Nonstoichiometric Rutile Research Articles

Electronic Structure and Magnetic Properties of Cobalt-Doped Nonstoichiometric Rutile Thin Films

The coherent potential method has been applied for the investigation of Co substitutional doping influence on the spectral and magnetic properties of TiO2 rutile (100) surface. Both stoichiometric and nonstoichiometric oxygen compositions have been considered. For the stoichiometric case, the energy spectrum corresponds to half-metallic ferromagnet. The Co ions are in 2 + formal oxidation state with a spin magnetic moment of 0.8 μB. The presence of oxygen vacancies makes the surface insulating and increases the Co spin magnetic moment value.

Electronic structure and magnetic properties of two-dimensional nonstoichiometric rutile

The coherent potential approximation is applied to study the influence of vacancies in the oxygen lattice on the electronic structure and magnetic properties of the TiO1.99 rutile (110) surface. Stoichiometric two-dimensional rutile is found to be a nonmagnetic semiconductor. Vacancies in the oxygen positions on the surface lead to the metallic type of the electronic spectrum. Additionally, they result in the appearance of spin magnetic moments on titanium atoms surrounded by only five oxygen atoms due to the surface formation. The vacancies in all the other oxygen positions except of the surface cause a nonmagnetic semiconducting character of the energy spectrum of two-dimensional TiO1.99. A mechanism that underlies the formation of spin magnetic moments of the titanium atoms, namely Stoner ferromagnetism of a defect related impurity band, is discussed.

Investigation of the influence of nonstoichiometry and doping with carbon and nitrogen on the electronic spectrum of rutile by the coherent potential method

The electronic spectra and magnetic properties of rutile (TiO2), nonstoichiometric rutile (TiO2 − δ), and stoichiometric and nonstoichiometric rutiles doped with carbon or nitrogen (TiO2 − y − δCy and TiO2 − y − δNy, y(δ) = 0, 0.03, 0.06) have been calculated using the coherent potential method. The used method of investigation has made it possible for the first time to calculate the disordered arrangement of impurity atoms and vacancies in the oxygen sublattice with their arbitrary concentration. The changes in the electronic spectrum and magnetic properties with variations in the concentration of dopants have been investigated. The possible photocatalytic activity of considered compositions has been analyzed.

Effect of on-site Coulomb repulsion termUon the band-gap states of the reduced rutile (110)TiO2surface

We present a study concerning the effect of the on site $d\text{\ensuremath{-}}d$ Coulomb interaction energy $U$ on the band-gap states of nonstoichiometric rutile (110) $\mathrm{Ti}{\mathrm{O}}_{2}$ surface. As well known, the excess electrons resulting from the formation of oxygen vacancies localize on the $\mathrm{Ti}\phantom{\rule{0.2em}{0ex}}3d$ orbitals forming band-gap states. Local density approximation (LDA) does not give a correct description of these band-gap states, either with or without gradient corrections. The failure of LDA is often attributed to an inadequate treatment of electron correlation in systems with localized orbitals and is commonly corrected with an empirical local Coulomb repulsion term, i.e., the $\mathrm{LDA}+U$ method. This study provides a completely general strategy to estimate the $U$ value in this kind of systems, illustrated here for reduced (110) $\mathrm{Ti}{\mathrm{O}}_{2}$ surface, well characterized from experiments. From ab initio embedded cluster configuration interaction calculations, combined with the effective Hamiltonian theory, a value of $U$ of $5.5\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is obtained, in good agreement with those reported for this system from x-ray photoemission spectroscopy experiments $(U=4.5\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. It is observed that when the ab initio estimate of $U$ is injected into the periodic $\mathrm{LDA}+U$ calculations, a correct description of the gap states is obtained from the periodic $\mathrm{LDA}+U$ calculations. Additionally, the results indicate that the position of these states on the band gap strongly depends on the level at which lattice relaxation is taken into account, with significant differences between the density of states curves at the $\mathrm{LDA}+U$ level obtained using the optimal generalized gradient approximation or $\mathrm{LDA}+U$ geometries. These results suggest that this combined strategy could be a useful tool for those systems where electron correlation plays a key role, and no experimental data are available for the on-site Coulomb repulsion.

