LOW AND HIGH-FREQUENCY DIELECTRIC RELAXATIONS IN A TETRAGONAL TUNGSTEN BRONZE TYPE CERAMICS

Strontium barium niobate Sr0.4Ba0.6Nb2O6 [SBN 40-60] ceramics have bee n synthesized by a mixed solid state oxide route. This composition rel atively rich in barium still exhibits a relaxer behavior. Dielectric m easurements have been performed in the frequency and temperature range s 20-10(9) Hz and, 250-450 K respectively. In addition to the broad lo w frequency relaxation [LF] (10(5) - 10(7) Hz) generally observed in r elaxer materials, a high frequency relaxation [HF] (10(8)-10(9) Hz) wa s evidenced in all the temperature range. Its characteristics are very similar to those observed in ferroelectric perovskites (i.e. minimum of the relaxation frequency, f(r) [HF], close to the temperature of th e maximum of permittivity, T-m). Below T-m, a strong deviation from a single Debye behavior appears. The corresponding broadening of the [HF ] relaxation time distribution can be related to the effect of the add itional low frequency contribution. At temperatures close to T-m, the two [LF] and [HF] relaxations are observed in spite of a strong epsilo n(r) peak overlap. The dielectric response is discussed in relation wi th the tetragonal tungsten bronze structure. Results are compared with the behavior observed in perovskite type compounds.