Electronic and lattice excitations in BaTiO3:La

We report lattice and electronical properties of BaTiO3:La powders and ceramics with 0<xLa<20 at.%. Dielectric measurements and x-ray scattering evidence a continuous decrease of the ferroelectric transition temperature when the La content is increased. For xLa=20 at.%, no dielectric maximum is found for temperatures higher than 20 K. At the same time, an impurity mode at 840 cm-1 increases linearly with xLa. We have modelled these data using a linear chain model with one polarizable ion. The electric excitations were probed using DC conductivity, EPR spectroscopy and optical absorption. This last technique was applied to BaTiO3:La single crystals, which were grown for the first time in our laboratory. Our main result is that the conductivity maximum at xLa approximately=0.5 at.% is related to an impurity level lying 2.1 eV below the conduction band.