Alexander Shluger - Academia.edu (original) (raw)
Papers by Alexander Shluger
We present a new technique for dielectric embedding for defects in SiO_2. In this scheme, a quant... more We present a new technique for dielectric embedding for defects in SiO_2. In this scheme, a quantum mechanical (QM) cluster is embedded into a polarizable lattice treated within the Mott-Littleton approximation. The QM cluster is treated using phab initio quantum chemical methods. This approach allows us to account for the effect of crystal polarization on the defect electronic structure and
Physical Review B, 2004
Calibration of embedded-cluster method for defect studies in amorphous silica. Andrey S. Mysovsky... more Calibration of embedded-cluster method for defect studies in amorphous silica. Andrey S. Mysovsky 1,2 , Peter V. Sushko 2 * , Sanghamitra Mukhopadhyay 2 , Arthur H. Edwards 3 , and Alexander L. Shluger 2 1 Institute of ...
Physica Status Solidi (c), 2006
We apply density functional Born-Oppenheimer molecular dynamics simulations to investigate adiaba... more We apply density functional Born-Oppenheimer molecular dynamics simulations to investigate adiabatic electron polaron in materials with high dielectric constant. In order to quantify electron-phonon coupling, we introduce single-particle autocorrelation and evaluate them for a polaron in the monoclinic HfO2. The analysis shows that the coupling of the conductivity electron to the atomic vibrations is strongly frequency selective with a dominant contribution from the infrared blind phonon band at –120 cm–1. Contrary to the expectation, no significant coupling to the longitudinal optical mode (∼770 cm–1) has been observed. Modelling implies the possibility of existence of self-trapped small electron polaron trapped to a short-range elastic rather than to a long-range polarisation field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physical Review B, 1990
The adiabatic potential surface for triplet exciton self-trapping and Frenkel-defect pair creatio... more The adiabatic potential surface for triplet exciton self-trapping and Frenkel-defect pair creation in SiO2 has been studied with use of a periodic supercell model and a self-consistent intermediate neglect of the differential overlap method for open shell. The results obtained confirm completely the self-trapped-exciton (STE) model obtained earlier [A. Shluger, J. Phys. C 21, L431 (1988)], making it possible to
Journal of Physical Chemistry B, 2007
A novel nanoporous material 12CaO.7Al2O3 (C12A7) offers a possibility of incorporating large conc... more A novel nanoporous material 12CaO.7Al2O3 (C12A7) offers a possibility of incorporating large concentrations (>1021 cm-3) of a wide range of extraframework anions inside its nanopores. We have investigated, both experimentally and theoretically, optical absorption associated with several types of such anions, including F-, OH-, O-, O2-, O2-, and O22-, and assigned their optical absorption bands. It is demonstrated that the chemical identity and concentration of extraframework anions can be controlled by an appropriate treatment of "as grown" C12A7. We also show that the position of the adsorption edge is, in turn, determined by the chemical identity of the extraframework species and can be varied in the range of approximately 4-6 eV. We suggest that C12A7 is a unique host material, which can be used as a playground for studying negatively charged species that are unstable in other environments.
Physica Status Solidi B-basic Solid State Physics, 1990
A new parametrization of the semiempirical quantum-chemical INDO method is proposed for the study... more A new parametrization of the semiempirical quantum-chemical INDO method is proposed for the study of the electronic structure of a wide class of perfect and defective ionic insulating crystals. The applicability of the “cut-off function” concept for the calculations of exchange interactions in the large unit cell (LUC) model is also briefly discussed. Both, the proposed calculation scheme and the parametrization are tested on a number of molecules, ionic crystals (LiH, LiF, LiCl, KCl, MgO, K2SO4), and (100) crystal surfaces of LiF, KCl, MgO.[Russian Text Ignored].
Journal of Physics-condensed Matter, 2005
... calculations Peter V Sushko, Sanghamitra Mukhopadhyay, Andrey S Mysovsky, Vladimir B Sulimov,... more ... calculations Peter V Sushko, Sanghamitra Mukhopadhyay, Andrey S Mysovsky, Vladimir B Sulimov, Adrian Taga and Alexander L Shluger1 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK E-mail: a.shluger@ucl.ac.uk ...
