Crystal growth and dielectric characterization of crystals derived from the solid-solution Ba (1x) Na x Ti (1x) Nb x O 3 (BTNN) Lakhdar Gacem a,b , Djamel Ouadjaout c , Jean-Pierre Chaminade a, *, Mario Maglione a , Re ´ gnault Von der Mu ¨ hll a , Stanislas Pechev a a ICMCB-CNRS, Universite ´ de Bordeaux, 87 av. Schweitzer, 33608 Pessac, France b De ´partement de physique, Universite ´ Mohamed Khider de Biskra, Biskra 07000, Algeria c UDTS, 2 Bd. Frantz Fanon, BP. 140 Alger-7 Merveilles, 16200 Alger, Algeria 1. Introduction In recent years, much attention is devoted to lead-free ferroelectric materials in both for basic research and for applica- tions [1–3]. Renewed interest on lead-free compounds like BaTiO 3 and NaNbO 3 that are respectively ferroelectric and/or antiferro- electric at room temperature, has been stimulated [4–8]. The solid solution (1  x)BaTiO 3 + xNaNbO 3 (BTNN) in the form of ceramics has been investigated in our laboratory [9–11]. A complete solid solution of perovskite type has been identified. Three ranges have been evidenced as a function of x: - 0  x < 0.075, close to BaTiO 3 , we observe a classical ferroelectric behaviour with a decrease in the Curie temperature (T c ) from 400 to 250 K as x increases from 0 to 0.075; - for 0.075  x  0.55 ceramics show relaxor properties with a minimum of T c at 150 K for x near 0.30; - when x > 0.55 for NaNbO 3 -rich compositions, ceramics show classical ferroelectric behaviour with an increase in Curie temperature T c towards 600 K near the one of NaNbO 3 . To fully understand the structural origin of such dielectric properties, single crystals are required. We have grown single crystals of BTNN for high value of x and investigated their dielectric properties. We targeted such compositions for the two following reasons: high content of Nb decreases the melting temperature and ‘‘a priori’’ favours crystal growth, high content of Nb in the BTNN solid solution involves a greater distortion of (Nb–Ti)O 6 octahedra and consequently an increase of T c , which is likely to stabilize the room temperature polarization and to increase the piezoelectric performances. 2. Experimental and results 2.1. Crystal growth The starting compositions were obtained from BaCO 3 , TiO 2 , Na 2 CO 3 , Nb 2 O 5 powders of high purity (>99.9%) by solid state reactions using the following scheme: ð1  xÞBaCO 3 þð1  xÞTiO 2 þ x=2Na 2 CO 3 þ x=2Nb 2 O 5 ! Ba ð1xÞ Na x Ti ð1xÞ Nb x O 3 þð1  x=2ÞCO 2 " The polycrystalline materials were ground and mixed with Na 2 B 4 O 7 as solvent and introduced in a Pt crucible for the growing Materials Research Bulletin 44 (2009) 2240–2245 ARTICLE INFO Article history: Received 5 June 2009 Received in revised form 22 July 2009 Accepted 27 July 2009 Available online 5 August 2009 Keywords: Oxides Crystal growth Crystal structure Dielectric properties Ferroelectricity Piezoelectricity Phase transition ABSTRACT Single crystals of (1  x)BaTiO 3 + xNaNbO 3 (BTNN) for x = 0.84 were obtained by high temperature solution growth using Na 2 B 4 O 7 as solvent. The room temperature crystal structure of BTNN 16/84-phase was determined from X-ray single crystal diffraction data, in the tetragonal system with space group P4bm. The refinement from 246 independent reflections led to the following parameters: a = b = 5.5845(3) A ˚ , c = 3.9453(2) A ˚ , V = 123.041(11) A ˚ 3 , Z = 2, with final cR wp = 0.150 and R B = 0.041. The structure of BTNN 16/84-phase can be described as a three-dimensional framework built up from (Nb–Ti)O 6 octahedra with Na and Ba in the dodecahedral site of perovskite-like type. Some mm 3 -sized crystals have been selected and various dielectric measurements (ferroelectric, pyroelectric, and piezoelectric) have been performed. Transition from paraelectric to ferroelectric state at around 460 K has been observed to be in good agreement with ceramics of closer composition. Dielectric, piezoelectric and pyroelectric measurements on crystal confirm the ferroelectric behaviour of BTNN 16/84. ß 2009 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +33 540006265; fax: +33 540002761. E-mail address: chamin@icmcb-bordeaux.cnrs.fr (J.-P. Chaminade). Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.07.019