Polymorphism, structural characterisation and electrical properties of Na2Nb4O11

Abstract

Structural characterisation of Na₂Nb₄O₁₁ powder has been carried out by Rietveld refinement of neutron powder diffraction data in the temperature range 10–573 K and measurements of second harmonic generation in the range 75 to 450 K. The space group changes from Rc to C2/c on cooling through a phase transition at ∼380 K. The origin of the phase transition in Na₂Nb₄O₁₁ is mainly related to distortion of some Nb(1)O₇ pentagonal bipyramids within the equatorial plane whereas other Nb(2)O₇ pentagonal bipyramids become almost regular. Although Na₂Nb₄O₁₁ ceramics show a maximum permittivity of ∼150 at ∼380 K and frequency-independent permittivity below this temperature, the room temperature polymorph is not ferroelectric. Comparison of the high and room temperature structures shows clear evidence of antiparallel atomic displacements in the room temperature structure and, therefore, a tendency towards low temperature antiferroelectric behaviour. A comparison is also made with the structurally-related ferroelectric Ag₂Nb₄O₁₁.