Nanocrystalline BaTiO 3 from freeze-dried nitrate solutions J. M. McHale, a) P. C. McIntyre, K. E. Sickafus, and N. V. Coppa Los Alamos National Laboratory, Los Alamos, New Mexico 85745 (Received 12 April 1995; accepted 16 January 1996) An aqueous, all nitrate, solution-based preparation of BaTiO 3 is reported here. Rapid freezing of a barium and titanyl nitrate solution, followed by low temperature sublimitation of the solvent, yielded a freeze-dried nitrate precursor which was thermally processed to produce BaTiO 3 . XRD revealed that 10 min at temperatures >600 ± C resulted in the formation of phase pure nanocrystalline BaTiO 3 . TEM revealed that the material was uniform and nanocrystalline (10 –15 nm). The high surface to volume ratio inherent in these small particles stabilized the cubic phase of BaTiO 3 at room temperature. It was also found that the average particle size of the BaTiO 3 produced was highly dependent upon calcination temperature and only slightly dependent upon annealing time. This result suggests a means of selection of particle size of the product through judicious choice of calcination temperature. The experimental details of the freeze-dried precursor preparation, thermal processing of the precursor, product formation, and product morphology are discussed. I. INTRODUCTION The wide range of electronic applications utilizing the high dielectric constant and ferroelectricity exhibited by BaTiO 3 , in conjunction with the difficulty in preparing morphologically homogeneous samples of the mate- rial by conventional ceramic processing, have resulted in the development of several solution-based methods for BaTiO 3 preparation. Solution-based routes to bar- ium titanate include coprecipitation of oxalates, 1,2 the Pechini process, 3 sol gel methods, 4–6 and the hydrother- mal method. 7,8 These processes have the common goal of achieving product formation under mild reaction con- ditions (low temperatures and short reaction times) in order to limit the extent of grain growth and control particle size. Small and uniform particle size BaTiO 3 powders allow for thinner layers of the ceramic to be used in multilayer capacitors without loss of dielec- tric properties. In addition, small and uniform particle morphology offers the advantage of lower sintering tem- peratures for such devices, which may allow for the use of less expensive electrode materials. 9 The permittivity and Curie temperature of BaTiO 3 have also been shown to be dependent upon sample morphology. 10,11 However, control of grain size is virtually impossible using conven- tional ceramic processing due to the high temperatures needed to facilitate ionic diffusion and reaction in the solid state. The low temperature synthesis of BaTiO 3 has been demonstrated using alkoxide precursors. 7,8,12 a) Present address: Princeton University, Department of Geological and Geophysical Sciences, Princeton, New Jersey 08544. These results suggest that other low temperature routes to BaTiO 3 synthesis may exist. It has been demon- strated that atomic level mixtures of nitrate salts, formed through a freeze-drying process, are highly reactive pre- cursors that yield ceramic products at low temperatures and pressures. 13,14 Here we apply this technique to the synthesis of BaTiO 3 . A complicating factor in the preparation of BaTiO 3 by solution methods is the commercial unavailability of a water soluble, easily handled titanium salt. The commercially available titanium tetrahalides, TiX 4 (X  F, Cl, Br, I), which form water soluble titanyl halides TiOX 2 , fume violently upon their addition to H 2 O and profusely fume even when exposed to slightly moist air. Complicating matters further is the chemical inactivity of strongly roasted TiO 2 (rutile or anatase). After igni- tion at 1000 ± C (a common procedure in commercially available samples of TiO 2 ), the oxide is practically insoluble in sulfuric, hydrochloric, or nitric acid. 15 How- ever, freshly precipitated titanium dioxide, or hydrous titania, 16,17 TiO 2 ? xH 2 O, obtained upon hydrolysis of Ti 41 salts (TiCl 4 , Ti[OCH(CH 3 ) 2 ] 4 , etc.), is readily solu- ble in mineral acids and can be used to prepare aqueous solutions of titanyl nitrate, TiO(NO 3 ) 2 . 18 Titanyl nitrate is an ideal precursor material for BaTiO 3 preparation due to its ease of handling, low de- composition temperature, and noncarbonaceous nature. Carbonaceous precursors (oxalates, etc.), upon decompo- sition, invariably result in the formation of intermediate BaCO 3 . Barium carbonate is a relatively stable phase that requires temperatures in excess of 800 ± C for complete decomposition. 2,19 This stability is often responsible for lower reaction rates. 20 In addition, carbonaceous pre- J. Mater. Res., Vol. 11, No. 5, May 1996  1996 Materials Research Society 1199 Help Comments Welcome Journal of MATERIALS RESEARCH