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

Abstract

NaNb_(1−x)Ta_xO₃ (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 ≈ c ≈ √2a_c, b ≈ 2a_c in the 0.4 ≤ x ≤ 0.6 composition range, a non-centrosymmetric ferroelectric phase (P2₁ma) 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)Ta_xO₃ (0.4 ≤ x ≤ 0.6) materials showing an enhanced dielectric response.