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Register a new userAnisotropic relaxations introduced by Cd impurities in Rutile TiO2: first-principles calculations and experimental support
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We present an ab initio study of the relaxations introduced in TiO2 when a Cd impurity substitutes a Ti atom and the experimental test of this calculation by a perturbed-angular-correlation (PAC) measurement of the orientation of the electric-field gradient (EFG) tensor at the Cd site. The ab-initio calculation predicts strong anisotropic relaxations of the nearest oxygen neighbors of the impurity and a change of the orientation of the largest EFG tensor component, V33, from the [001] to the [110] direction upon substitution of a Ti atom by a Cd impurity. The last prediction is confirmed by the PAC experiment that shows that V33 at the Cd site is parallel to either the [110] or the [1bar{1}0] crystal axis.
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First-principles calculations based on density functional theory and the pseudopotential method have been used to investigate the energetics of H$_2$O adsorption on the (110) surface of TiO$_2$ and SnO$_2$. Full relaxation of all atomic positions is performed on slab systems with periodic boundary conditions, and the cases of full and half coverage are studied. Both molecular and dissociative (H$_2$O $rightarrow$ OH$^-$ + H$^+$) adsorption are treated, and allowance is made for relaxation of the adsorbed species to unsymmetrical configurations. It is found that for both TiO$_2$ and SnO$_2$ an unsymmetrical dissociated configuration is the most stable. The symmetrical molecularly adsorbed configuration is unstable with respect to lowering of symmetry, and is separated from the fully dissociated configuration by at most a very small energy barrier. The calculated dissociative adsorption energies for TiO$_2$ and SnO$_2$ are in reasonable agreement with the results of thermal desorption experiments. Calculated total and local electronic densities of states for dissociatively and molecularly adsorbed configurations are presented and their relation with experimental UPS spectra is discussed.
Using the first-principles density-functional approach, magnetic properties of Mn-, Fe-, Co-, and Ni-doped rutile TiO2 were investigated for two different impurity concentrations (25% and 6.25%). Calculations were performed with the Full-Potential Linearized-Augmented Plane Waves (FLAPW) method, assuming that the magnetic impurities substitutionally replace the Ti ions. Our results show that the systems (with the exception of Ni-doped TiO2) are ferromagnetic. We also found that polarization mainly occurs at the impurity sites, and the magnetic moments of the impurities are independent of the impurity concentration.
Elucidating the details of the electron-phonon coupling in semiconductors and insulators is a topic of pivotal interest, as it governs the transport mechanisms and is responsible for various phenomena such as spectral-weight transfers to phonon sidebands and self-trapping. Here, we investigate the influence of the electron-phonon interaction on the excitonic peaks of rutile TiO$_2$, revealing a strong anisotropic polarization dependence with increasing temperature, namely an anomalous blueshift for light polarized along the a-axis and a conventional redshift for light polarized along the c-axis. By employing many-body perturbation theory, we identify two terms in the electron-phonon interaction Hamiltonian that contribute to the anomalous blueshift of the a-axis exciton. Our approach paves the way to a complete ab initio treatment of the electron-phonon interaction and of its influence on the optical spectra of polar materials.
We determine the structural relaxations around paramagnetic impurities (Ti, Cr, Fe) in corundum (alpha-Al2O3), by combining x-ray absorption near edge structure (XANES) experiments and ab initio calculations. The structural relaxations are found to be very local. We then show that XANES is sensitive to small variations in interatomic distances within the coordination shell of the absorbing atom. The experiments were carried out on single crystals of ruby and sapphires. Linear dichroic signals are essential to characterize the geometry of the impurity site. The calculations were performed within a self-consistent ``non muffin-tin framework, that uses pseudopotentials, plane-wave basis set, and the continued fraction for the absorption cross section.
We study the magnetic properties of single crystals of rutile TiO2 implanted with cobalt for various fluences. The temperature variation of zero field cooled(ZFC) and field cooled (FC) magnetization shows a much higher blocking temperature (TB) along [1-10]. Similarly the scaling of magnetization isotherms above TB is seen only when the field is parallel to [1-10] direction. With field along this direction, the magnetization shows near saturation at a much smaller field compared to that of[001] direction. The Co nanoclusters possess an easy and hard axis of magnetization coupled by the magneto crystalline anisotropy of secondary phases of cobalt with TiO2. In addition, at T=2 K we observe a crossover in the magnetization vs field isotherms between the two field directions in the samples which has been attributed to the anisotropic paramagnetism arising from cobalt present in 2+ ionic state with S = 3/2.
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