Structural, optical and dielectric properties of relaxor- ferroelectric Pb0.78Ba0.22Sc0.5Ta0.5O3 Running head: Properties of relaxor-ferroelectric Pb0.78Ba0.22Sc0.5Ta0.5O3

We report on the synthesis of a new relaxor compound with chemical formula Pb0.78Ba0.22Sc0.5Ta0.5O3 (PBST). Single crystals were obtained by the high-temperature solution growth method. The structure of the new compound is of double-perovskite type with face-centred cubic symmetry at room temperature. The frequency dependence of the dielectric constant of PBST shows a strong dielectric dispersion in a wide temperature range and a dielectric-constant maximum near 200 K at 10 kHz. The local atomic environment was probed by Raman scattering and optical absorption spectroscopy. The results show that the incorporation of Ba deforms the BO6 octahedra adjoining the BaO12-polyhedra along the <111> direction and shortens the Pb-O bond lengths next to the BaO12-polyhedra within the {111} planes. The random substitution of Ba for Pb leads to a wider distribution in the size and shape of the ferroic species in PBST as compared to stoichiometric PbSc0.5Ta0.5O3. The addition of Ba shifts the optical absorption edge to lower energies and gives rise to extra absorption peaks at 460 nm and 730 nm. The latter peak is related to polar atomic rearrangements in the vicinity of the Ba ions embedded into the PbSc0.5Ta0.5O3 matrix.