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H atom in BaTiO<inf>3</inf> and CaTiO<inf>3</inf> crystals: Structure, electronic properties and diffusion

Abstract:

We investigate effects that an H impurity produces upon the geometry and the electronic structure in the BaTiO3 and CaTiO3 crystals considering several lattices of these materials. In order to study the H-doped barium and calcium titanates we use a quantum-chemical method based on the Hartree-Fock formalism and a periodic large unit cell (LUC) model. As a result, the interstitial H is found to bind to one of the O atoms forming the so-called OH group. In equilibrium, O-H distance is found to be 0.89 and 0.91 Å for BaTiO3 cubic and tetragonal lattices, respectively. These results thus pbkp_redict reduction of the O-H bond-length compared to a free radical, obviously due to rather compact crystalline lattice. In the case of the CaTiO3 crystals, the O-H distance is found to be 0.89 and 1.04 Å for cubic and orthorhombic phases, respectively. We also study the impurity effect upon the lattice distortion and analyze the ferroelectric polarization in the tetragonal BaTiO3. A qualitative study shows that the absolute values of the effective charges decrease while the Ti-O bond-lengths increase considerably in the defective region. Combining these two effects we obtain that the bulk ferroelectric polarization increases 1.27 times. Therefore, one can conclude that the ferroelectric degradation is not a bulk effect. This could also provide an explanation for the oxygen loss in the ferroelectric films near the surface under thermal annealing in hydrogen atmosphere. Diffusion of the hydrogen in the BaTiO3 cubic lattice is also studied in the present work considering different types of trajectories. The diffusion path that gives the lowest potential energy barrier for the hydrogen motion is found to be of a parabolic form.