Structural characterisation of ferroelectric Ag2Nb4O11 and dielectric Ag2Ta4O11†
Abstract
Structural characterisation of Ag2Nb4O11 and Ag2Ta4O11 powders has been carried out by Rietveld refinement of c to R3c as the temperature decreases through TC with a further transition from R3c to R3 below ∼200 K. The origin of the polar structure is related to displacement of Nb atoms in pentagonal bipyramids towards apical oxygens and in octahedra towards faces. This displacement towards octahedra faces is observed for some Ag atoms too. Ag2Ta4O11 ceramics show temperature-independent permittivity of ∼20, with a centrosymmetric crystal structure, R
c. Comments on possible phase transition(s) of several compounds similar to Ag2(Nb,Ta)4O11 are made.