Study on the annealing effects of proton-implanted rutile

The annealing effects of proton-implanted rutile single crystal have been investigated with positron annihilation technology (PAT). The stoichiometric and non-stoichiometric rutile crystals were implanted by protons, with an energy of 4.9 MeV and a fluence of 1×10 18 m −2. Implanted samples were annealed from room temperature to 500 °C with steps of 50 °C. Positron annihilation lifetime spectra were used to study the dynamic variation of defects. Corresponding optical absorption spectra were also measured for comparison. The results show that, implanted H + ions induce defects being different in stoichiometric and non-stoichiometric samples. The defects aggregate to bigger, complex clusters during annealing, then decompose into small, simple defects again. The aggregating temperature of different defects is different. For non-stoichiometric rutile, the results of PAT measurements are satisfactorily consistent with those of optical absorption. The mechanism of annealing effects is also discussed.

Influence of proton implantation on optical absorption of rutile

The effects of proton implantation on the optical properties of rutile single crystals were studied for the first time. The stoichiometric and non-stoichiometric rutile samples were irradiated with protons, with energy of 4.9 MeV and fluence of 1×10 15 and 1×10 18 m −2, respectively. UV-VIS, FT-IR and LRS were used to study the variation of optical absorption spectra and lattice vibration before and after implantation. The results showed that, for stoichiometric rutile, the optical absorption in the visible region is almost not influenced by proton implantation but that in ultraviolet region is influenced. For non-stoichiometric rutile, proton implantation induces a novel optical absorption peak in visible region. The intensity of the absorption peak depends on the fluence of implanted protons. The formation mechanism of novel optical absorption and defect structure in implanted crystals have also been discussed.

Cluster simulation of bulk properties for stoichiometric and non-stoichiometric rutile

Model clusters are used for the study of geometric, energetic and electronic properties of the titanium dioxide form rutile. The calculations are performed with the semi-empirical MO method SINDO1. A systematic increase of the model clusters is presented with an extrapolation to bulk data. The influence of oxygen defects at the (110) surface on the electronic structure is investigated.

Formation energies of small defects in non-stoichiometric rutile

The enthalpies of formation of traditional and reconstructed small defects, having both oxygen vacancy and cation interstitial natures, have been calculated for non-stoichiometric rutile. Reconstructed interstitial defects, consisting of two pairs of face-shared octahedra linked by a string of cation Frenkel defects, prove to have significantly lower formation enthalpies (per eliminated oxygen), confirming recent interpretations of electrical conductivity and high-resolution electron microscopic results.

Clustering of small defects in non-stoichiometric rutile

Abstract A simple electrostatic model for the effective charges associated with small linear defects in non-stoichiometric mile is introduced. This is used to predict relative stabilities of clusters of linear defects. The results are in good agreement with recent high-resolution electron microscopic observations of the presence of small cluster defects in non-stoichiometric rutile (TiO2-x; 0≤x≤0·0035) and chromia-doped rutile ((Ti, Cr)O2-x; 0≤x≤0·05), and with observations of the precipitation of pairs of crystallographic shear planes and {100} platelet defects.

Approaching atomic-resolution electron microscopy

The Cambridge University 600 kV high-resolution electron microscope, supported by local facilities for image simulation and processing, has been successfully used to characterise the microstructure of a wide range of materials at resolutions on, or approaching, the atomic scale. After short descriptions of the microscope and computer facilities, attention is first directed towards the problems of imaging artefacts and beam misalignment. The usefulness of Weak-Phase-Object images for preliminary assessments of image detail, such as dumbbells and possible oxygen atom resolution, is stressed. It is shown that the well-known imaging artefact, namely dumbbells, is an interference phenomenon which can be generated even when the crystal being imaged does not have the classic “atom-pair” projected structure. Methods for assessing resolving power and the validity of information beyond the resolution limit are considered. Particular applications to the study of surface rearrangements, nonstoichiometric rutile and defect annealing in cadmium telluride are briefly described.

Defect Behaviour in Oxides

ABSTRACTWe describe computer-based atomistic simulation studies of defect behaviour in oxides. In illustrating the value of the technique, contact with experiment is discussed through comparison with measured energies of ion migration and other energies. Since the success of these methods is dependent on the extent to which the interatomic potentials used provide a realistic model of the material, consideration is given to transferability of potentials, especially to tenary compounds where the widely adopted practice of fitting potential parameters to available crystal data is not feasible. Our principal examples will include ionic conductivity in gadolinium zirconate, electronic defect properties of BaTiO3 and grossly nonstoichiometric rutile.

