Dielectric properties, polymorphism, structural characterisation and phase diagram of Na2Nb4O11–Ag2Nb4O11 solid solutions

•The polymorphism in the solid solution (Na1−xAgx)2Nb4O11 has been studied.•NaAgNb4O11 transforms from R  3¯c to P21/n at ~340 K and P  1¯ at ~270 K.•Na1.50Ag0.50Nb4O11 transforms from R  3¯c to C2/c at ~380 K.•Na0.25Ag1.75Nb4O11 transforms from R  3¯c to R3c at ~360 K and R3 at ~210 K.•A revised phase diagram for the system Na2Nb4O11–Ag2Nb4O11 is proposed.

The cross-over between ferroelectric Ag2Nb4O11 and anti-ferroelectric Na2Nb4O11 has been characterised by fixed frequency permittivity data and Rietveld refinement of X-ray and neutron powder diffraction data in the temperature range 10–450 K of (Na1−xAgx)2Nb4O11 ceramics. The permittivity passes through a maximum between ~90 and 160 over the temperature range 330 to 400 K that depends on composition and is frequency-independent below this temperature. Ceramics with composition x≿0.50 also show a shoulder in the ε′ vs temperature profile at ~200–275 K. Structural characterisation of powders with x=0.25, 0.50 and 0.875 show that, on cooling, NaAgNb4O11 (x=0.50) transforms from R  3¯c to P21/n at ~340 K with a further transition from P21/n to P  1¯ below ~270 K; Na1.50Ag0.50Nb4O11 (x=0.25) transforms from R  3¯c to C2/c at ~380 K; Na0.25Ag1.75Nb4O11 (x=0.875) transforms from R  3¯c to R3c at ~360 K and R3 at ~210 K. A revised phase diagram for the system Na2Nb4O11–Ag2Nb4O11 is proposed.

Graphical abstractWorking phase diagram for the system Na2Nb4O11–Ag2Nb4O11.Figure optionsDownload full-size imageDownload as PowerPoint slide