Phase coexistence in NaNb (1x) Ta x O 3 materials with enhanced dielectric properties† Almudena Torres-Pardo, a Ricardo Jim enez, b Ester Garc  ıa-Gonz alez a and Jos e M. Gonz alez-Calbet * a Received 3rd April 2012, Accepted 21st May 2012 DOI: 10.1039/c2jm32078a NaNb (1x) Ta x O 3 (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 NaNbO 3 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 z c z O2a c , b z 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) Ta x O 3 (0.4 # x # 0.6) materials showing an enhanced dielectric response. 1 Introduction The large number of applications of ferroelectric perovskite (ABO 3 ) materials, particularly those derived from PbTiO 3 , has motivated a tremendous amount of research aimed at under- standing the correlation between composition, structure and their functional properties. 1–3 The control of the doping process on the A and/or B sites of the perovskite structure in PbTiO 3 - related systems has culminated in the improvement of the dielectric properties, the enhanced dielectric response being usually identified in those materials in which either the coexis- tence of two different phases 4–7 or the stabilization of a lower symmetry phase 8–11 is involved in a structural transition. Nevertheless, the increasing demand for environment-friendly functional materials has driven the research work to the synthesis of lead-free ferroelectric oxides, with compounds derived from BaTiO 3 and NaNbO 3 oxides being chosen for this purpose. 12–16 The phase diagram of NaNbO 3 shows the antiferroelectric phase (space group Pbma, a z c z O2a c , b z 4a c ) as the stable poly- morph at room temperature 17 which can coexist with a two-fold ferroelectric phase (space group P2 1 ma, a z c z O2a c , b z 2a c ) for temperatures in the approximate range of 393–533 K (ref. 18 and 19). The ferroelectric phase can be induced by applying an electric field 20 or through the formation of adequate solid solu- tions. Several works have succeeded in the stabilization of ferro- electric materials through the partial substitution of Na in the A sublattice 21–23 of the antiferroelectric NaNbO 3 . However, the tunability of the dielectric properties through the doping process in the B sublattice of NaNbO 3 oxide has aroused less attention in the literature. The substitution of Nb by the isovalent Ta cation has been reported to allow the stabilization of NaNb (1x) Ta x O 3 ferroelectric phases from a threshold composition x $ 0.55 (ref. 24 and 25) and thermal hysteresis of the dielectric constant has been observed for materials with the composition x # 0.50. 13 In these works, the information concerning the structural characterization of the different NaNb (1x) Ta x O 3 (0.1 # x # 0.9) materials is quite limited, with the electric characterization being the main moti- vation of these investigations. In the present work, we have carried out the synthesis and the structural characterization of NaNb (1x) Ta x O 3 oxides in the whole composition range. The stabilization of ferroelectric materials from a threshold dopant concentration of 40% has been established from the electric characterization performed. Additionally, the structural information obtained by combining X-ray diffraction and high resolution transmission electron microscopy has allowed us to unambiguously probe that the ferroelectric and the paraelectric phases coexist at room temperature in the 0.4 # x # 0.6 composition range. The phase coexistence seems to be related to the change from a first-order- like to a second order ferroelectric phase transition that provokes a Departamento de Quimica Inorganica, Facultad de Quimicas, Universidad Complutense Madrid, Madrid, 28040 Spain. E-mail: atorresp@quim.ucm. es; Tel: +34 91 3945234 b Instituto de Ciencia de Materiales de Madrid, CSIC Cantoblanco, Madrid, 28049 Spain † Electronic supplementary information (ESI) available: Average cationic composition of NaNb (1x) Ta x O 3 samples obtained by means of wavelength-dispersive X-ray spectroscopy. See DOI: 10.1039/c2jm32078a 14938 | J. Mater. Chem., 2012, 22, 14938–14943 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2012, 22, 14938 www.rsc.org/materials PAPER Downloaded by CSIC - Instituto Quimica Y Fisica on 13 July 2012 Published on 21 May 2012 on http://pubs.rsc.org | doi:10.1039/C2JM32078A View Online / Journal Homepage / Table of Contents for this issue