Phase Transformations of Anatase TiO2 on Cation Intercalation from Firstprinciples Simulation.
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Phase Transformations of Anatase TiO2 on Cation Intercalation from First Principles Simulation. Marina V. Koudriachova1, Nicholas M.Harrison2, Simon W. de Leeuw1 1 Computational Physics, Dept. of Applied Physics, TU Delft, Lorentzweg 1, 2628 CJ Delft, The Netherlands 2 Department of Chemistry, Imperial College of Science, Technology and Medicine, London, SW7 2AY, U.K. and CLRC, Daresbury Laboratory, Daresbury, Warrington WA4 4AD,UK ABSTRACT The phase transformations of anatase on Li-, H- and Na- intercalation are analyzed, for a variety of different insertion concentrations, using first principle calculations. Predicted structures are based on symmetry unconstrained optimization of all internal degrees of freedom and the unit cell shape and volume. The maximum insertion concentrations are determined, the phase stability of the predicted structures examined and the mechanism of the phase transformations discussed. INTRODUCTION Anatase-structured TiO2 has many technologically important applications including solar cells, Li-rechargeable batteries and electrochromic devices which are based on its intercalation properties [1-7]. The electron density donated upon intercalation may be accommodated by the flexible electronic structure of titanium ions while the open structure provides space for the charge compensating cations such as Li+, H+ and possibly Na+. Lithium intercalation into anatase has been studied extensively with a variety of experimental and theoretical techniques [3-8]. The electrochemical insertion of Li-ions at room temperature proceeds through a two-phase equilibrium of a Li-poor (tetragonal) and a Li-rich (orthorhombic) phase. The latter has a composition Li0.5TiO2 [6,7]. The structure of Li0.5TiO2 phase has been determined from neutron diffraction data [6] and confirmed in first principles calculations [9]. Controversy exists about the possibility of intercalation of considerably larger Na+ions. Intercalation from a Na+ containing electrolyte in Li-electrochemical cells was believed to be impossible [10] while diffusion barriers for Na+ ions migrating through anatase TiO2 without major structural changes have been reported [11]. On the basis of previous ab initio calculations, which imposed symmetry restrictions on the structural relaxations, a tetragonal structure for Na0.5TiO2 has been proposed [11]. Electrochemical and photochemical insertion of H+-ions into anatase is well established [12-15]. However, to the authors knowledge the structure of H+intercalated anatase has not previously been reported. H+-ions are believed to intercalate into anatase from protic electrolytes or photochemically with the formation of Ti+3 sites and interstitial protons involved in hydroxyl (Ti-OH) or hydride (Ti-H) bonds [12-15]. An understanding the mechanisms of the phase transformations and the structures adopted upon cation intercalation is a key issue in the prediction and engineering of new devices and this understanding is currently far from complete. In this paper the phase behavior of anatase on cation intercal
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