Papers by Keyword: Isotope Effect

Abstract: In this paper, the diffusion isotope effect and the Manning factor are investigated by means of Molecular Dynamics simulations in liquid Cu-Ag alloys. The values for the diffusion isotope effect parameter allow for the estimate of the number of atoms that are moving cooperatively in a basic diffusion event as ‘seen’ by a given (tracer) atom. On average, in the considered alloys and considered temperatures, this is limited to between 5 and 15. This is consistent with results of Molecular Dynamics simulations on the average coordination number calculations. This would suggest that, together with a given atom, a majority of the neighbouring atoms are involved in a basic diffusion event. Results for the Manning factor (MD simulation) for Cu-Ag liquid alloys are seemingly in agreement with the direct exchange mechanism where only two atoms are involved in the elementary diffusion event. This is not in apparent agreement with the isotope effect results. It was shown, however, that any ring mechanism, or, more complex, cage mechanism are, in fact, a combination of several simultaneously happening direct exchanges. Any other possible mechanisms for diffusion in liquids is most likely a combination of direct exchanges as well. It can be seen then that the collective nature of all considered mechanisms is very similar and follows the direct exchange signature properties.

Abstract: We remind the reader to some common features of metallic and oxide glasses. We then introduce the radiotracer method for diffusion studies, which can be applied for both types of glasses. We provide an overview on diffusion in metallic glasses in which we consider both types of metallic glasses – conventional and bulk metallic glasses. In the last part we discuss diffusion and ionic conduction in oxide glasses. For ionic glasses, conductivity measurements are an important complement to tracer diffusion studies. We remind the reader to the method of impedance spectroscopy. We discuss results for soda-lime silicate glasses, single alkali borate glasses and mixed alkali borate glasses and present evidence for collective jump processes in glasses.

Abstract: In this paper, the diffusion isotope effect and diffusion mechanism are investigated by means of molecular dynamics simulations in two liquid alloys, Ni-Ag and Ni-Cu. The values for the diffusion isotope effect parameter allow for the estimate of the number of atoms which are moving cooperatively in a basic diffusion event as experienced by a given atomic species. It is shown that the composition dependence of N_D is typically very small. However, the temperature dependence of this parameter is much more pronounced. In addition, it is shown that, on average, in these alloys and temperatures considered, N_D is limited to the range: 5<N_D<17. This is consistent with results of molecular dynamics simulations on the average coordination number calculations. This would suggest that, together with a given atom, depending on temperature, the neighbouring atoms are all involved in the basic diffusion event.

Abstract: Firstly, this paper reminds the reader of some basic facts about the glassy state, then of the various ways to produce amorphous metals with particular emphasis on the route of vitrification from the melt. Vitrification of an undercooled melt is the most important route from the viewpoint of the application of metallic glasses. We compare diffusion in some metallic glasses with related crystalline metals. Glassy metals, also called metallic glasses, comprise conventional [1] and bulk metallic glasses [2,3]. We remind the reader of the major experimental techniques for diffusion studies in metallic glasses. The paper then reviews our current understanding of diffusion in glassy metals (see also [4,5,6]), including conventional as well as bulk metallic glasses and undercooled melts. We cover the temperature dependence of diffusion in metallic glasses and discuss the spectrum of activation parameters of glassy metals and its difference to the corresponding one of crystalline metals. We mention the pressure dependence and the isotope effect and we discuss tracer diffusion and viscosity diffusion for a bulk metallic glass and its undercooled melt. Finally we mention computer simulations of atomic jump processes. The diffusion mechanism in metallic glasses differs from that in crystalline metals and involves thermally activated, highly collective (chain-like or caterpillar-like) diffusion jumps. Finally, we mention diffusion along shearbands in a plastically deformed glassy metal.

Abstract: The oxygen isotope effect in ordered half-doped manganite PrBaMn₂^16-18O₆ has been investigated. Real and imaginary parts of ac magnetic susceptibility, dc magnetization and magnetoresistance were measured at temperatures from 4.2 to 320 K in applied magnetic field up to 35 kOe. Substantial increase of charge ordering (CO) transition temperature T_CO under oxygen isotope substitution ¹⁶O ¹⁸O and small decrease of ferromagnetic (FM) transition temperature T_FM were found. Small systematic shift of ferromagnetic transition temperature in oxygen reduced manganite PrBaMn₂^{16 18}O₅ is also considered. The obtained results are compared with those observed in other manganites demonstrating the large isotope effect.

Abstract: Effect of the partial oxygen isotope substitution on (Pr_1-yEu_y)_0.7Ca_0.3CoO₃ cobaltites was studied in detail for the crossover region (0.12<у<0.26) of the phase diagram. We prepared a series of the cobaltite samples with the different degrees of enrichment by the heavy oxygen isotope ¹⁸O, namely, containing 90%, 67%, 43%, 17%, and 0% of ¹⁸O. The Eu doping of the samples was chosen to be near and on the both sides of the crossover doping level y_cr. Based on measurements of the ac magnetic susceptibility χ(Т) and electrical resistivity ρ(Т), we were able to analyze the evolution the sample properties with the change Eu and ¹⁸O content.

Abstract: Self-diffusion, both cationic and anionic diffusion, in ZnS nanoparticles is studied here following reaction coordinate theory. The jump frequencies, at various temperatures are computed. The isotope effect reveals the self-diffusion in nano ZnS is mainly through interstitial migration.

Abstract: The diffusion coefficients of hydrogen (H) and deuterium (D) in Pd were determined in the frame of a quantum mechanical consideration. The three dimensional (3D) wave functions and eigenenergies of H and D at a stable octahedral (o) site and a metastable tetrahedral (t) site were determined by solving the Schrödinger equation with 3D potentials obtained by the first principles calculation. The states of H and D along the diffusion path were also determined by using transient potentials which were obtained with an aid of the nudged elastic band method. The magnitudes of tunneling matrix elements (J) were evaluated from the eigenenergy curves along the diffusion path. In the Pd-H system, thermally activated tunneling transitions are dominant jump processes. On the other hand, in the Pd-D system, it is revealed that a transition via an extended state where the wave function spreads both to o- and t-sites also has a considerable contribution to the diffusion. The calculated diffusion coefficients for H and D quantitatively agreed with experimental values and the so-called inverse isotope effect was reproduced.

Abstract: The magnetic/spin-state phase diagram of the (Pr1-yEuy)0.7Ca0.3CoO3 series was obtained on the basis of measurements of the specific heat, thermal expansion, magnetization and resistivity. The phase diagram reveals three different states depending on the static distortions (Eu content), the oxygen-isotope mass, and the temperature. The samples with the lower Eu concentrations are ferromagnetically ordered up to moderate temperatures (about 50 K),, most probably, due to the low-spin Co4+ – intermediate-spin Co3+ interaction of the double-exchange type. As the Eu doping increases, the Co3+ LS (S = 0) state becomes stabilized and the magnetic ordering of the Co4+ ions is suppressed up to temperatures well below 5 K, resulting in a low-temperature anomaly in Cp. At higher temperatures, we observe a first-order spin-state transition from the LS to the IS state of Co3+, which is accompanied by a decrease in the electrical resistivity, an increase in the magnetization, and a strong lattice expansion.

Abstract: Self-diffusion, both cationic and anionic, in ZnO nanoparticles was studied here in accord with reaction coordinate theory. The jump frequencies at various temperatures were computed. The isotope effect revealed that self-diffusion occurred mainly via a vacancy mechanism in nano ZnO; a result not previously reported in the literature.