Optical properties of semiconductor strontium and barium titanates

At small concentrations (approximately 0.003 at.%) rare-earth elements Eu, Sm enter the crystal lattice of barium and strontium titanates in the two-valent state. In this case an isovalent substitution takes place and no increase in the electric conductivity is observed. With an increase in the impurity concentration in the phonon spectra there appear absorption bands which may be ascribed to local impurity oscillations. When crystals are alloyed and reduced, additional bands at 0.5-0.6 μ appear in the spectra, their intensity increasing monotonously with the rate of reduction and concentration of the alloyed impurity. The nature of these bands may be connected with the formation of Ti3+, and more complex defects of the crystal lattice, for instance, of Z-centres