Phase coexistence in NaNb(1-x)TaxO3 materials with enhanced dielectric properties

NaNb(1-x)TaxO3 (0.1 <= x <= 0.9) materials have been synthesized and investigated by combining diffraction methods and high resolution electron microscopy with dielectric measurements. The stabilization of lead-free ferroelectric materials has been established from a threshold dopant composition of 40%, probing the doping process in the B-cationic sublattice of the perovskite structure to be a suitable approach for tuning the dielectric properties of the antiferroelectric NaNbO3 oxide. Rietveld analysis of the room temperature powder X-ray diffraction data for ferroelectric compositions has provided unambiguous evidence for the coexistence of two different orthorhombic phases with lattice parameters a approximate to c approximate to root 2a(c), b approximate to 2a(c) in the 0.4 <= x <= 0.6 composition range, a non-centrosymmetric ferroelectric phase (P2(1)ma) and a centrosymmetric paraelectric phase (Pcmn). The change in the order of the phase transitions from first order-like to second order seems to be related to the coexistence of ferroelectric and paraelectric phases, with the NaNb(1-x)TaxO3 (0.4 <= x <= 0.6) materials showing an enhanced dielectric response.