Relaxor Ferroelectric Behavior on Ag-Substituted K 5 Nb 9 O 25 (= (5/6) K 6 Nb 10.8 O 30 ) Single Crystals

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.