First Principles Hartree–Fock Description of Lithium Insertion in Oxides: I. The End Members TiO2and LiTiO2of the System LixTiO2
✍ Scribed by W.C. Mackrodt
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 548 KB
- Volume
- 142
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
First principles periodic Hartree-Fock calculations are reported for the P4 2 /mnm (rutile), I4 1 /amd (anatase), Pbca (brookite), Pnma (ramsdellite), Pcbn (colombite), Fd3 m (spinel), and Imma (orthorhombic) polymorphs of TiO 2 , from which the predicted order of stability is
The calculated difference in energy between the rutile and anatase structures is 0.02-0.06 eV, in good agreement with a recent local density approximation (LDA) estimate of 0.033 eV and an experiment enthalpy difference of 0.05 eV. The corresponding Hartree-Fock and LDA differences for the brookite structure are 0.06 and 0.058 eV, respectively. The calculated volumes, which are based on isotropic volume-optimized Hartree-Fock energies, are also in good agreement with recent LDA calculations and with experiment. Spin-unrestricted calculations are reported for the Fm3 m, Imma, Pnma, and P4 2 /mmm of LiTiO 2 , where the stability is in the order
The only reported phase for LiTiO 2 is Fm3 m, for which the calculated volume is in good agreement with experiment. From the relative stabilities of TiO 2 and LiTiO 2 , the relative lithium insertion potentials corresponding to TiO 2 Li P LiTiO 2 are deduced, with a maximum variation of 1.6 eV for the different polymorphic routes. The maximum voltage predicted is that for the Imma route which is &1 eV larger than that for Pnma. Direct comparisons with the calculated energy for C2/m Li 0.5 MnO 2 Li P LiMnO 2 lead to an estimate of the voltage for Imma TiO 2 Li P LiTiO 2 of &1.3 eV, which is &2.5 eV anodic to the Mn system. The corresponding values for the Pnma polymorphic route are &3 and &3.5 eV, respectively. Mulliken population analyses indicate that lithium is completely ionized in LiTiO 2 and that the charge transfer is predominantly to the oxygen sublattice. There is a rehybridization of the titanium valence orbitals leading to a slight increase in the 3d population and strong localization of spin density at the titanium sites with local moments of &1 B .