Extended versus small defect equilibria in non-stoichiometric rutile. II. Structural mechanisms for dissolution and reprecipitation of extended defects

High-resolution electron micrographs are presented of a specimen of TiO1.9966, which is previously heated at 750 °C in an electron microscope, in order to dissolve extended defects. This reveals the size and distribution of the small (⪅ 1 nm diameter) defects produced following dissolution of crystallographic shear planes, and also during cooling of the specimen to room temperature. The observed dissolution and reprecipitation phenomena are discussed in terms of new models for the small defects, traditionally cation interstitial and/or oxygen vacancy defects, responsible for the nonstoichiometric phase TiO2−x. Asymmetry between precipitation and dissolution mechanisms should lead to temperature hysteresis. On présente ici les résultats d'une étude par microscopie électronique à haute résolution effectuée sur un échantillon de rutile réduit TiO1,9966 auparavant chauffé à 750 °C de façon à dissoudre les défauts étendus qu'il contenait. Les micrographies révèlent la taille (⪅ 1 nm) et la distribution des petits défauts formés durant la dissolution des plans de cisaillement cristallographique et pendant le refroidissement de l'échantillon jusqu'à température ambiante. Les phénomènes observés lors de la dissolution et la re-précipitation sont discutés en termes de nouveaux modèles structuraux pour les petits défauts, traditionnellement dénommés interstitiel cationique et/ou lacune d'oxygène, existant dans la phase non-stoechiométrique TiO2−x. L'assymétrie entre les mécanismes de dissolution et de précipitation conduit à un phénomène d'hystérésis.

Extended versus small defect-equilibria in non-stoichiometric rutile. I. Dissolution and re-precipitation of extended defects observed by in-situ transmission electron microscopy

Extended defects accomodating non-stoichiometry in rutiles reduced slightly to compositions of TiO1.9966, TiO1.9979, and TiO1.9986 are seen to dissolve in situ in an electron microscope using a goniometer at temperatures of up to 1000 °C. This allows to determine as directly as possible the stoichiometry dependence of the dissolution temperature, and hence the phase limits of nonstoichiometric TiO2−x. The defect structures formed during dissolution, and also the reprecipitation of extended defects upon cooling of the specimens to room temperature, are also studied. Dans un microscope electronique muni d'un porte-object chauffant jusqu' a des temperatures de 1000 °C environ, on a observe in situ la dissolution des defauts etendus accommodant la nonstoichiometrie de cristaux de rutile reduits aux compositions TiO1,9966, TiO1,9979 et TiO1,9986. Ceci a permis de determiner la temperature de dissolution en fonction de l'ecart a la stoichiometrie, et par la -měme la limite de la phase nonstoichiometrique TiO2−x aussi directement que possible. La structure des defauts formes durant la dissolution ainsi que la re-precipitation des defauts etendus (apres dissolution) ont ete egalement etudiees.

Structure of small oxygen vacancy defects in nonstoichiometric rutile

New structural models are derived for oxygen vacancy defects, which occur in impurity and dopant-controlled regimes of slightly substoichiometric rutiles. Two reconstructions of the traditional vacancy model are derived, which offer a natural explanation for high-resolution electron paramagnetic (ESR) observations. Both reconstructions involve simply displacement of one cation from (000) to(−½00), for a vacancy at ( uu0) of the rutile structure. They differ in the distribution of charge-compensating defects associated with the different sites in the immediate vicinity of the defect. Analysis of the electrostatic valencies of the oxygen ions leads to the suggestion that oxygen vacancies in rutile should readily trap H + ions. ESR evidence is reviewed offering confirmation of these predictions. Diffusion mechanisms are also described. These new small defect models offer natural explanations for many conflicting interpretations of physical property measurements of reduced and doped rutiles (e.g., electrical conductivity, thermogravimetry, and anelastic dielectric relaxation). The possibility of directly observing and positively identifying such small defects using high-resolution electron microscopic techniques is considered briefly, as is their relevance to the interpretation of plastic deformation and dynamic strain ageing experiments.

