H atom in BaTiO3 and CaTiO3 crystals: structure, electronic properties, and diffusion

Johnny Chimborazo; Milagros Castillo; Carmen Velasco; Arvids Stashans

doi:10.1117/12.515776

8 August 2003 H atom in BaTiO₃ and CaTiO₃ crystals: structure, electronic properties, and diffusion

Johnny Chimborazo, Milagros Castillo, Carmen Velasco, Arvids Stashans

Proceedings Volume 5122, Advanced Organic and Inorganic Optical Materials; (2003) https://doi.org/10.1117/12.515776
Event: 2003 Chapter books, 2003, Bellingham, WA, United States

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 predict 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.

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Johnny Chimborazo, Milagros Castillo, Carmen Velasco, and Arvids Stashans "H atom in BaTiO₃ and CaTiO₃ crystals: structure, electronic properties, and diffusion", Proc. SPIE 5122, Advanced Organic and Inorganic Optical Materials, (8 August 2003); https://doi.org/10.1117/12.515776

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