Publication No.52



J. Chimborazo, E. Patiño, A. Stashans. Effect of interstitial hydrogen
on structural and electronic properties in BaTiO3 - 19th General Conf. 
Condens. Matter Division Europ. Phys. Soc., Brighton (United Kingdom), 
2002, p. 230. 

Abstract:
We investigate geometry and electronic structure of interstitial H atom 
in the BaTiO3 crystal considering both, cubic and tetragonal crystallographic 
lattices. A quantum-chemical method based on the Hartree-Fock formalism is 
used throughout the study. Hydrogen impurity in BaTiO3 is found to be an 
autoionised positively charged ion since its donor level lies above the 
conduction band bottom. In this positively charged state, the valence bands 
are fully occupied and there is no state in the band-gap. Interstitial H 
behaves, therefore, as a shallow donor. It is found that interstitial H 
binds to one of the O atoms forming the so-called OH group. At equilibrium, 
O-H distance is found to be 0.89 Å and 0.91 Å for cubic and tetragonal 
lattices, respectively. The role of H impurity on ferroelectric polarisation 
in the BaTiO3 crystal is analysed. Host Ti-O bonds are found to be 
significantly weakened around a hydrogen impurity. This may provide an 
explanation for oxygen loss in ferroelectric films near the surface 
under thermal annealing in a hydrogen atmosphere. As a result of our study 
we can conclude that the degradation is not a bulk effect since H impurity 
augments the bulk ferroelectric polarisation by about 1.27 times. This 
explains some contradictory experimental data on oxygen loss, which is 
enhanced around defective regions and especially at an interface where 
the hydroxyl ion can chemically react with the electrode. 




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