First principles calculations of Pb and O vacancies in perovskite compounds performed.

Theorists at NIST have performed first principles calculations that have demonstrated that Pb-O vacancy pairs can be a significant source of "random" electric fields in Pb-based perovskite compounds, materials under development for electrostrictive transducers, actuators, and sonar devices for medical imaging and military applications.

Relaxor ferroelectrics exhibit useful physical properties, such as a high dielectric constant over a wide range of temperature and large electromechanical coupling constants. The known relaxors with the best properties are Pb-based compounds with the perovskite-type structure. Experiments indicate that inducing a 1 % to 5 % concentration of Pb and O vacancies significantly affects their physical properties. Because theories of relaxor behavior are based on interactions between polar nanoregions, and with local random electric fields, it is essential to characterize the polarization and electric fields generated by Pb and O vacancies.

First principles calculations were used to study the electrostatics of Pb and O vacancies in the Pb-based perovskite PbTi[O.sub.3]. The calculations suggest that at low concentrations Pb and O vacancies are paired into nearest neighbor divacancy pairs. Calculating the dipole moment of this pair required a novel application of the modern theory of polarizability. The dipole moment of the divacancy is greater than the dipole moment per unit cell in defect-free Pb-based ferroelectrics, indicating that Pb-O vacancy pairs can be a significant source of local random fields in Pb-based perovskites. This result can have important implications for the performance of relaxor ferroelectric materials in electronic devices.

CONTACT: Eric Cockayne, [phone]; eric. [e-mail] or Benjamin Burton, [phone]; [e-mail].