Publication No.40



P. Sánchez, A. Stashans. Computational studies and comparison 
of Nb and La doped SrTiO3 - PASI Int. Conf. Phys. Technol. 
Nanometer Scale, San José (Costa Rica), 2001, p. sat42.

Abstract:
	Due to the extensive applications in the electronic industry 
and its unusual properties, SrTiO3 crystal has attracted a great deal 
of attention. The modified for crystals quantum-chemical method and a 
periodic large unit cell (LUC) model was used to study this material. 
	One of the Ti atoms was substituted by Nb atom in LUCs of 40 
and 135 atoms. This kind of doping increases the number of valence 
electrons by one leading to the relaxation of the Nb-surrounding O and 
Sr atoms. As it is known, the directions <100> and <010> are completely 
equivalent due to the symmetry of the tetragonal phase of the crystal.  
That is why the relaxation of atoms along these axes is expected to be 
the same and was found to be equal to 0.06 Å for the Sr atoms in the xy 
plane under the impurity and 0.11 Å for the Sr atoms in the plane above 
it.  The oxygen atoms have movements of 0.08 Å, 0.07 Å and 0.05 Å, 
respectively, depending on their relative position with respect to the 
impurity.
	The lattice relaxation was performed taking into account the 
fact that the tetragonal phase of the crystal presents a ferroelectric 
behavior.  This implies the presence of a spontaneous electric dipole 
along the z-axis, which changes due to the doping.  This produces a 
different kind of motion along the z-axis for both kinds of atoms.  We 
have found that the strontium and oxygen atoms have the opposite 
displacements along the ferroelectric axis.  We suppose that this fact 
evidences a local change in the electric dipole of the crystal due to 
the impurity presence.
	Besides, we have been studying La doping using LUCs of 40, 80 
and 160, where two of the Sr atoms were substituted by two La atoms 
producing two different system states.  The singlet state corresponds 
to the case when two extra electrons occupy the same local energy level 
within the band-gap and the triplet state is when two electrons occupy 
two different local energy levels.
	Trying to explain some effects in the superconducting La-doped 
strontium titanate under 14 K is our main reason to study this specific 
kind of doping.  Experiments carried out recently in Japan showed that 
two peaks in the band gap region are induced by La substitution for Sr, 
a metallic peak and a ~ 1.5 eV peak above the top of the upper valence 
band (UVB). Those two system states guided us not only to support the 
idea that these peaks are produced by electrons of the La 6s atomic 
orbital (AO), but also to find electron transfer to the Ti 3d AOs, 
which form the lower part of the conduction band. The equilibrium atomic 
configurations were studied, applying also an automated geometry 
optimization procedure. The obtained atomic displacements, in particular, 
the lost of symmetry in the crystal, pointed to the possibility that we 
could have been dealing with the Jahn-Teller effect. It seems that the 
ferroelectricity phenomenon in the SrTiO3 crystal has an important role 
in the way the host atoms reach their new equilibrium positions in both 
singlet and triplet states.  




Back to the main page of our research centre