Cluster Models of Doped Ionic Crystal Scintillators. Quantum Calculations+

1994 ◽  
Vol 348 ◽  
Author(s):  
M. Berrondo ◽  
J.F. Rivas-Silva ◽  
J.B. Czirr
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Ionic Crystal ◽  
Energy Gap ◽  
Alkali Halides ◽  
Alkali Cation ◽  
Quantum Calculations ◽  
Pure Crystal ◽  
Absorption And Emission ◽  
Quantum Electronic

ABSTRACTWe have performed quantum electronic ab initio calculations of clusters of alkali halides, both pure and doped. In order to estimate the pure crystal energy gap and the dopant excitation, we have considered a central cation and four shells of ions surrounding it. We have studied KC1, Nal, NaCl and KI, both pure and with a Tl+ ion replacing thecentral alkali cation. Encouraging results are presented for absorption and emission.

ab initio MO Optimization of Molecular Structures of Fluoro- and Chloro-Substituted Dibenzo-p-dioxines and the Effect of Halogen Substitution at the 2,3,7,8-Positions on Metabolic Attack

1997 ◽  
Vol 52 (11) ◽  
pp. 1418-1431 ◽  
Author(s):  
Roland Weber ◽  
Till Kühn ◽  
Hanspaul Hagenmaier ◽  
Günter Häfelinger
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Energy Gap ◽  
Substituent Effects ◽  
Molecular Structures ◽  
X Ray ◽  
Degree Pattern ◽  
Chlorine Substituent ◽  
Homo Lumo ◽  
Good Agreement

Full ab initio optimizations were performed on the molecular structures of 24 fluorinated and chlorinated dibenzodioxines (PFDD/PCDD ) and dibenzofurans (PFDF/PCDF). Reasonable agreement was found by comparing the geometries of four calculated structures with known X-ray data from the literature. For the fluorine substituent, calculated electron densities (Mulliken total charges and π-electron charges) clearly demonstrate the opposite influence of the inductive (I) and mesomeric (M) effect. The changes in π-densities at carbons in ortho-, meta- and para-position are constant for each fluorine substituent (independent of degree, pattern, and position of substitution). It is thus possible to calculate the π-densities of the substituted dioxines by increments starting from dibenzodioxine. π-Charges from quantum mechanical calculations and the increment system show good agreement even for OctaFDD (O8FDD ), where eight substituent effects are acting additively. Compared with fluorine, the chlorine substituent exercises a smaller -I-effect and a clearly weaker +M-effect. The HOMO coefficients of the unsubstituted dibenzodioxine and dibenzofuran, extracted from ab initio calculations, yield a good explanation for the observed regioselective metabolic attack at the 2,3,7,8-positions. The squares of the HOMO-coefficients of the 2,3,7,8-positions in dibenzodioxine (DD ) are about ten times greater than those of the 1,4,6,9-positions. These HOMO coefficients are practically unaffected by halide substitution. But halogen substitution reduces strongly the electron density at the halogen-bound carbon, which, however, is a necessary prerequisite for the electrophilic oxygen transfer during metabolism. One would therefore expect halogen substitution of dibenzodioxine and dibenzofuran (DF) at the 2,3,7,8-position to hinder metabolism, as is indeed found. This provides a plausible explanation for the highly selective tissue retention of 2,3,7,8-substituted PCDDs and PCDFs. Our ab initio calculations of five tetra CDDs (T4CDDs) confirm the postulate of Kobayashi et al. [1 ] who, using semiempirical calculations, found a correlation between the toxicity of a dioxine congener and its absolute molecular hardness. The 2,3,7,8-T4CDD also exhibits the smallest absolute hardness (derived from the HOMO-LUMO energy gap) in our calculations.

Ab initio calculations of SrTiO3, BaTiO3, PbTiO3, CaTiO3, SrZrO3, PbZrO3 and BaZrO3 (001), (011) and (111) surfaces as well as F centers, polarons, KTN solid solutions and Nb impurities therein

2014 ◽  
Vol 28 (17) ◽  
pp. 1430009 ◽  
Author(s):  
R. I. Eglitis
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Solid Solutions ◽  
Alkali Halides ◽  
Shallow Donor ◽  
Surface Energies ◽  
F Center ◽  
Bonding Properties ◽  
Almost All ◽  
F Centers