Emergence of small defect contrast within HREM images of nonstoichiometric rutile

Calculations of HREM images have been used to define the conditions for which interpretable information may be obtained concerning the nature of small defects (traditionally called “point defects”) and their aggregation to form extended defects in nonstoichiometric rutile. At specific crystal thickness and lens defocus conditions as few as three small defects (aligned along the projection axis) become visible in lattice images. The emergence of stronger contrast, as extended defects develop, should allow the interstitial or vacancy nature of the defects to be distinguished. Experimental HREM images are also presented which show a variety of “small defect contrasts” for [001], [010] and [111] projections of nonstoichiometric rutiles (TiO 2- x ; 0.006 ≦ x ≦ 0.02). None of these allow interstitial versus vacancy defects to be distinguished directly, although some show evidence for aggregates of less than 10 small defects.

Point, linear and extended defect structures in nonstoichiometric rutile

Abstract TiO2-x crystals (0 < x ≤ 0.0035) prepared at 1323 K and given controlled thermomechanica treatment were examined by high-resolution electron microscopy (HREM). Crystallographic shear planes (CSP) were not present in non-deformed specimens quenched from 1323 K but appeared, mostly as closely-spaced pairs, in slowly-cooled or deformed and reduced specimens. Lateral and longitudinal disorder in the fine structure of the CSP, which was strongly dependent upon cooling rate was observed. Platelet defects, approximately parallel to {100}, formed at about 400–600°C. Defects intermediate in size between point defects and CSP were also discovered. In situ observations revealed some details of the mechanisms of formation and dissolution of CSP and platelets. New interstitial cation structural models were derived using the above observations and electrostatic energy arguments. Linear defects, consisting of two face-shared pairs of octahedra containing trivalent cations are proposed. These have very much lowe...

Structure of cation interstitial defects in nonstoichiometric rutile

Structure of cation interstitial defects in nonstoichiometric rutile

Recalculations of Madelung Potentials and Polarisabilities of Ions in Stoichiometric Rutile (TiO2)

AbstractThe Madelung potentials in stoichiometric rutile (TiO2) are calculated and found to be 29.53 eV at the Ti4+ site and −41.20 eV at the O2‐ site. These potentials lead to the value of 141.5 eV for the Madelung energy per TiO2 “molecule” and also the value of 4.810 for the Madelung constant. Using the Madelung potential at the Ti4+ ion, the electronic polarisability of the Ti4+ ion is estimated to be 0.368 Å3. This result is used to determine the ionic polarisability of the Ti4+ ion and the electronic and ionic polarisabilities of the O2‐ ion. The electronic polarisability of the O2‐ ion is found to be large in value compared with that in other oxides. The possible connection between polarisability and defect structure formation in nonstoichiometric rutile is discussed.

ChemInform Abstract: Defect Structure and Related Thermodynamic Properties of Nonstoichiometric Rutile (TiO2‐x) and Nb2O5 Doped Rutile. Part 2. Defect Structure of TiO2 Solid Solution Containing 0.1‐3.0 mole% Nb2O5 and Partial Molar‐Properties for Oxygen Solution at 1273 K.

Chemischer InformationsdienstVolume 10, Issue 39 Physical Inorganic Chemistry ChemInform Abstract: Defect Structure and Related Thermodynamic Properties of Nonstoichiometric Rutile (TiO2-x) and Nb2O5 Doped Rutile. Part 2. Defect Structure of TiO2 Solid Solution Containing 0.1-3.0 mole% Nb2O5 and Partial Molar-Properties for Oxygen Solution at 1273 K. R. T. DIRSTINE, R. T. DIRSTINESearch for more papers by this authorC. J. ROSA, C. J. ROSASearch for more papers by this author R. T. DIRSTINE, R. T. DIRSTINESearch for more papers by this authorC. J. ROSA, C. J. ROSASearch for more papers by this author First published: September 25, 1979 https://doi.org/10.1002/chin.197939007Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume10, Issue39September 25, 1979 RelatedInformation

Defect Structure and Related Thermodynamic Properties of Nonstoichiometric Rutile (TiO,_,) and Nb,0O, Doped Rutile

Defect Structure and Related Thermodynamic Properties of Nonstoichiometric Rutile (TiO,_,) and Nb,0O, Doped Rutile