Spontaneous polarisation induced effects, band structure and interband transition momentum evolution at the para-ferroelectric phase change of BaTiO3 and KNbO3

The tight-binding energy band structures of BaTiO3 and KNbO3 have been determined in the cubic and tetragonal phases. The modifications at the para-ferroelectric change are analysed through the orbital interactions. The evaluation of the interband transition moment at different points of the Brillouin zone reveals the main interband transition, giving rise to the first optical structure. The Delta -band pairs which run nearly parallel determine the optical and dielectric properties. An optical matrix element investigation provides the parameters which determine the anisotropy. The evolution of the refractive index during the phase change is described through the decrease of interband transition strengths along the polar axis and the upward shift of the transition energies. The polarisation-induced band edge effects are estimated and connected with Bloch orbital interactions. Experimental results on the band gap and refractive index near the transition for T>Tc associated with precursor polarisation can be interpreted from this model. The long- and short-range interactions, expressed through the Bloch orbital interactions. allow the para-ferroelectric phase change to be described.