Electrical characterisation of ferroelectric oxides
Two groups of ferroelectric oxides have been studied using a.c. impedance techniques. These were donor-doped BaTiO3 ceramics showing the positive temperature of resistance, PTCR, effect and single crystal LiTaO3. Existing theories of the PTCR effect in BaTiO3 ceramics assume that it is associated with the grain boundary regions. An in-depth analysis of a.c. data, using combined impedance and modulus spectroscopy revealed the presence of at least two components, both of which exhibited PTCR effects. These were attributed to bulk and grain boundary effects because of the different temperature dependence of their associated capacitances: grain boundary effects have temperature independent capacitances whereas bulk effects show a capacitance maximum at the Curie pont and Curie-Weiss behaviour above the Curie point. An explanation for the bulk PTCR effect is proposed. The a.c. data handling techniques used and developed here provide information regarding the inhomogeneous nature of the grain boundary and bulk components which cannot be obtained from d.c. measurements. An equivalent circuit to model the a.c. response of PTCR BaTiO3 ceramics is presented. The influence of processing conditions on the various bulk and grain boundary PTCR effects, such as the sample cooling rate from the sintering temperature and low temperature anneals, < 400 °C, in various reducing and oxidising atmospheres is discussed. For quickly cooled samples, the PTCR response is dominated by the bulk impedance, whereas for slowly cooled samples, the grain boundary component dominates. Annealing in reducing atmospheres destroys the grain boundary PTCR effect whereas bulk PTCR effects are relatively insensitive to the atmosphere at low temperatures. Information regarding the conductive core of the grains and the behaviour of component resistances below the Curie point for slowly cooled samples is presented. A general model is proposed which explains the PTCR behaviour of BaTiO3 ceramics. The effects of both cooling rate and atmosphere are incorporated in this model. A.c. impedance data for single crystal LiTaO3 with the crystal c axis oriented parallel and perpendicular to the electric field were recorded above and below the Curie point, 590oC. With the polar c axis parallel to the electric field, the following were measured: the charge polarisation associated with the ferroelectric domains, the intrinsic lattice polarisation, the resistance associated with domain re-orientation and the resistance due to lithium ion migration. The correct choice of equivalent circuit is crucial to the determination of these parameters. With the c axis parallel to the electric field, no ferroelectric behaviour was observed and the crystal was a modest electronic conductor at elevated tempeatures, 500-700oC. The LiTaO3 results presented indicate the potential of a.c. impedance techniques for probing the electrical properties of ferroelectric single crystals. Domain re-orientation may be characterised by macroscopic resistance and capacitance values. This allows the temperature dependence of domain re-orientation phenomena to be characterised as the crystal is heated through the Curie point. Such detailed characterisation cannot be obtained from existing fixed-frequency or d.c. measurements.