DIELECTRIC BEHAVIOR OF MATERIALS FOR WIRELESS COMMUNICATIONS PREDICTED FROM FIRST PRINCIPLES.

Dielectric materials for wireless communications applications must have high dielectric constant, low loss, and temperature stability. For high-power base station resonators, the only ceramic known with the required dielectric properties is [Ba.sub.3][ZnTa.sub.2][O.sub.9] (BZT). The drive to find low-cost alternatives to BZT motivates our interest in determining the microscopic origin of useful electronic properties. As part of this effort, NIST researchers have performed first-principles calculations of the dielectric properties of [CaTiO.sub.3] (CT) and [CaAl.sub.1/2][Nb.sub.1/2][O.sub.3] (CAN). These systems are both components of solid solutions that contain phases with favorable dielectric properties. CT and CAN have similar perovskite-related crystal structures but very different room temperature dielectric constants: [element of] = 170 for CT; [element of] = 27 for CAN. With lattice parameters and space groups as the only experimental inputs, the researchers used density-functional theory methods to co mpute for CT and CAN as a function of temperature, and obtained room temperature values of 140 and 25, respectively, in good agreement with experiment. Low-frequency phonons dominate the dielectric properties, and their calculations predict important differences between the properties of the low-frequency phonons in CT and CAN. In CT, frequencies are lower and all cations move in opposition to the oxide ions; in CAN however, frequencies are higher and Al and Nb move with the oxide ions. Their calculations of phonon properties have been verified experimentally by infrared reflectivity measurements. Ultimately, determination of the microscopic origin of dielectric behavior will permit the rational, efficient discovery and development of advanced ceramics needed for next-generation applications.