Physical Review B, 1999
We demonstrate a dramatic dependence on the oxygen coordination of the calculated optical absorpt... more We demonstrate a dramatic dependence on the oxygen coordination of the calculated optical absorption and luminescence energies of low-coordinated sites at the surfaces and in nanoclusters of MgO. The calculations for (MgO) 256 cubic nanoclusters were performed using an embedded molecular cluster model and both semiempirical and ab initio Hartree-Fock methods. The optical-absorption energies were calculated using the configuration interaction technique for single ͑CIS͒ and double electronic excitations. The luminescence energies were calculated using the CIS method. The low-coordinated sites included corners, kinks, step and cluster edges, and corner vacancy defects. We have also studied the zigzag steps and monatomic steps at infinite surfaces using a periodic density-functional theory method and a plane-wave basis set. For both the nanoclusters and infinite surfaces, the results show a consistent significant reduction of the exciton excitation energies and of the luminescence energies of relaxed excitons as the oxygen coordination decreases. We also demonstrate the possibility of exciton transfer from the sites with higher coordination to those with lower coordination and finally to the localization centers. Selective optical excitation of low-coordinated surface sites could be used to study molecular adsorption at surface sites, photocatalytic surface processes and desorption induced by electronic transitions.
Physical Review B, 2002
Asymmetry and long-range character of lattice deformation by neutral oxygen vacancy in α-quartz. ... more Asymmetry and long-range character of lattice deformation by neutral oxygen vacancy in α-quartz. Vladimir B. Sulimov 1 , Peter V. Sushko 1 , Arthur H. Edwards 2 , Alexander L. Shluger 1 , and A. Marshall Stoneham 1 1 Department ...
Journal of The American Chemical Society, 2007
The electron-and hole-trapping and optical properties of a wide variety of interfaces between MgO... more The electron-and hole-trapping and optical properties of a wide variety of interfaces between MgO nanocrystallites are investigated for the first time using a quantum-mechanical embedded-cluster method and time-dependent density functional theory. We conclude that delocalized holes can be transiently trapped at a large number of places within a powder. However, it is more energetically favorable for holes to trap on low-coordinated anions on the nanocrystallite surface, forming Ospecies. Electrons are trapped at few interfaces but are readily trapped by surface kink and corner sites. Contrary to common perception, our calculations of optical absorption spectra indicate that a variety of features buried within a powder can be exited with photon energies less than 5 eV, usually used to selectively excite low-coordinated surface sites.
Surface Science, 2004
We have developed a new method for determining surface exciton band energies in alkali halides ba... more We have developed a new method for determining surface exciton band energies in alkali halides based on velocityresolved atomic desorption (VRAD). Using this new method, we predict the surface exciton energies for KI, KBr, KCl, and NaCl within ±0.15 eV. Our data, the measured and calculated shifts of the surface exciton energies with respect to the bulk excitons, combined with the available EELS data for alkali fluorides, demonstrate a universal linear correlation with the inverse inter-atomic distance in these materials. The results suggest that surface excitons exist in all alkali halides and their excitation energies can be predicted from the known bulk exciton energies and the obtained correlation plot.
Physical Review B, 2006
Site-specific laser modification of MgO nanoclusters: Towards atomic-scale surface structuring. K... more Site-specific laser modification of MgO nanoclusters: Towards atomic-scale surface structuring. Kenneth M. Beck 1 , Matthias Henyk 1 , Chongmin Wang 1 , Paolo E. Trevisanutto 2 , Peter V. Sushko 2 , Wayne P. Hess 1 , and ...
Physical Review B, 2004
Calibration of embedded-cluster method for defect studies in amorphous silica. Andrey S. Mysovsky... more Calibration of embedded-cluster method for defect studies in amorphous silica. Andrey S. Mysovsky 1,2 , Peter V. Sushko 2 * , Sanghamitra Mukhopadhyay 2 , Arthur H. Edwards 3 , and Alexander L. Shluger 2 1 Institute of ...
Physical Review Letters, 2003
The nanoporous main group oxide 12CaO.7Al(2)O3 (C12A7) can be transformed from a wide-gap insulat... more The nanoporous main group oxide 12CaO.7Al(2)O3 (C12A7) can be transformed from a wide-gap insulator to an electride where electrons substitute anions in cages constituting a positive frame. Our ab initio calculations of the electronic structure of this novel material give a consistent explanation of its high conductivity and optical properties. They show that the electrons confined in the inert positive frame are localized in cages and undergo hopping between neighboring cages. The results are useful for the understanding of behavior of confined electron gas of different topology and electron-phonon coupling, and for designing new transparent conductors, electron emitters, and electrides.
