ELECTRONIC TRANSPORT-PROPERTIES OF SR1-XLAXTIO3 CERAMICS

The electrical conductivity of Sr1-,La,TiO3 ceramics (x less than or e qual to 0.4) and single crystals (x less than or equal to 0.1) was inv estigated in the temperature range between 19 and 1673 K. The mobility was calculated from the carrier concentration, n, which was determine d by Hall measurements as well as by a chemical Ti3+ titration. The mo bility of the single crystals agrees well with a model originally deve loped for undoped strontium titanate. At low T and high n scattering b y ionized donor centers predominates. Above room-temperature phonon sc attering becomes predominant. Sr1-xLaxTiO3 ceramics follow the same mo del, but only if they had been previously reduced in water-free and ox ygen-free hydrogen atmospheres. The behavior during the reducing and c ooling process can be explained by a defect chemical model, using a se t of constants developed in a former work. However, reduced in water s aturated hydrogen the conductivity at low T decreases by decades compa red to single-crystal data and cannot be explained anymore by the abov e-mentioned models. In conjunction with impedance spectroscopy experim ents it was found, that in this case the electrical behavior of the ce ramics is completely governed by grain-boundary phenomena. These high- ohmic grain boundaries may be depletion layers, which are formed or an nihilated depending on the conditions of the preceding high-temperatur e process. (C) 1996 American Institute of Physics.