Relaxor Ferroelectric Behavior on Ag-Substituted K ₅ Nb ₉ O ₂₅ (= (5/6) K Yukikuni Akishige, Shinya Tsukada, Harutoshi Asakawa, Ryuichi Komatsu

Single crystals of Ag-substituted K ₅ Nb ₉ O ₂₅ , which belongs to the partially filled tetragonal tungsten bronze (space group P4/mbm), were prepared by the AgNbO ₃ flux method. Dielectric constant ε′, ac electrical conductivity σ, and a pyroelectric charge Δp were measured over a wide temperature range of 10 to 900 K, in both directions along and perpendicular to the tetragonal c-axis. The specific heat C _p was also measured in a range of 2 to 300 K. At temperatures above ca. 500 K, σ increases rapidly with increasing temperature owing to a migration of oxygen vacancies. At low temperatures, on the other hand, ε′ shows a frequency-dependent broad maximum at around T _m = 100 K and Δp appears below ca. 150 K. However, no anomaly was observed in the temperature dependence of C _p around T _m . The experimental results are explained as a ferroelectric phase transition accompanied by relaxor behavior.