Journal of The American Chemical Society, 2003
On the basis of embedded cluster calculations, we propose a new model for the structure of parama... more On the basis of embedded cluster calculations, we propose a new model for the structure of paramagnetic color centers at the MgO surface usually denoted as F(S)(H)(+) (an electron trapped near an adsorbed proton). These centers are produced by exposing the surface of polycrystalline MgO to H(2) followed by UV irradiation. We demonstrate that properties of H atom absorbed at surface sites such as step edges (MgO(step)) and reverse corner sites (MgO(RC)), formed at the intersection of two step edges, are compatible with a number of features observed for F(S)(H)(+). Our calculations suggest that (i) H(2) dissociates at the reverse corner site heterolytically and that there is no barrier for this exothermic reaction; (ii) the calculated vibrations of the resulting MgO(RC)(H(+))(H(-)) complex are fully consistent with the measured ones; (iii) desorption of a neutral H atom from the diamagnetic precursor requires UV light and leads to the formation of stable neutral paramagnetic centers at the surface, MgO(step)(H(+))(e(-))(trapped) and MgO(RC)(H(+))(e(-))(trapped). The computed isotropic hyperfine coupling constants and optical transitions of these centers are in broad agreement with the existing experimental data. We argue that these centers, which do not belong to the class of "oxygen vacancies", are two of the many possible forms of the F(S)(H)(+) defect center.
Superconductor Science & Technology, 2008
Undoped LaFeAsO, parent compound of the newly found high-Tc superconductor, exhibits a sharp decr... more Undoped LaFeAsO, parent compound of the newly found high-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at ~160 K. The anomaly can be suppressed by F doping and the superconductivity appears correspondingly, suggesting a close associate of the anomaly with the superconductivity. We examined the crystal structures, magnetic properties and superconductivity of undoped (normal conductor) and 14 at.% F-doped LaFeAsO (Tc = 20 K) by synchrotron X-ray diffraction, DC magnetic measurements, and ab initio calculations to demonstrate that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160 K as well as an antiferromagnetic transition at ~140 K. These transitions can be explained by spin configuration-dependent potential energy surfaces derived from the ab initio calculations. The suppression of the transitions is ascribed to interrelated effects of geometric and electronic structural changes due to doping by F- ions.
Physical Review B, 2006
ABSTRACT
Physical Review B, 1994
We have performed a thermodynamical analysis of the phase diagrams for ZrO 2 -CaO and ZrO 2 -MgO ... more We have performed a thermodynamical analysis of the phase diagrams for ZrO 2 -CaO and ZrO 2 -MgO solid solutions which has demonstrated that differential heats of mixing are important parameters determining the stabilization of the cubic phase of ZrO 2 by impurities. It is ...
Physical Review B, 2006
We performed a theoretical analysis of O2- diffusion mechanisms in a nanoporous complex oxide 12C... more We performed a theoretical analysis of O2- diffusion mechanisms in a nanoporous complex oxide 12CaO•7Al2O3 (C12A7). This material can be viewed as a positively charged framework, arranged in subnanometer sized cages, hosting extra-framework O2- ions occupying one in six cages. Using both classical molecular-dynamics simulations and ab initio calculations we demonstrate that the diffusion of O2- species is dominated by
Surface Science, 2000
We present the results of calculations of the energy levels of defects at the (001) surface of Mg... more We present the results of calculations of the energy levels of defects at the (001) surface of MgO relative to the top of the valence band and values of defect ionisation potentials and electron affinities. The calculations were made using an embedded cluster method in which a cluster of several tens of ions treated quantum mechanically is embedded in a finite array of polarisable and point ions modelling the crystalline potential and the classical polarisation of the host lattice. The calculated ionisation potential of the ideal surface, which fixes the position of the top of the valence band with respect to the vacuum level, is about 6.7 eV. This value is used as a reference for positioning the energy levels of three charge states of a surface anion vacancy, which are also calculated as ionisation energies with respect to the vacuum level. The surface and vacancy electron affinities are calculated using the same method. As a prototype low-coordinated surface site, we have considered a cube corner. Our calculations predict the splitting of the corner states from the top of the surface valence band by about 1.0 eV. Both unrelaxed and relaxed holes are strongly localised at the corner oxygen ion. The ionisation energies and electron affinities of the corner anion vacancy are calculated. The electrons in the F and F+ centres at the corner are shown to be significantly delocalised over surrounding Mg ions.
