Electrical and pyroelectric properties of lanthanum based niobate R. Padhee, Piyush R. Das n , B.N. Parida, R.N.P. Choudhary Department of Physics, Institute of Technical Education and Research, Siksha ‘O’ Anusandhan University, Jagamohan Nagar, Jagamara, Khandagiri, Bhubaneswar 751030, India article info Article history: Received 25 June 2012 Received in revised form 27 September 2012 Accepted 24 October 2012 Available online 2 November 2012 Keywords: A. Ceramics C. Thermogravimetric analysis C. X-ray diffraction D. Electrical properties D. Ferroelectricity abstract A lanthanum based new tungsten bronze (TB) ferroelectrics (K 2 Pb 2 La 2 W 2 Ti 4 Nb 4 O 30 ) was synthesized by a mixed-oxide method at high temperature. Thermogravimetry analysis (TG) technique was used to decide the material preparation conditions. The formation of desired compound was confirmed by preliminary X-ray structural analysis. The SEM micrograph of the sintered sample exhibits uniform rod- like grain distribution without many voids. Detailed studies of the nature of variation of dielectric parameters with temperature and frequency shows dielectric anomaly at 310 1C. The temperature dependence of electrical parameters (impedance, modulus, conductivity, etc) of the material exhibits a strong correlation between its micro-structure (i.e., bulk, grain boundary, etc) and electrical properties. The dc conductivity follows the Arrhenius equation, and thus its variation with rise in temperature reveals the negative temperature coefficient of resistance (NTCR) behavior of the material. The material obeys Jonscher’s universal power law which is evident from the frequency dependence of ac conductivity. The variation of current with temperature shows that the material has high pyroelectric co-efficient and figure of merit. Hence the material is useful for pyroelectric sensors. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Tungsten bronze (TB) compounds belong to an important family of dielectric materials, which display interesting ferro- electric, piezoelectric, pyroelectric and non-linear optical beha- vior because of which they have attracted much attention of the scientific community and find use in wide ranging industrial applications such as transducers, multi-layered capacitors, micro- wave dielectric resonators, pyro-electric detectors, actuators, etc [1–5]. As TB structure compounds usually have moderate dielec- tric constant and low tangent loss, they are suitable for many device applications. The complex and disorder TB structure is derived from the simple perovskite structure of ABO 3 type. Thus TB structure has arrays of distorted BO 6 octahedral sharing corners in such a way that three different types of interstices (A, B and C) can be utilized for designing and developing a large number of new compounds of a general formula [(A 1 ) 2 (A 2 ) 4 (C) 4 ] [(B 1 ) 2 (B 2 ) 8 ]O 30 or [(A 1 ) 2 (A 2 ) 2 (A 3 ) 2 (C) 4 ][(B 1 ) 2 (B 2 ) 4 (B 3 ) 4 ]O 30 . In this formula A 1 and A 2 (also A 3 ) sites are usually occupied by mono- trivalent cations, B 1 and B 2 (also B 3 ) sites by tetra-hexavalent (i.e., Ti þ 4 , Nb þ 5 , Ta þ 5 ,V þ 5 and W þ 6 ) ions, and C site (being smaller) often remains empty. Because of the complexity of the above formula it is possible to make different types of ionic substitu- tions at the above mentioned sites to tailor the physical proper- ties of the materials for different applications. Detailed literature survey on TB structure compounds reveals that lot of work on synthesis and characterization of large number of ferroelectric oxides of this family (with different chemical compositions) have been investigated and reported in the past. Ferroelectric phase transitions in some rare-earth ions containing TB compounds have been reported by several research groups [1–5]. We were the first to report structural and ferroelectric phase transition in some complex compounds having mono-divalent and rare earth ions substituted at the A site; and tetra-, penta- and hexa-valent ions substituted at the B site [6–11]. Recently, some other groups have reported interesting properties in niobium based TB compounds such as Ba 5 NdTi 3 Nb 7 O 30 [12], Ba 5 SmTi 3 Nb 7 O 30 [13], Ba 5 SmTi 3 Nb 7 O 30 [14], Pb 2 Na 0.8 R 0.2 Nb 4.8- Fe 0.2 O 15 (R ¼ Dy, Eu, Sm, Nd, La) [15]. Very recently we also reported ferroelectric properties in K 2 Pb 2 Dy 2 W 2 Ti 4 Nb 4 O 30 [16] and Li 2 Pb 2 Pr 2 W 2 Ti 4 Nb 4 O 30 [17]. Pyroelectricity has been reported in the famous barium sodium niobate abbreviated as BNN (or Banana), Pb x Ba 1x Nb 2 O 6 and Ba 1x Sr x Nb 2 O 6 [18]. In order to search for new materials for device applications, we have prepared a new complex TB structured niobate: K 2 Pb 2 La 2 W 2 Ti 4 Nb 4 O 30 (KLN) and studied its structural, dielectric, electrical and pyroelectric characteristics. The chemical formula of the proposed compound is well fitted with the above general formula of TB structure where A 1 ¼ K, A 2 ¼ Pb, A 3 ¼ La, B 1 ¼ W, B 2 ¼ Ti, B 3 ¼ Nb and C is empty. 2. Experimental The polycrystalline sample of K 2 Pb 2 La 2 W 2 Ti 4 Nb 4 O 30 (KLN) was prepared by a high-temperature solid-state reaction method with Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jpcs Journal of Physics and Chemistry of Solids 0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.10.017 n Corresponding author. Tel.: þ91 9438047597; fax: þ91 674 2351217. E-mail addresses: prdas63@yahoo.com, prdas63@gmail.com (P.R. Das). Journal of Physics and Chemistry of Solids 74 (2013) 377–385