Search results for "Corundum"
showing 9 items of 39 documents
Radiation-induced point defects in simple oxides
1998
We present a survey of recent theoretical studies of radiation-induced point defects in simple oxides with emphasis on highly ionic MgO, partly-covalent corundum (Al2O3) and ferroelectric KNbO3. The atomic and electronic structure of the electronic (F a and F centers) and hole centers, as well as interstitial atoms therein are discussed in light of the available experimental data. Results for defect diAusion and photo-stimulated F a fi F center conversion are also ana
Molecular dynamics study of high-pressure alumina polymorphs with a tight-binding variable-charge model
2016
Abstract A tight-binding variable-charge model aimed at performing large-scale realistic simulations of bulk, surfaces and interfaces of aluminum oxides have been developed. This model is based on the charge equilibration (QEq) method and explicitly takes into account the mixed iono–covalent character of the metal–oxygen bond by means of a tight-binding analytical approach in the second-moment approximation of the electronic structure. The parameters of the model were optimized to reproduce structural and energetic properties of the α-Al2O3 corundum structure at room temperature and pressure. The model exhibits a good transferability between five alumina polymorphs: corundum, Rh2O3(II)-type…
Luminescence study of defects in synthetic as-grown and HPHT diamonds compared to natural diamonds
2005
The optically active defects in as-grown, high-pressure high-temperature-treated (HPHT), boron-doped, and synthetic diamonds (SD) grown with a nitrogen-getter, as well as of natural diamonds (ND), were characterized by absorption and luminescence spectroscopies using different excitation sources. The laser-excited photoluminescence (PL) spectra of SDs show numerous sharp lines characteristic for nickel-related centers, whereas NDs yield mainly broad PL bands. The emission from the nickel-related defects in NIR range increases and the maxima of the bands shift to lower energies with increasing temperature. Under UV and electron beam excitation, the yellow synthetic diamonds display green lum…
Calculations of the ground and excited states ofF-type centers in corundum crystals
1994
The semiempirical intermediate neglect of differential overlap method was used for calculating optical properties of ${\mathit{F}}^{+}$ and F centers (oxygen vacancy trapped one and two electrons, respectively) embedded into large quantum clusters, ${\mathrm{Al}}_{26}$${\mathrm{O}}_{39}$. The geometry was optimized for both the ground and excited states of defects. Calculated absorption and luminescence energies obtained for ${\mathit{F}}^{+}$ and F centers are in good agreement with experimental data. Their energy levels lie in the gap between the upper valence band and conduction band, like for similar centers in MgO and alkali halides. It is shown that the oxygen vacancy in corundum crys…
Quantum-chemical simulations of free and bound hole polarons in corundum crystal
1997
Abstract The semi-empirical method of the so-called intermediate neglect of differential overlap (INDO) has been applied to the calculations of the hole small-radius polarons in corundum crystals. Results for optimized atomic and electronic structure using two different approaches (the molecular cluster and periodic, supercell model) are critically compared. It is shown that the main results are similar in both cases.
Origin of Blue Sapphire in Newly Discovered Spinel–Chlorite–Muscovite Rocks within Meta-Ultramafites of Ilmen Mountains, South Urals of Russia: Evide…
2019
Blue sapphire of gem quality was recently discovered in spinel&ndash
Atomic and Electronic Structure of the Corundum (0001) Surface
1996
AbstractThe electronic structure and geometry of the Al terminated corundum (0001) surface were studied using a slab model within the ab-initio Hartree-Fock technique. The distance between the top Al plane and the next O basal plane is found to be considerably reduced on relaxation (by 0.57 Å, i.e. by 68% of the corresponding interlayer distance in the bulk). An interpretation of experimental photoelectron spectra (UPS Hel) and metastable impact electron spectra (MIES) is given. Calculated projected densities of states exhibit a strong dependence on the relaxation of surface atoms.
Calculations of the geometry and optical properties ofFMgcenters and dimer (F2-type) centers in corundum crystals
1995
CALCULATIONS OF THE GEOMETRY AND OPTICAL-PROPERTIES OF F-MG CENTERS AND DIMER (F-2-TYPE) CENTERS IN CORUNDUM CRYSTALS
Ti Alloyed α-Ga2O3: Route towards Wide Band Gap Engineering
2020
The suitability of Ti as a band gap modifier for &alpha