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Synthesis of textured Bi5Ti3FeO15 and LaBi4Ti3FeO15 ferroelectric layered Aurivillius phases by molten-salt flux methods
Materials Research Bulletin 41 (2006) 1513–1519 www.elsevier.com/locate/matresbu
Synthesis of textured Bi5Ti3FeO15 and LaBi4Ti3FeO15 ferroelectric layered Aurivillius phases by molten-salt flux methods Digamber G. Porob, Paul A. Maggard * Department of Chemistry, North Carolina State University, Raleigh, NC 27695, USA Received 23 September 2005; received in revised form 13 January 2006; accepted 18 January 2006 Available online 13 February 2006
Abstract The ferroelectric layered Bi5Ti3FeO15 and LaBi4Ti3FeO15 Aurivillius phases were synthesized in high purity and textured microstructures in a molten Na2SO4/K2SO4 (1:1 molar ratio) flux in much shortened reaction times, 1 h minimum compared to conventional techniques. The particle growth and microstructure of both phases were investigated as a function of temperature and reaction duration, and yielded plate-like particles that could be synthesized in sizes from <1 mm to >20 mm. The product crystallinity, purity and microstructures were characterized via powder X-ray diffraction and scanning electron microscopy. The UV–vis diffuse reflectance of the products were measured and analyzed with respect to the resultant particle sizes. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; A. Oxides; C. Electron microscopy; C. X-ray diffraction; D. Microstructure
1. Introduction Solid state materials in which long range magnetic (ferro/antiferromagnetic) and electric (ferroelectric) ordering coexists [1–4], termed ‘magnetoelectrics’, have recently become the focus of much research. This class of materials exhibits a spontaneous magnetization/polarization that can be switched by an applied magnetic/electric field, respectively, and often coupling between the two [5,6]. These solids hold promise as a new generation of electronic devices, owing to the possibility of applications such as combined information storage/logic operations, multiple-state memory devices, electric-field-controlled ferromagnetic resonance devices, and transducers with magnetically modulated piezoelectricity [7]. Among magnetoelectric solids, the Aurivillius series of compounds, e.g. Bi4Ti3O12nBiMO3 (M = Fe, Mn; n = 1, 2) or Bi4Ti3O12nLaFeO3, have been among some of the most heavily investigated for their magnetic and electronic properties [8–18]. In the former, the structure is comprised of n perovskite layers of magnetic BiMO3 (M = Fe, Mn) that have been inserted into the three-layer [Bi2Ti3O10]2 perovskite slab of the parent Bi4Ti3O12 ferroelectric [11]. The resultant perovskite sheets are 3 + n layers in thickness, with disordered Ti4+/Fe3+ (75%:25% ratio for n = 1) on the M sites, and these are in turn separated by fluorite-like [Bi2O2]2+ layers [18]. The related Bi4Ti3O12LaFeO3 phase is similarly constructed, but contains mixed Bi3+/La3+ cations in the perovskite sheet [11].
* Corresponding author. Tel.: +1 919 515 3616; fax: +1 919 515 5079. E-mail address: [email protected] (P.A. Maggard). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.01.020
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Despite the growing number of reports on their properties, all current syntheses of these magnetoelectric Aurivillius phases are based on conventional high-temperature ‘grind and heat’ solid-state techniques, which often leads to compositional and structural inhomogeneities owing to the high synthesis temperatures (>1000 8C), inadequate mixing, and long reaction times (>2–4 days). This is especially the case for solid-state reactions with relatively volatile bismuth oxides, and as well, for solids with different intergrowths that are sensitive to small compositional changes. As an alternate approach, molten-salt (or flux) synthetic methods have shown the ability