Behaviour of the complex electronic dielectric constant of cubic ABO3 perovskites from tight-binding energy bands

The electronic structures of KNbO3 and KTaO3 have been determined by the tight-binding method, using a 17 atomic orbital basis set. The Hamiltonian parameters have been taken from the previous APW-LCAO Mattheiss computations. Additional interaction and overlap integrals have been introduced and their influence on the band scheme was determined. The influence on the behaviour of the imaginary part of the dielectric constant epsilon 2 has been studied when interband transition moments are evaluated for each k wavevector in the Brillouin zone. The introduction of transition moments revealed the second maximum of epsilon 2 about 8 eV and pointed out the importance of O-O and B-B parameters. The real part of the dielectric constant epsilon 1 and the related optical constants such as the refractive index, the reflectivity and absorption coefficients deduced from epsilon 1 and epsilon 2 spectra were compared with experimental data.