Electronic International Standard Serial Number (EISSN)
Structural characterisation of Ag2Nb4O11 and Ag2Ta4O11 powders has been carried out by Rietveld refinement of neutron powder diffraction data in the temperature range 10&-573 K, symmetry-adapted mode analysis and second-harmonic generation. Ag2Nb4O11 ceramics appear to be ferroelectric with a maximum permittivity of 150 at TC ≈ 400 K, frequency-independent permittivity below this temperature and a shoulder in the epsilon′ vs. temperature profile at 200 K. The space group changes from Rc 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, Rc. Comments on possible phase transition(s) of several compounds similar to Ag2(Nb,Ta)4O11 are made.