to achieve improved homogeneity, purity and control over particle sizes, and has been found a suitable method of choice to synthesize pure homogeneous powders, or large single crystals, of Bi4Ti3O12 [19], ferrites [20], PZT [21], Bi2WO6 [22] and PbNb2O6 [23], for example. Some of the more common fluxes used in the syntheses of these oxides include mixtures of Na2SO4/ K2SO4, NaCl/KCl or Na2SO4/Li2SO4. Moreover, metal-oxide powders with anisotropic particle morphologies [24], i.e. textured microstructures, can be prepared by this technique and used as starting materials for grain-oriented ceramics. Compared to ceramics with a randomly oriented grain structure, those with textured microstructures and an appropriate alignment of the polar axes down the poling direction can exhibit maximal polarization responses [25,26], such as for materials with non-cubic symmetries and plate-like morphologies. It is further desirable to obtain these ceramics with good compositional and structural homogeneity for physical property reliability, and which might impose serious limitations on their study and application. In this paper, we report one of the first investigations into the molten-salt syntheses of the magnetoelectric Bi5Ti3FeO15 and LaBi4Ti3FeO15 layered Aurivillius solids, and the effects of synthesis temperatures and reaction times on the particle microstructures and sizes. Platelet-like morphologies with controlled particle sizes could be obtained for both solids that are homogeneous and in high purity. Further, the first diffuse reflectance measurements for these solids are reported herein and are analyzed in relation to the particle sizes. 2. Experimental Molten-salt syntheses of Bi5Ti3FeO15 and LaBi4Ti3FeO15 were performed by combining stoichiometric mixtures of AR grade Bi2O3, TiO2, Fe2O3 and La2O3 (preheated to 850 8C) to a eutectic mixture of Na2SO4/K2SO4 salts (1:1 molar ratio). The molar ratio of Bi5Ti3FeO15 or LaBi4Ti3FeO15 to the flux mixture was 1:7, which was excess in flux. The reactant mixtures were placed inside an alumina crucible and heated at temperatures ranging from 800 to 1000 8C for 1–20 h. Finally, the resulting powder was washed several times with hot deionized water to remove the alkali metal salts. Synthetic attempts at using a NaCl/KCl molten flux, in place of the equivalent sulfates, resulted in a detectable amount of corrosion to the alumina crucible. High-resolution powder X-ray diffraction (PXRD) spectra of the products ˚ ) radiation from a sealed tube X-ray generator were collected on an INEL diffractometer using Cu Kal (l = 1.54056 A
Fig. 1. The PXRD patterns of Bi5Ti3FeO15 synthesized at different reaction temperatures and times: (a) 800 8C for 1 h, (b) 900 8C for 1 h, (c) 1000 8C for 1 h, (d) 1000 8C for 2 h, (e) 1000 8C for 5 h, (f) 1000 8C for 20 h, and (g) the theoretical pattern of Bi5Ti3FeO15.
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(35 kV, 30 mA) in transmission mode and using a curved position sensitive detector (CPS120). Scanning electron microscopy (SEM) on a JEOL JEM 6300 was performed to examine the particle microstructures and sizes of the reaction products and energy dispersive X-ray (EDX) analyses were used to verify the elemental compositions. The UV–vis diffuse reflectance spectra were taken on a CARY 3E spectrophotometer equipped with an integrating sphere. 3. Results and discussion The powder X-ray diffraction (PXRD) patterns of the Bi5Ti3FeO15-loaded flux products, Fig. 1, shows that the targeted solid was prepared in high purity and good crystallinity in as little as 1 h at 800–1000 8C. This compares with the conventional solid-state reaction that requires 48 h at 1000 8C, or other similar multistep grind and reheat
Fig. 2. SEM micrographs of the Bi5Ti3FeO15 products synthesized at (a) 900 8C for 1h, (b) 1000 8C for 1 h, (c) 1000 8C for 2 h, (d) 1000 8C for 5 h, and (e) 1000 8C for 20 h.
