Synthesis, structure, and dielectric properties of (Bi1/2Ag1/2)TiO3

The compound Bi1/2Ag1/2TiO3 has been synthesized, for the first time, at 14 -14.5 GPa and 1000 degrees C using the uniaxial split sphere anvil type pre ss (USSA-2000). The structure of the quenched powder sample was refined fro m synchrotron X-ray diffraction data using the Rietveld method. At room tem perature, Bi1/2Ag1/2TiO3 exhibits an orthorhombic distortion of the cubic p erovskite structure and was assigned to a space group of Ibam and lattice p arameters of a = 5.48970(4) Angstrom, b = 5.51954(5) Angstrom, and c = 7.17 585(7) Angstrom. The refinements are consistent with a random distribution of Bi3+ and Ag+ on the A-sites. Diffraction results at various temperatures show two phase transitions as the temperature is increased. At 573 K, tetr agonal symmetry is observed and was assigned to a space group of I4/mcm wit h lattice parameters of a = 5.51664(8) Angstrom and c = 7.8134(2) Angstrom, while at 673 K, the cubic perovskite structure, with space group <Pm(3)ove r bar m> and a = 3.90607(8) Angstrom, was determined. Plane capacitor diele ctric contact measurements at 100 kHz gave a dielectric constant of 354 at ambient temperature for the sample. The dielectric constant increases with temperature and reaches a local maximum of 834 at 578 K. After heating the sample to 778 K and cooling back to room-temperature, both dielectric and d iffraction measurements provide evidence for a small but irreversible chang e in the stoichiometry.