We present a new technique for dielectric embedding for defects in SiO_2. In this scheme, a quant... more We present a new technique for dielectric embedding for defects in SiO_2. In this scheme, a quantum mechanical (QM) cluster is embedded into a polarizable lattice treated within the Mott-Littleton approximation. The QM cluster is treated using phab initio quantum chemical methods. This approach allows us to account for the effect of crystal polarization on the defect electronic structure and
Physical Review B, 2004
Calibration of embedded-cluster method for defect studies in amorphous silica. Andrey S. Mysovsky... more Calibration of embedded-cluster method for defect studies in amorphous silica. Andrey S. Mysovsky 1,2 , Peter V. Sushko 2 * , Sanghamitra Mukhopadhyay 2 , Arthur H. Edwards 3 , and Alexander L. Shluger 2 1 Institute of ...
Physica Status Solidi (c), 2006
We apply density functional Born-Oppenheimer molecular dynamics simulations to investigate adiaba... more We apply density functional Born-Oppenheimer molecular dynamics simulations to investigate adiabatic electron polaron in materials with high dielectric constant. In order to quantify electron-phonon coupling, we introduce single-particle autocorrelation and evaluate them for a polaron in the monoclinic HfO2. The analysis shows that the coupling of the conductivity electron to the atomic vibrations is strongly frequency selective with a dominant contribution from the infrared blind phonon band at –120 cm–1. Contrary to the expectation, no significant coupling to the longitudinal optical mode (∼770 cm–1) has been observed. Modelling implies the possibility of existence of self-trapped small electron polaron trapped to a short-range elastic rather than to a long-range polarisation field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physical Review B, 1990
The adiabatic potential surface for triplet exciton self-trapping and Frenkel-defect pair creatio... more The adiabatic potential surface for triplet exciton self-trapping and Frenkel-defect pair creation in SiO2 has been studied with use of a periodic supercell model and a self-consistent intermediate neglect of the differential overlap method for open shell. The results obtained confirm completely the self-trapped-exciton (STE) model obtained earlier [A. Shluger, J. Phys. C 21, L431 (1988)], making it possible to
Journal of Physical Chemistry B, 2007
A novel nanoporous material 12CaO.7Al2O3 (C12A7) offers a possibility of incorporating large conc... more A novel nanoporous material 12CaO.7Al2O3 (C12A7) offers a possibility of incorporating large concentrations (>1021 cm-3) of a wide range of extraframework anions inside its nanopores. We have investigated, both experimentally and theoretically, optical absorption associated with several types of such anions, including F-, OH-, O-, O2-, O2-, and O22-, and assigned their optical absorption bands. It is demonstrated that the chemical identity and concentration of extraframework anions can be controlled by an appropriate treatment of "as grown" C12A7. We also show that the position of the adsorption edge is, in turn, determined by the chemical identity of the extraframework species and can be varied in the range of approximately 4-6 eV. We suggest that C12A7 is a unique host material, which can be used as a playground for studying negatively charged species that are unstable in other environments.
Physica Status Solidi B-basic Solid State Physics, 1990
A new parametrization of the semiempirical quantum-chemical INDO method is proposed for the study... more A new parametrization of the semiempirical quantum-chemical INDO method is proposed for the study of the electronic structure of a wide class of perfect and defective ionic insulating crystals. The applicability of the “cut-off function” concept for the calculations of exchange interactions in the large unit cell (LUC) model is also briefly discussed. Both, the proposed calculation scheme and the parametrization are tested on a number of molecules, ionic crystals (LiH, LiF, LiCl, KCl, MgO, K2SO4), and (100) crystal surfaces of LiF, KCl, MgO.[Russian Text Ignored].
Journal of Physics-condensed Matter, 2005
... calculations Peter V Sushko, Sanghamitra Mukhopadhyay, Andrey S Mysovsky, Vladimir B Sulimov,... more ... calculations Peter V Sushko, Sanghamitra Mukhopadhyay, Andrey S Mysovsky, Vladimir B Sulimov, Adrian Taga and Alexander L Shluger1 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK E-mail: a.shluger@ucl.ac.uk ...
