Structural characterization and Curie temperature determination of a sodium strontium niobate ferroelectric nanostructured powder Silvania Lanfredi a , Diego H.M. Gˆ enova a , Iara A.O. Brito b , Alan R.F. Lima c , Marcos A.L. Nobre a,n a Faculdade de Ciˆ encias e Tecnologia, FCT, Univ Estadual Paulista, UNESP, P.O Box 467, Presidente Prudente-SP, Brazil b Instituto de Biociˆ encias, Letras e Ciˆ encias Exatas, Univ Estadual Paulista. UNESP, P.O Box 6154, S ~ ao Jose´ do Rio Preto-SP, Brazil c Departamento de Quı ´mica, Univ Estadual de Ponta Grossa, UEPG, Ponta Grossa, PR, Brazil article info Article history: Received 28 October 2010 Received in revised form 2 February 2011 Accepted 2 March 2011 Available online 10 March 2011 Keywords: Nanostructured powder Rietveld method Infrared spectroscopy Impedance spectroscopy Ferroelectric Curie temperature abstract The Curie temperature and its correlation with the magnitude of the displacement of the niobium atom from the center of [NbO 6 ] octahedra in NaSr 2 Nb 5 O 15 nanostructured powder were investigated. A single powder was prepared by high-energy ball milling. A powder with an average crystallite size of 37 nm was prepared by calcining the precursor at 1423 K. The refinement of the structural parameters was carried out by the Rietveld method. NaSr 2 Nb 5 O 15 exhibits tetragonal symmetry with the tungsten bronze structure (a ¼b ¼12.3495 (6) ˚ A, c ¼3.8911 (2) ˚ A, V ¼593.432 (5) ˚ A 3 , and Z ¼2). The site occupancy of the Na þ and Sr 2 þ cations and the interatomic distances between the niobium and oxygen atoms were derived. The [NbO 6 ] octahedron undergoes both rotation and tilting depending on the crystallographic site. The Curie temperature of the powder was derived using both the impedance and infrared spectroscopy methods. & 2011 Elsevier Inc. All rights reserved. 1. Introduction Niobates with a tetragonal tungsten bronze (TTB)-type struc- ture are of great scientific, technical, and industrial interest as materials for laser modulation, frequency multiplicity, and the generation of second harmonics for applications in pyroelectric detectors and piezoelectric transducers [1]. Some polycation ferroelectric oxides are also important for microwave telecom- munications involving satellite broadcasting and related devices [2]. In some cases, these materials have the potential to replace members of the classic set of ferroelectric ceramics, such as Pb(Zr,Ti)O 3 (PZT), [(Pb(Mg 1/3 Nb 2/3 )O 3 ] (PMN), and [(Pb,La) (Zr,Ti)O 3 ] (PLZT) [3]. Furthermore, dielectric, thermistor, and chemical sensor properties can be expected. In the last few years, alkaline and alkaline earth niobates with TTB-type structures, such as KSr 2 Nb 5 O 15 , NaSr 2 Nb 5 O 15 , KBa 2 Nb 5 O 15 , NaBa 2 Nb 5 O 15 , and K 3 Li 2 Nb 5 O 15 , have been studied due to the high anisotropy of their crystalline structure [4]. The TTB structure can be consid- ered as a derivative of the classical perovskite structure. It can be described by the chemical formula (A1) 2 (A2) 4 C 4 Nb 10 O 30 . A1, A2, and C denote different oxygen sites in the crystal structure. The A1 cavities have cuboctahedral coordination, the A2 cavities have pentacapped pentagonal prismatic coordination, and the C cav- ities have tricapped trigonal prismatic coordination. The cavity size decreases in the following order: A2 4A1 4C. In TTB-type compounds, alkaline and/or alkaline-earth metals are located in the A1 and A2 sites, while only small cations like Li are found in the C site [5]. Taking into account the TTB-type structure, a wide variety of cation substitutions is possible. In a broad sense, TTB-type compounds with the A 6 Nb 10 O 30 formula, where A ¼ Sr, Ba, are semiconductor oxides containing Nb 5 þ cations. In a TTB- type structure, the coexistence of cations is favorable in both the tetragonal and pentagonal sites. In some of them, cation reparti- tion disorder has been correlated to relaxer behavior [6]. Among TTB-type oxides, the classical ferroelectric quaternary niobates are of particular relevance. The size and type of replacement ions at different sites of the structure and the degree of disorder have a significant effect on the dielectric properties. In particular, the Curie temperature (T C ) is influenced by these parameters. In fact, the value of T C depends on the octahedron distortion [7] because the T C can be changed by application of hydrostatic pressure, which gives rise to further octahedral distortion [8]. In addition, the crystallite size can affect the domain size, and the degree of cooperation between domains can modify the Curie temperature [9]. As a matter of fact, the method of preparation or synthesis and its macroscopic variables, such as the cooling rate and calcination temperature, can change structural properties such as the space group [10]. Alkali–metal Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry 0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2011.03.001 n Corresponding author. Fax: þ55 18 3221 5682. E-mail address: nobremal@fct.unesp.br (M.A. Nobre). Journal of Solid State Chemistry 184 (2011) 990–1000