Publication No.62



J. Chimborazo, M. Castillo, C. Velasco, A. Stashans. H atom 
in BaTiO3 and CaTiO3 crystals: structure, electronic properties 
and diffusion - SPIE Proceedings, 2003, Vol. 5122, p. 280-286. 

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 AA 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 AA 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.  




Back to the main page of our research centre