Physical Review B, 1999
We demonstrate a dramatic dependence on the oxygen coordination of the calculated optical absorpt... more We demonstrate a dramatic dependence on the oxygen coordination of the calculated optical absorption and luminescence energies of low-coordinated sites at the surfaces and in nanoclusters of MgO. The calculations for (MgO) 256 cubic nanoclusters were performed using an embedded molecular cluster model and both semiempirical and ab initio Hartree-Fock methods. The optical-absorption energies were calculated using the configuration interaction technique for single ͑CIS͒ and double electronic excitations. The luminescence energies were calculated using the CIS method. The low-coordinated sites included corners, kinks, step and cluster edges, and corner vacancy defects. We have also studied the zigzag steps and monatomic steps at infinite surfaces using a periodic density-functional theory method and a plane-wave basis set. For both the nanoclusters and infinite surfaces, the results show a consistent significant reduction of the exciton excitation energies and of the luminescence energies of relaxed excitons as the oxygen coordination decreases. We also demonstrate the possibility of exciton transfer from the sites with higher coordination to those with lower coordination and finally to the localization centers. Selective optical excitation of low-coordinated surface sites could be used to study molecular adsorption at surface sites, photocatalytic surface processes and desorption induced by electronic transitions.
Physical Review B, 2002
Asymmetry and long-range character of lattice deformation by neutral oxygen vacancy in α-quartz. ... more Asymmetry and long-range character of lattice deformation by neutral oxygen vacancy in α-quartz. Vladimir B. Sulimov 1 , Peter V. Sushko 1 , Arthur H. Edwards 2 , Alexander L. Shluger 1 , and A. Marshall Stoneham 1 1 Department ...
Journal of The American Chemical Society, 2007
The electron-and hole-trapping and optical properties of a wide variety of interfaces between MgO... more The electron-and hole-trapping and optical properties of a wide variety of interfaces between MgO nanocrystallites are investigated for the first time using a quantum-mechanical embedded-cluster method and time-dependent density functional theory. We conclude that delocalized holes can be transiently trapped at a large number of places within a powder. However, it is more energetically favorable for holes to trap on low-coordinated anions on the nanocrystallite surface, forming Ospecies. Electrons are trapped at few interfaces but are readily trapped by surface kink and corner sites. Contrary to common perception, our calculations of optical absorption spectra indicate that a variety of features buried within a powder can be exited with photon energies less than 5 eV, usually used to selectively excite low-coordinated surface sites.
Surface Science, 2004
We have developed a new method for determining surface exciton band energies in alkali halides ba... more We have developed a new method for determining surface exciton band energies in alkali halides based on velocityresolved atomic desorption (VRAD). Using this new method, we predict the surface exciton energies for KI, KBr, KCl, and NaCl within ±0.15 eV. Our data, the measured and calculated shifts of the surface exciton energies with respect to the bulk excitons, combined with the available EELS data for alkali fluorides, demonstrate a universal linear correlation with the inverse inter-atomic distance in these materials. The results suggest that surface excitons exist in all alkali halides and their excitation energies can be predicted from the known bulk exciton energies and the obtained correlation plot.
Physical Review B, 2006
Site-specific laser modification of MgO nanoclusters: Towards atomic-scale surface structuring. K... more Site-specific laser modification of MgO nanoclusters: Towards atomic-scale surface structuring. Kenneth M. Beck 1 , Matthias Henyk 1 , Chongmin Wang 1 , Paolo E. Trevisanutto 2 , Peter V. Sushko 2 , Wayne P. Hess 1 , and ...
Physical Review B, 2004
Calibration of embedded-cluster method for defect studies in amorphous silica. Andrey S. Mysovsky... more Calibration of embedded-cluster method for defect studies in amorphous silica. Andrey S. Mysovsky 1,2 , Peter V. Sushko 2 * , Sanghamitra Mukhopadhyay 2 , Arthur H. Edwards 3 , and Alexander L. Shluger 2 1 Institute of ...
Physical Review Letters, 2003
The nanoporous main group oxide 12CaO.7Al(2)O3 (C12A7) can be transformed from a wide-gap insulat... more The nanoporous main group oxide 12CaO.7Al(2)O3 (C12A7) can be transformed from a wide-gap insulator to an electride where electrons substitute anions in cages constituting a positive frame. Our ab initio calculations of the electronic structure of this novel material give a consistent explanation of its high conductivity and optical properties. They show that the electrons confined in the inert positive frame are localized in cages and undergo hopping between neighboring cages. The results are useful for the understanding of behavior of confined electron gas of different topology and electron-phonon coupling, and for designing new transparent conductors, electron emitters, and electrides.
