Properties of Perovskite Systems
Perovskite-phase metal oxides exhibit a variety of interesting physical properties which include ferroelectric, dielectric, pyroelectric, and piezoelectric behavior. Specifically, linear dielectric materials exhibit linear polarization behavior as a function of applied field. Ferroelectric ceramics are dielectrics with a permanent electric dipole, which can be oriented upon the application of an electric field. Pyroelectric materials yield a spontaneous polarization, but the direction of such polarization cannot be reversed upon application of an electric field. Piezoelectrics either exhibit an electrical charge when mechanically stressed or undergo mechanical deformation upon the application of an electric field. In general, these various properties arise from the crystal symmetry adopted by these materials.
Of these various classes of structures, ferroelectric materials are both pyroelectric and piezoelectric and are generating a great deal of current interest because their electric dipoles can be reoriented by the application of an electric field. However, all of these materials lose their inherent polar properties above their Curie temperature (Tc) transforming into a nonpolar state known as the paraelectric phase. For example, barium titanate (BaTiO3, Aldrich Prod.
ers from a tetragonal ferroelectric phase to a paraelectric cubic phase at a Curie temperature of 120 °C. The properties of ferroelectric materials change significantly and show a maximum as the temperature approaches Tc.
bove this temperature, there is a gradual decrease in the dielectric constant. Hence, the main applications of BaTiO3 are based around this phase transition, especially with respect to its potential as a dielectric. The temperatures at which these phase transitions occur can also be modified by the partial substitution of other metal oxides into BaTiO3 so as to yield solid solutions. For example, dopant substitutions can change cell dimensions and hence the temperatures at which these phase changes occur. Doped BaTiO3 has found wide applications in PTC (positive temperature coefficient) thermistors, semiconductors, and piezoelectric devices.
