Core exciton energies of bulk MgO,Al2O3,and

Abstract

Ab initio cluster model wave functions are used to predict the existence of localized excited states in MgO, Al 2 O 3 , and SiO 2 arising from metal 2p core-level excitations. Theoretical values obtained at different levels of theory result in a quantitative agreement with experiment, and the use of different models permits us to quantify the different contributions to the final excitation energy. The most important contribution is atomic in nature; a meaningful zero-order approximation is that in MgO and Al 2 O 3 the exciton can be assigned to a M (2p 6 )→M (2p 5 3s 1 )-like excitation, where M ϭMg or Al. For the atomic models, the singlet-triplet exchange in the excited configuration is in good agreement with experiment. In addition, the solid-state effects on this exchange energy predicted by experiment are well reproduced by the cluster models representing MgO and SiO 2 , whereas a less clear situation appears in Al 2 O 3 . The open-shell orbital in the final state has, however, important contributions from the ions near the atomic site where excitation occurs. Nevertheless, the final state appears to be localized in space without any a priori assumption, the localization following from the holeparticle interaction implicitly induced in the final-state wave function. The Madelung field reduces the excitation energy with respect to the atomic value; the effect of neighboring atoms, mainly Pauli repulsion, acts in the opposite way; and electronic correlation effects decrease it again. In agreement with the covalent nature of SiO 2 , the exciton cannot be simply understood as arising from a Si(2p 6 )→Si(2p 5 3s 1 ) in a fully oxidized Si cation.

Optical spectroscopy of (C2H5NH3)2CdCl4:Cu2+ under pressure: Study of Cu2+ local structure from theoretical calculations

“…The CASSCF/CASPT2 methodology has not only been proven to be a very accurate method to study the spectroscopy of organic and inorganic molecules 22, but has also been successfully applied to study different types of excitations in ionic solids. Examples are the localized d ‐ d excitations in a series of transition metal oxides 23, 24, the excitonic states in MgO, Al 2 O 3 , and SiO 2 25, and the spectroscopy of the KCl: Ag 0 impurity 26. The CASSCF/CASPT2 calculations were performed with MOLCAS version 4 27.…”

Time Dependent Density Functional Theory of X-ray Absorption Spectroscopy of Alkaline-Earth Oxides

“…The interpretation of the EELS experimental data of condensed systems requires first principle calculations, and although many efforts have been spent to reproduce the spectra, a standard and robust method has not yet emerged clearly. − Recently, the time dependent density functional theory (TDDFT) has been extended to the treatment of the core electron excitations , and its application to calculate the core excitation spectra of solid MgO using a cluster model to mimic the bulk has been tested. The agreement with the experimental data indicates the reliability of the TDDFT approach also in the case of solid samples which may be twofold.…”

Color shift inAl2O3⋅xCr2O3solid solutions and the electroneutrality principle

“…18 In highly dilute samples of Al 2 O 3 :Cr 3+ all cations close to a given CrO 6 9− complex are Al 3+ ions whose actual charge has been shown to be not far from +3e. 20 However, when the chromium content in the series of Al 2 O 3 · xCr 2 O 3 crystals increases some of the close sites can be occupied by a Cr 3+ ion substituting Al 3+ . The central idea of our model is that the actual charge of chromium cations in CrO 6 9− complexes is much smaller than that corresponding to aluminum in AlO 6 9− complexes because Cr 3+ is an open shell ion, and thus it can receive additional electronic charge from closed shell oxygen ligands ͑dative bonding͒.…”