Publication No.56



L. M. Prócel, R. Viteri, F. Tipán, D. F. Ortiz, A. Stashans. 
A quantum-chemical study of excitons in tetragonal BaTiO3 
and SrTiO3 crystals - 3rd Int. Conf. Advanced Opt. Mater. 
Devices, Riga (Latvia), 2002, p. 41. 

Abstract:
	Barium and strontium titanates are very important materials 
having a number of high-tech applications and fundamental scientific 
interests. On the other hand, the studies of excitons in the perovskites 
are very few. The present research tries to explain the effects in 
BaTiO3 and SrTiO3 tetragonal phases when they have an exciton. The 
exciton is an electron-hole pair and might be responsible for the 
photoluminescence. Within the present work we study the triplet excitons 
using a quantum-chemical method based on the Hartree-Fock formalism and 
the periodic large unit cell (LUC) model [1]. The LUC of 135 atoms are 
exploited for both crystals. 
	As a result, for both materials we obtain the defect structure 
corresponding to the so-called Mott-Wannier-type exciton having a 
considerable distance between the hole and electronic parts of the 
defect. The hole is localised on one of the O atoms while the electron 
is localised on one of the Ti atoms. The calculated distance between 
the hole and electron centres is about 4.0 Å. The obtained lattice 
relaxation energies are close to 5.0 eV in both cases. 
	The calculated luminescence energy using the so-called delta SCF 
method is found to be equal to 0.94 eV and 1.10 eV for BaTiO3 and in the 
SrTiO3 crystals, respectively. As one can notice it corresponds to the 
infrared part of the spectrum. So, in our mind, the experimentally 
detected green luminescence due to the excitons should be attributed 
to the singlet excitons. 

[1] E. V. Stefanovich, E. K. Shidlovskaya, A. L. Shluger and M. A. Zakharov, 
    Phys. Status Solidi (b), 160, 529 (1990).




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