A COMPARISON OF DEFECT ENERGIES IN MGO USING MOTT-LITTLETON AND QUANTUM-MECHANICAL PROCEDURES

The authors compare the predictions of Mott-Littleton calculations, based on empirical interatomic potentials, with predictions based on self-consistent solutions of the Schrodinger equation for embedded clusters. Simple vacancy and substitutional defects in MgO are modelled using both the classical Mott-Littleton and quantum mechanical methods. Particular attention is paid to the size of the quantum mechanical cluster, the different ways that polarisation is taken into account and the choice of basis set. Results are presented for closed-shell systems only, namely V"Mg and Vo vacancies and for Li'Mg, Na'Mg, AlMg, Fo and Clo substitutional impurities. They find a respectable level of agreement between the quite distinct approaches. This both validates the classical calculations and indicates useful generalisations combining the two approache

Keyword binding as a method of reducing the length of indexes in library catalogues (based on the experience of Digital Library of Wielkopolska)

This paper presents a relatively simple and cheap method for shortening the subject indexes in library catalogs. The method involves taking a set of several dozen general concepts, characterized by a low semantic awareness barrier. Built around these words are subindexes made up of the words which appear in descriptions containing a particular general concept. The effectiveness of the method was studied by analyzing the content of fragments of subject indexes of the NUKAT central catalog of Polish libraries, the University Library in Poznań and the Library of Congress. Compared with the subject headings language method, this method reduces the length of an index by an average of two-thirds, and makes it significantly easier for readers to navigate the vocabulary used by the cataloger. This method has been developed for the needs of Digital Library of Wielkopolska, and will probably be used in all regional digital libraries in Poland

GEOMETRY AND CHARGE-DISTRIBUTION OF H-CENTERS IN THE FLUORITE STRUCTURE

The analysis of experimental optical and spin-resonance data for the H centre gives a consistent picture of the local geometry and one-electron wavefunctions. One of the two ions in the F2- molecular ion remains very close to the perfect lattice site the other is at a distance close to that found in other F2- centres. This analysis is confirmed by atomistic calculations using the HADES code. The results are used to give a preliminary analysis of the self-trapped exciton data

Simulation study of copper(I) and copper(II) species in ZSM-5 zeolite

Low energy configurations of CuI and CuII species in the ZSM-5 zeolite, probed by energy minimisation techniques, are found to be bound strongly to framework aluminium or copper species

Defect structures and migration mechanisms in oxide pyrochlores

Using computer simulation techniques the defect structure and oxygen ion migration mechanism of oxide pyrochlores (eg. Gd2Zr2O7) was investigated in order to explain the decreased activation enthalpy for oxygen ion conductivity as a function of order. Shell model potentials were found to be necessary in order to obtain sufficiently accurate physical properties for the pyrochlore compound. The oxygen Frenkel defect consisting of ‘a split 48f vacancy’ and 8b interstitial appeared to be the most stable instrinsic defect, but vacancies related to extended defect structures may play an important role in the diffusion mechanism too. The migration mechanism of oxygen ions is mainly based on 48f-48f jumps and involve 0.9 eV barrier energy, comparable with the experimental activation enthalpies of 70–85 kJ/mol

The determination of infrared optical constants of some liquids and thin-film solids

Fourier transform infrared techniques have been applied in order to determine the optical constants of several liquid and thin-film solid systems over infrared frequencies. In the far infrared region, dispersive interferometry has been employed leading to the derivation of both real and imaginary parts of the complex refractive index in one experiment. As an extension to this, a method has been devised for the analysis of ‘full interferograms' involving multiple internal reflections that inevitably arise for normal transmission through thin parallel specimens. A program has been written which constrcts a theoretical complex insertion loss for the system understudy and then performs an iteration on estimated values for absorption coefficient and refractive index. Results are presented for several liquids analysed using this technique. The bandshapes of pure acetonitrile and benzene have been studied in the far infrared region of the spectrum. This was as a consequence of claims in the literature [83,84] that discernable structure in such bandshapes indicated the presence of long-ranged order in molecular liquids. Bandshapes have been produced using both non-dispersive and dispersive transmission, and spectra from both experiments have been averaged. These are presented together with the standard deviation on the mean. It is concluded that there is no evidence for real structure above the level of the noise in the spectra. The collision-induced spectrum of iodine in benzene has been studied in the far infrared region. Subsequent analysis has been done in both the frequency domain and the time domain in order to examine the nature of the intermolecular interactions and the time evolution of the relaxation processes. Experimental intensity and second moment data have been found to agree well with those predicted by a theory based only on classical electrostatic forces of attraction. Spectra have been modelled using two approaches for solving the generalised Langevin equation. The fits were found to be reasonable and the fitted parameters have been used to study the interactions involved in the system. A mid infrared study of Langmuir-Blodgett films has been undertaken, involving layers of w-tricosenoic acid on a silicon substrate. Anomalies in the carbonyl intensities for the first few layers have been attributed to the partially polarised nature of the incident radiation, and have been used to study the orientation of the fatty acid molecules relative to the substrate surface. A variable temperature cryostat with sample-holder has been interfaced with a Beckmann FS720 interferometer. This has been used to measure the optical constants of several solids over a limited frequency range in the far infrared

Concluding remarks: Reaction mechanisms in catalysis: perspectives and prospects

We consider the current status of our understanding of reaction mechanisms in catalysis in the light of the papers presented in this Discussion. We identify some of the challenges in both theoretical and experimental studies, which we illustrate by considering three key reactions

Microscopic origin of the optical processes in blue sapphire

Al2O3 changes from transparent to a range of intense colours depending on the chemical impurities present. In blue sapphire, Fe and Ti are incorporated; however, the chemical process that gives rise to the colour has long been debated. Atomistic modelling identifies charge transfer from Ti(III) to Fe(III) as being responsible for the characteristic blue appearance