Publication No.54



J. Chimborazo, M. Castillo, C. Velasco, A. Stashans. H atom 
in BaTiO3 and CaTiO3 crystals: structure, electronic properties 
and diffusion - 3rd Int. Conf. Advanced Opt. Mater. Devices, 
Riga (Latvia), 2002, p. 39. 

Abstract:
	We investigate the effects that an H impurity produces to 
the geometry and electronic structure in the BaTiO3 and CaTiO3 
crystals considering various crystallographic lattices of these 
materials.
	In order to the study the H-doped barium and calcium titanates 
we use a quantum-chemical method based on the Hartree-Fock formalism 
and the so-called LUC approximation [1]. In our analysis, the 
interstitial H is found to bind to one of the O atoms forming the 
so-called O-H group. At equilibrium the O-H distance is found to 
be equal to 0.89 � and 0.91 � for cubic and tetragonal lattices of 
BaTiO3 crystal, respectively. These results predicting the shortening 
of the O-H bond-length due to the crystalline environment are in an 
agreement with another studies on H-doped titanates [2]. In the case 
of the CaTiO3 crystal, the O-H distance is found to be equal to 0.89 
� for cubic and 1.04 � for orthorhombic lattices, respectively.
	We also study impurity effect on structure distortion and 
analyse the ferroelectric polarisation due to the impurity doping in 
BaTiO3. A qualitative study shows that the absolute values of the 
effective charges reduce while the Ti-O bond-lengths increase 
considerably in the defective region. As a result of the above-
mentioned two effects we obtain that the bulk ferroelectric 
polarisation augments 1.27 times. Thus one can conclude that the 
ferroelectric degradation is not a bulk effect. This might also 
provide an explanation for oxygen loss in ferroelectric films near 
the surface under thermal annealing in a hydrogen atmosphere. 
	Hydrogen diffusion in the crystalline lattice is studied 
considering different types of trajectories. The trajectory giving 
the lower potential energy barrier for the H atom movement is found 
to be of a parabolic form. 

[1] E. V. Stefanovich, E. K. Shidlovskaya, A. L. Shluger, and M. A. 
    Zakharov, Phys. Status Solidi (b), 160, 529 (1990).
[2] C. H. Park and D. J. Chadi, Phys. Rev. Lett., 84, 4717 (2000). 




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