J ournal J. Am. Ceram. Soc., 89 [2] 737–739 (2006) DOI: 10.1111/j.1551-2916.2005.00757.x r 2005 The American Ceramic Society Chemical Synthesis of Nanocrystalline Strontium Bismuth Tantalate Powders Using Tantalum–Tartarate Complex Asit Baran Panda, Amita Pathak, and Panchanan Pramanik w Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal 721 302, India Nanocrystalline strontium bismuth tantalate (SrBi 2 Ta 2 O 9 ; SBT) powders have been synthesized through complete dehy- dration of an aqueous solution mixture of tantalum tartarate, strontium salt of ethylenediaminetetraacetic acid, and bismuth– triethanolamine complex. Single-step calcination of the fluffy, mesoporous, carbonaceous dehydrated precursor mass at 7001C/2 h results in nanosized SBT powders, with average par- ticle size B15 nm. When palletized and sintered at 9501C/4 h these powders show a relative density of 97.6% of theoretical value, and a maximum dielectric constant value of 1387 at T c (Curie temperature) 5 2791C, when measured at 100 kHz. I. Introduction I N recent years, strontium bismuth tantalates, i.e., SrBi 2 Ta 2 O 9 (SBT), have attracted wide attention as potential ferroelectric materials for nonvolatile ferroelectric random access memory (NVFeRAM) applications, 1 because of their excellent ferroelec- tric properties, anti-fatigue behavior, 2,3 and the possibility of using low-polarization switching voltage. SBT has a layer-type perovskite crystal structure with an orthorhombic lattice struc- ture where a unit cell 4,5 consists of a stack of alternating layers of (Bi 2 O 2 21 ) interleaved between two pseudo-perovskite layers of oxygen octahedra (SrTa 2 O 7 ) 2À along the c-axis. Extensive studies have been reported on the preparation of SBT thin films through various deposition techniques for the NVFeRAM application. 6–10 But there is limited literature on the preparation and characterization of SBT powders. 3,11–14 The properties of the ferroelectric ceramic materials are great- ly affected by their powder characteristics, such as particle size, morphology, purity, chemical composition, and homogeneity. The ‘‘wet’’ or chemical processes ensure chemical homogeneity and stoichiometric control through molecular-level mixing of the starting materials in solution, 15,16 and for this reason, they are considered to be appropriate methods for their preparation. However, the major drawback in the preparation of SBT pow- ders through the reported chemical processes is the use of metal alkoxide (sol–gel) 9 /metal halides (colloid emulsion process). 13 These reagents are not only expensive and scarcely available, but are also very moisture sensitive. At the same time, the alternative aqueous-based chemical processes get complicated because of the scarcity of stable, wa- ter-soluble salts of the Vb group metals. However, the use of water-soluble coordinated complexes of the Vb group metal ions has recently been reported as the perfect starting materials for the preparation of these types of ceramic compositions. 17,18 These water-soluble coordinated complexes are stable, do not undergo hydrolysis in an aqueous medium, and can be prepared using cheap, easily available inorganic salts. In the present paper, we report the preparation of nanocrys- talline powders of SBT through an aqueous-based chemical process using tantalum tartarate as the tantalum source. II. Experimental Procedure (1) Materials and Reagents Tantalum pentoxide (Ta 2 O 5 ) (Aldrich, 99.99%, Aldrich Chemi- cals, Milwaukee, WI), ammonium tartarate (Loba Chemicals Ltd., Mumbai, India), hydrofluoric acid (HF), hydrogen perox- ide (H 2 O 2 ), strontium nitrate (Sr(NO 3 ) 2 ), bismuth nitrate (Bi(NO 3 ) 3 ), triethanolamine (TEA) (E. Merck, Mumbai, India), and ammonium hydroxide (NH 4 OH) (Quest, Kolkata, India) are the raw materials required for the preparation. (A) Preparation of Aqueous Solution of Tantalum Tarta- rate: The aqueous solution of tantalum–tartarate complex was prepared in the laboratory starting from its hydrous oxide. At first, a clear solution of the tantalum–fluoride complex (i.e., TaOF 5 2À or TaF 7 2À complexes) was obtained by dissolving tan- talum oxide (Ta 2 O 5 ) (1M, 499%) in HF (47M) by heating the mixture over a water bath for 28–30 h. The hydrous tantalum oxide (Ta 2 O 5 Á nH 2 O) was then precipitated from this solution by adding dilute ammonia in the presence of a freshly prepared aqueous solution of ammonium oxalate (42M). The precipitate was filtered, washed with 5% ammonia solution to make it flu- oride free, and then assayed at 10001C/2 h to estimate the tan- talum oxide. A stock of the aqueous solution of tantalum tartarate was then obtained by dissolving desired amounts of Ta 2 O 5 Á nH 2 O in tartaric acid (42M per unit mol of Ta 51 ion) through contin- uous stirring and heating of the mixture over a water-bath for 38–40 h. Normally, the hydrous tantalum oxides are insoluble in tartaric acid because of the formation of polymeric polynuclear chains, but the presence of ammonium oxalate in the tantalum fluoride solution before their precipitation served to increase their solubility in tartaric acid. The complexing tendency of oxalate ions led them to occupy the coordinating site of the tantalum ions during the neutralization process, 19 and this trun- cated the formation of polynuclear chains by reducing the number of hydroxy bridges. Furthermore, it was observed that the addition of catalytic amounts (2–5 mol%) of hydrogen per- oxide to the aqueous solution of tartaric acid enhanced the rate of solubility of the hydrous tantalum oxide by about tenfold through the formation of soluble yellowish peroxo complexes. 20 Consequentially, the formation of the tantalum–tartarate com- plex in the presence of H 2 O 2 was accomplished without heating and with stirring time reduced to 5–6 h. (B) Preparation of Aqueous Solutions of Sr-EDTA and Bi- TEA Complexes: Stocks of aqueous solutions of Sr-EDTA and Bi-TEA complexes were separately prepared by adding an aqueous ammoniacal solution of EDTA (1M) to a strontium 737 L. Klein—contributing editor The authors are grateful for the financial assistance provided by CSIR, Government of India, for carrying out this study. w Author to whom correspondence should be addressed. e-mail: ami@chem.iitkgp. ernet.in Manuscript No. 186723. Received September 17, 2002; approved May 26, 2005.