Biomedical subjects

Christian Minot

Publications and source records attributed to Christian Minot.

3 recordsLinked to original sources

PubMed2006

Ionic conductivity of Li2B4O7.

The formation and mobility of Li point defects in Li(2)B(4)O(7) are investigated theoretically with periodic quantum chemical calculations. Calculated defect formation energies obtained with a density functional theory/Hartree-Fock hybrid method and with the Perdew-Wang density functional method are compared. The basis set effect is investigated by comparison of results obtained with atom-centered basis functions and plane waves. With both methods only a moderate relaxation is observed for the atoms surrounding the Li defect position. The defect-induced change of electronic properties is investigated by calculating the density of states for the stoichiometric and defective supercells. The activation energy for the movement of a Li(+) ion along the (001) direction is calculated. It is observed that Li(+) ion migrates through a one-dimensional channel formed by the five-vertex lithium-oxygen (LiO(5)) polyhedra. The calculated activation energies are in excellent accord with experiment.

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PubMed2006

Theoretical analysis of structural, energetic, electronic, and defect properties of Li2O.

The structural, energetic, and electronic properties of stoichiometric and defective Li(2)O were studied theoretically. The reliability of the Perdew-Wang method in the framework of density functional theory (DFT), and of two DFT/Hartree-Fock hybrid methods (PW1PW and B3LYP), was examined by comparison of calculated and available experimental data. Atom-centered orbitals and plane waves were used as basis functions for the crystalline orbitals. For both cases, the basis set dependence of calculated properties was investigated. With most of the methods, good agreement with the experimental Li(2)O lattice parameter and cohesive energy was obtained. In accordance with experiment, the analysis of electronic properties shows that Li(2)O is a wide gap insulator. Among the considered methods, the hybrid methods PW1PW and B3LYP give the best agreement with experiment for the band gap. The formation of an isolated cation vacancy defect and an F center in Li(2)O were studied. The effect of local relaxation on the calculated defect formation energies and the defect-induced changes of electronic properties were investigated and compared to available experimental results. The migration of a Li(+) ion in Li(2)O bulk was investigated. The activation energy for the migration of a Li(+) ion from its regular tetrahedral site to an adjacent cation vacancy was calculated, including the effect of local relaxation. The calculated activation barriers, 0.27-0.33 eV, are in excellent agreement with experiment.

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PubMed2005

Structural and electronic properties of Li(2)b(4)O(7).

The reliability of various quantum-chemical approaches for the calculation of bulk properties of lithium tetraborate Li(2)B(4)O(7) was examined. Lattice parameters and the electronic structure obtained with density-functional theory (DFT), with DFT-Hartree-Fock (HF) hybrid methods, and with the semiempirical method MSINDO were compared to available experimental data. We also compared the results at DFT level using different wave functions, either based on linear combinations of atom-centered orbitals (LCAO), or on plane waves, as implemented in the crystalline orbital programs CRYSTAL and VASP. The basis set dependence of calculated properties was investigated for the LCAO method. In the plane wave approach ultrasoft pseudopotentials (US PP), and projector-augmented wave (PAW) potentials were used to represent the core electrons. For all methods under consideration, the calculated Li(2)B(4)O(7) structure parameters are close to each other and agree within a few percent with measured values. A more pronounced method dependence was found for the band structure, the band gap and the cohesive energy. Closest agreement between theoretical and experimental results for the band gap was obtained with the DFT-HF hybrid methods while pure DFT methods underestimate and HF based methods overestimate the measured value. It was found that the calculated band gap strongly depends on the atomic basis set in the LCAO approach. The description of the core electrons considerably affects the cohesive energy obtained with the plane wave approach. Atomic charges based on a Mulliken analysis were compared to effective charges obtained from Raman spectroscopy. Electron density maps are used to analyze the character of B-O and Li-O interactions.

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