Dielectric properties of Ca- and Ba-doped SrTiO3

Dielectric properties of high quality polycrystalline Ca- and Ba- doped SrTiO$_{3}$ perovskite were studied by means of dielectric constant, dielectric loss and ferroelectric hysteresis measurements. Low frequency dispersion of the dielectric constant was found to be very small and a simple relaxor model may not be able to explain its dielectric behavior. A few relaxation modes were found in these samples and they were all interpreted as thermally activated dipolar re-orientation across energy barriers. In Sr$_{1-x}$Ca$_{x}$TiO$_{3}$ (x=0-0.3), two modes were found and its concentration dependence implies a competition between these two relaxation processes. In Sr$_{1-x}$Ba$_{x}$TiO$_{3}$ (x=0-0.25), relaxation modes were found to be related to the structural transitions and the relaxation modes existed even at low doping levels (x$<$0.1) at which the structural ordering were not identified by previous Neutron scattering studies. Application of various models was discussed and both of the Ca- and Ba-doped samples can be understood qualitatively.