Journal of The American Chemical Society, 2003
On the basis of embedded cluster calculations, we propose a new model for the structure of parama... more On the basis of embedded cluster calculations, we propose a new model for the structure of paramagnetic color centers at the MgO surface usually denoted as F(S)(H)(+) (an electron trapped near an adsorbed proton). These centers are produced by exposing the surface of polycrystalline MgO to H(2) followed by UV irradiation. We demonstrate that properties of H atom absorbed at surface sites such as step edges (MgO(step)) and reverse corner sites (MgO(RC)), formed at the intersection of two step edges, are compatible with a number of features observed for F(S)(H)(+). Our calculations suggest that (i) H(2) dissociates at the reverse corner site heterolytically and that there is no barrier for this exothermic reaction; (ii) the calculated vibrations of the resulting MgO(RC)(H(+))(H(-)) complex are fully consistent with the measured ones; (iii) desorption of a neutral H atom from the diamagnetic precursor requires UV light and leads to the formation of stable neutral paramagnetic centers at the surface, MgO(step)(H(+))(e(-))(trapped) and MgO(RC)(H(+))(e(-))(trapped). The computed isotropic hyperfine coupling constants and optical transitions of these centers are in broad agreement with the existing experimental data. We argue that these centers, which do not belong to the class of "oxygen vacancies", are two of the many possible forms of the F(S)(H)(+) defect center.
Superconductor Science & Technology, 2008
Undoped LaFeAsO, parent compound of the newly found high-Tc superconductor, exhibits a sharp decr... more Undoped LaFeAsO, parent compound of the newly found high-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at ~160 K. The anomaly can be suppressed by F doping and the superconductivity appears correspondingly, suggesting a close associate of the anomaly with the superconductivity. We examined the crystal structures, magnetic properties and superconductivity of undoped (normal conductor) and 14 at.% F-doped LaFeAsO (Tc = 20 K) by synchrotron X-ray diffraction, DC magnetic measurements, and ab initio calculations to demonstrate that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160 K as well as an antiferromagnetic transition at ~140 K. These transitions can be explained by spin configuration-dependent potential energy surfaces derived from the ab initio calculations. The suppression of the transitions is ascribed to interrelated effects of geometric and electronic structural changes due to doping by F- ions.
Physical Review B, 2006
ABSTRACT
Physical Review B, 1994
We have performed a thermodynamical analysis of the phase diagrams for ZrO 2 -CaO and ZrO 2 -MgO ... more We have performed a thermodynamical analysis of the phase diagrams for ZrO 2 -CaO and ZrO 2 -MgO solid solutions which has demonstrated that differential heats of mixing are important parameters determining the stabilization of the cubic phase of ZrO 2 by impurities. It is ...
Physical Review B, 2006
We performed a theoretical analysis of O2- diffusion mechanisms in a nanoporous complex oxide 12C... more We performed a theoretical analysis of O2- diffusion mechanisms in a nanoporous complex oxide 12CaO•7Al2O3 (C12A7). This material can be viewed as a positively charged framework, arranged in subnanometer sized cages, hosting extra-framework O2- ions occupying one in six cages. Using both classical molecular-dynamics simulations and ab initio calculations we demonstrate that the diffusion of O2- species is dominated by
Surface Science, 2000
We present the results of calculations of the energy levels of defects at the (001) surface of Mg... more We present the results of calculations of the energy levels of defects at the (001) surface of MgO relative to the top of the valence band and values of defect ionisation potentials and electron affinities. The calculations were made using an embedded cluster method in which a cluster of several tens of ions treated quantum mechanically is embedded in a finite array of polarisable and point ions modelling the crystalline potential and the classical polarisation of the host lattice. The calculated ionisation potential of the ideal surface, which fixes the position of the top of the valence band with respect to the vacuum level, is about 6.7 eV. This value is used as a reference for positioning the energy levels of three charge states of a surface anion vacancy, which are also calculated as ionisation energies with respect to the vacuum level. The surface and vacancy electron affinities are calculated using the same method. As a prototype low-coordinated surface site, we have considered a cube corner. Our calculations predict the splitting of the corner states from the top of the surface valence band by about 1.0 eV. Both unrelaxed and relaxed holes are strongly localised at the corner oxygen ion. The ionisation energies and electron affinities of the corner anion vacancy are calculated. The electrons in the F and F+ centres at the corner are shown to be significantly delocalised over surrounding Mg ions.