration gradient along a [100] direction is discussed, with the direct exchange mechanism for atomic migration assumed. The path probability g for change of state during a short time inrerval is written in terms of the path parameters { Y ij } using the pair approximation of the cluster-variation method. The most probable path is determined by maximizing g with respect to { Y ij }. The system is isothermal and a flow of atoms is maintained by two particle reservoirs of different chemical potentials placed at the two ends of the system. When there is no chemical potential gradient, the system is in an equilibrium state identical to that obtained by the pair approximation, which is equivalent to Bethe's approximation or the quasi-chemical approximation. The diffusion coefficient is obtained from the stationary state by expanding the state parameters in terms of the concentration gradient. The expression of the diffusion coefficient is given for an arbitrary value of the concentration, and is written in terms of a probability parameter for a unit interchange of atoms.