Massimo Tomellini

X-ray photoelectron spectra of defective nickel oxide

Cation-deficient nickel oxides have been investigated by X-ray photoelectron spectroscopy. The high-binding-energy features in the O(1s), Ni(2p) X.p.s. of defective NiO are described as multielectron final states, induced by defective sites. These sites are described as (NiO6)9– molecular clusters because of the d hole. The energy shift between the main peak and the satellite, in both the O(1s) and Ni(2p) X.p.s. spectra, is due to a change in the ionic charge and in the oxygen coordination induced by cation vacancies.

Film growth viewed as stochastic dot processes

In this article some results regarding film growth considered as a stochastic process of dots are reviewed. The central concept of the theory described in the initial part of the article is the evaluation of the exclusion probability, i.e. the probability that no dots are found in a given region of the surface. This is reviewed to a certain extent for both correlated and uncorrelated dots and, moreover, for distinguishable classes of dots. This theoretical framework allows one to tackle the nucleation and growth of films ruled by diffusion of adspecies. In this specific instance the theory has been employed for computing the coverage dependent characteristic times for monomer capture from islands and for island collision, in the case of impingement and/or coalescence mechanisms. The ultimate aim is to model, by means of rate equations, the kinetics of film formation over the whole range of coverage.

Mean field rate equation for diffusion-controlled growth in binary alloys

Abstract A system of rate equations is developed for modeling the kinetics of diffusional phase transitions in binary alloys. By employing a mean field assumption and the quasi static approximation a novel expression for the growth law of the nuclei is obtained in terms of supersaturation and diffusion length of component in the parent phase. The mean field rate equations are integrated for the model case of a transformation ruled by simultaneous nucleation and the behaviour of both Avrami’s exponent and rate constant investigated as a function of the initial value of the supersaturation. The expression of the activation energy, as extracted by the Arrhenius plot of the rate constant, is determined and is shown to be a function of the activation energies for nucleation and diffusion, and of the initial value of the supersaturation. The limiting case of infinite diffusion length is also analysed and discussed.

EXAFS and XANES joint analyses for semiconducting vanadium phosphate glasses

Abstract Vanadium EXAFS spectra of 50V2O5-50P2O5 glasses with different C = V+4/Vtot have been measured. The V-O distances increase by ΔR 1 = (0.03 ± 0.02) A to ΔR 2 = (0.07 ± 0.03) A going from a glass with C = 0.64 to C = 0.84 and from C = 0.51 to C = 0.84, respectively. The EXAFS data show a basically similar structure of the vanadium sites for both the V4+ and V5+ ionic states. The density of the glasses increases with C from the value d1 = 2.81 g/cm3 for C = 0.51 to d2 = 2.92 g/cm3 for C = 0.84 indicating a more random packing of glass units.

Determination of mixing of 4f-ligand orbitals in Ce(SO4)2 by Xanes is Ce(SO4)2 a mixed valent insulating system?

Abstract High resolution XANES spectra of insulating lanthanum and cerium 4f 0 and 4f 1 compounds have been measured. We demonstrate that final state effects like charge transfer satellites (configuration interactions in the final state) are quenched in XANES, and that the splitting of core excitations in L III- and L I- XANES of Ce(SO 4 ) 2 like in CeO 2 is due to configuration interaction in the initial state. We show that the ground state electronic structure of formally Ce 4+ compounds is given by mixing of multielectron configurations involving oxygen ligand orbitals. These materials (with NiO) can be classified in a new class of materials “inter-atomic intermediate valent” (IAIV) described by configuration interactions 4f 0 x 4f 1 L -1 .

Beyond the Kolmogorov Johnson Mehl Avrami kinetics: inclusion of the spatial correlation

Abstract.The Kolmogorov-Johnson-Mehl-Avrami model, which is a nucleation and growth Poissonian process in space, has been implemented by taking into account spatial correlation among nuclei. This is achieved through a detailed study of a system of distinguishable and correlated dots (nuclei). The probability that no dots be in a region of the space has been evaluated in terms of correlation functions. The central point of the paper is the application of the theory to describe nucleation and growth in two dimensions under constant nucleation rate, where correlation among nuclei depends upon the size of the nucleus. This is a typical case occurring in transformation governed by particle diffusion. We also propose a simple formula for describing the phase transition kinetics under this circumstance.

Kinetics of the total cluster perimeter in thin film nucleation on solid surfaces

Abstract The time evolution of the total perimeter of clusters growing on a surface has been described on the basis of the JMAK (Johnson-Mehl-Avrami-Kolmogoroff) statistical theory. A general formula, which can be easily extended to any space dimension, is obtained. When particular nucleation functions and the cluster growth law are considered the kinetics of the perimeter can be explicitly calculated and moreover, it can be expressed as a function of the covered surface. Experimental data on the efficiency of a Cu/CuO x model catalyst, towards imide formation, have been satisfactorily described by the model. Moreover, the growth of Ag on a GaAs(110) surface studied via photoelectron spectroscopy has been qualitatively explained by the proposed model. In the model case of cylindrical clusters the knowledge of the evolution of the total perimeter allows the entire area of the film to be evaluated.

On the kinetics of nucleation and growth reactions in inhomogeneous systems

Nucleation and growth kinetics in systems with a small degree of inhomogeneity are usually modeled through the KJMA (Kolmogorov–Johnson–Mehl–Avrami) theory, that is by using the local values of the nucleation and growth rates which are proper to the region where the transition takes place. In this study, a general expression for the kinetics is derived which applies, in principle, to any degree of inhomogeneity and conforms to previous approaches. The model is employed to study, analytically, first order corrections to the KJMA formula in the case of simultaneous nucleation and interface-limited growth. It is shown that under these circumstances, the nucleus shape is a circle (two-dimensional) whose center is displaced with respect to the point where the nucleation event occurs. The displacement of the center and the radius of the nucleus are both functions of time. The behavior of the Avrami exponent and the impingement factor as a function of the fraction of transformed volume is investigated and discussed.

Oxygen diffusion in YBa2Cu3O7–δ mixed conductors: interpretation of T-jump measurements and experiments on hysteresis of conductivity

Oxygen diffusion in YBCO has been investigated by means of non-equilibrium T-jump data. Kinetic curves previously published have been analysed in the temperature range 873–1023 K on the basis of a bipolar transport model leading to the evaluation of the activation energy for oxygen vacancy diffusion. New experimental data on hysteresis cycles of the conductivity, at constant heating rate, are also presented providing further evidence for the oxygen exchange between the oxide and the gas phase.

Kinetics of coadsorption of dioxygen and ammonia at a Zn(0001) surface: a theoretical model

The experimentally observed kinetics of the coadsorption of dioxygen and ammonia at a Zn(0001) surface have been modelled theoretically. The rate-limiting step involves a precursor dioxygen—ammonia surface complex, formed via ammonia and dioxygen surface diffusion. The system of differential equations describing the model provides a solution that is in good agreement with the experimental results when account is taken of the dependence of the enthalpy of ammonia adsorption on the coverage of chemisorbed oxygen O2–(a). Thermodynamic arguments are shown to account for the observed large increase (ca. 103) in the probability of dioxygen dissociation during coadsorption, compared with that observed with dioxygen alone. Electrostatic interactions within the precursor {NH3–O–2} zinc complex are suggested to provide a low-energy pathway to the observed chemisorbed products.