In this paper, the review of recent results of calculations of surface relaxations, energetics, and bonding properties for ABO 3 perovskite (001), (011) and (111) surfaces using mostly a hybrid description of exchange and correlation is presented. Both AO and BO 2-terminations of the nonpolar (001) surface and A , BO , and O terminations of the polar (011) surface, as well as B and AO 3-terminations of the polar (111) surface were considered. On the AO -terminated (001) surface, all upper-layer A atoms relax inwards, while all second layer atoms relax outwards. For the BO 2-terminated (001) surface, in most cases, the largest relaxations are on the second-layer metal atoms. For almost all ABO 3 perovskites, the surface rumpling is much larger for the AO -terminated than for the BO 2-terminated (001) surface, but their surface energies are always quite similar. In contrast, different terminations of the (011) ABO 3 surface lead to very different surface energies for the O -terminated, A -terminated, and BO -terminated (011) surface, respectively. A considerable increase in the Ti – O or Zr – O , respectively, chemical bond covalency near the (011) surface as compared both to the bulk and to the (001) surface in ABO 3 perovskites were predicted. According to the results of ab initio calculations for Nb doped SrTiO 3, Nb is a shallow donor; six nearest O ions are slightly displaced outwards from the Nb ion. The F center in ABO 3 perovskites resembles electron defects in the partially-covalent SiO 2 crystal rather than usual F centers in ionic crystals like MgO and alkali halides. The results of calculations for several perovskite KNb x Ta 1-x O 3 (KTN) solid solutions, as well as hole and electron polarons in ABO 3 perovskites are analyzed.

Ab initio calculations on the local structure and the 4f–5d absorption and emission spectra of -doped YAG

2008 ◽  
Vol 128 (8) ◽  
pp. 1248-1254 ◽  
Author(s):  
Jose Gracia ◽  
Luis Seijo ◽  
Zoila Barandiarán ◽  
Daniel Curulla ◽  
Hans Niemansverdriet ◽  
...  
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Local Structure ◽  
Emission Spectra ◽  
Absorption And Emission ◽  
Absorption And Emission Spectra

Dissociation of the COS+ ion by photoionisation: experiment and ab initio calculations The Franck-Condon energy gap an the vibronic state Franck-Condon energy regions

1996 ◽  
Vol 209 (2-3) ◽  
pp. 143-157 ◽  
Author(s):  
M.-J. Hubin-Franskin ◽  
J. Delwiche ◽  
P.-M. Guyon ◽  
M. Richard-Viard ◽  
M. Lavollée ◽  
...  
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Energy Gap ◽  
Vibronic State ◽  
Franck Condon

FREQUENCY-DEPENDENT HYPERPOLARIZABILITY OF BENZENE DERIVATIVES: AB-INITIO CALCULATIONS

2007 ◽  
Vol 16 (03) ◽  
pp. 367-380
Author(s):  
DIKSHA MAKWANI ◽  
R. VIJAYA
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Energy Gap ◽  
Lone Pair ◽  
Benzene Derivatives ◽  
Structure Property ◽  
Disubstituted Benzene ◽  
Structure Property Relationship ◽  
Donor Acceptor ◽  
Acceptor Capacity

Structure–property relationship and dispersion effects for disubstituted benzene molecules have been investigated. Ab-initio calculations of the first hyperpolarizabilities (β) of donor–acceptor benzene derivatives show that the magnitude of β depends upon the availability of the lone pair of electrons on the nitrogen atom to conjugate with the benzene ring. The HOMO–LUMO energy gap and β have an inverse relationship. From the dispersion studies, it is observed that the first resonance peak shifts towards lower frequencies as the donor/acceptor capacity increases.

Calculation of Energy Levels of Cerium in Inorganic Scintillator Crystals

1994 ◽  
Vol 348 ◽  
Author(s):  
J. Andriessen ◽  
P. Dorenbos ◽  
C.W.E. van Eijk
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Energy Gap ◽  
Energy Levels ◽  
Fair Agreement ◽  
High Symmetry ◽  
Inorganic Scintillator ◽  
Low Symmetry

ABSTRACTFully relativistic ab initio calculations have been performed on energy levels of cerium in BaF2, LaF3, YAP and YAG. Also nonrelativistic calculations were done on cerium in LSO. The results are in fair agreement with experiment. Both the splitting of the 4f and 5d levels and the 4f–5d energy gap can be explained in the high symmetry crystal BaF2 as well as in the low symmetry crystals LaF3, YAP, YAG and LSO.

Sign in / Sign up

Export Citation Format

Share Document