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procedures [18,27,28]. Significant peak broadening is observed at 800 8C in the PXRD pattern, in particular for the [0 0 l] reflections, indicating relatively thin (<500 nm) Bi5Ti3FeO15 particle sizes. As the synthesis temperature is increased from 800 to 900 8C the peak widths become narrower, and further increases in the reaction temperature to 1000 8C, and longer reaction times, gave no further narrowing of the peak widths. The narrow peak widths at higher temperatures indicate good crystallinity even after only a 1 h reaction period. The evolution of microstructure and particle sizes in the Bi5Ti3FeO15 products was investigated using SEM, shown in Fig. 2. All micrographs revealed a distinct plate-like morphology of Bi5Ti3FeO15, typical of layered compounds belonging to the Aurivillius phases. The samples synthesized at 900 8C show the formation of aggregates of small plate-like particles, along with a few discrete platelets, and a particle size that varied approximately between 1 and 5 mm. The individual platelets were an order of magnitude thinner, corresponding to the c-axis direction of the structure that is perpendicular to the stacked perovskite layers. Samples synthesized at 1000 8C for 1 h exhibited a further increase in particle sizes with a large number of distinct platelets that are clearly visible in a size range of 5– 10 mm, as well as a few randomly distributed platelets as large as 10–15 mm. Next, the effect of the reaction duration on the particle morphologies and sizes was studied by holding the reaction at 1000 8C for 2, 5 and 20 h, shown in Fig. 2(c)–(e). At reaction times greater than 2 h, the particles transformed to discrete platelets and increasingly grew in size at longer reaction times. The particle dimensions reached much larger than 10–20 mm after a reaction time of 20 h. An energy-dispersive X-ray (EDX) analysis was performed (Section 5) and did not reveal any detectable amounts of alkali metals from the flux in the products. These observations confirm that phase purity and homogeneity of Bi5Ti3FeO15 particles can be achieved rapidly in the alkali metal sulfate flux, and that the particle growth can be controlled through an appropriate selection of the reaction temperature and reaction time. The molten-salt synthesis of the related LaBi4Ti3FeO15 Aurivillius phase was optimal under slightly different reaction conditions as compared to Bi5Ti3FeO15. The PXRD, shown in Fig. 3, revealed that the products obtained at 800 8C for a reaction time of 1 h contained a minor amount of LaFeO3 impurity along with the major phase of LaBi4Ti3FeO15. The diffraction peaks showed significant broadening, and indicated a small crystallite size within the product, similar to that found for Bi5Ti3FeO15. Raising the temperature to 900 8C at the same reaction time of 1 h gave a phase pure product with no detectable LaFeO3. Sharper diffraction peaks are observed at 900 8C and higher reaction temperatures, compared to that found at 800 8C, and demonstrates that good crystallinity and purity is obtainable in as short a reaction time as 1 h. The SEM micrographs of the LaBi4Ti3FeO15 products, shown in Fig. 4, reveal a plate-like morphology similar to that for Bi5Ti3FeO15. The products obtained at 900 8C show the formation of aggregates of small particles that are individually sized at 1–3 mm. At a higher reaction temperature of 1000 8C, the particle sizes increase and a small number of larger discrete platelets, 5–10 mm, are visible. As before, the synthesis temperature was held constant at 1000 8C, and the
Fig. 3. The PXRD patterns of LaBi4Ti3FeO15 synthesized at different reaction temperatures and times: (a) 800 8C for 1 h, (b) 900 8C for 1 h, (c) 1000 8C for 1 h, (d) 1000 8C for 2 h, (e) 1000 8C for 5 h, (f) 1000 8C for 20 h, and (g) the theoretical pattern of LaBi4Ti3FeO15.
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Fig. 4. SEM micrographs of the LaBi4Ti3FeO15 products synthesized at (a) 900 8C for 1 h, (b) 1000 8C for 1 h, (c) 1000 8C for 2 h, (d) 1000 8C for 5 h, and (e) 1000 8C for 20 h.
reaction durations were increased from 2, 5 to 20 h. The micrographs show that as the reaction time increases the percentage of discrete plate-like particles increases and which paralleled an increase in their sizes. For the LaBi4Ti3FeO15 products synthesized at 1000 8C for 20 h, all particles had transformed to discrete platelets and particle sizes of approximately 10–20 mm were observed. No detectable amounts of alkali metal from the flux were found in the products by EDX analyses. A comparison of the LaBi4Ti3FeO15 and Bi5Ti3FeO15 products, Figs. 2 and 4, reveals that under identical reaction conditions the particles of the latter were 2 larger and crystallized more quickly. Molten-salt syntheses of pure LaBi4Ti3FeO15 was possible at the slightly higher temperature 900 8C, while increasing reaction times and temperatures had slightly smaller effects on the particle growth processes. The UV–vis diffuse reflectance spectra (DRS) were taken for each sample in order to probe the relationship between particle sizes and percent reflectance (%R), shown in Fig. 5(a) and (b) for Bi5Ti3FeO15 and LaBi4Ti3FeO15, respectively. For weakly absorbing solids, the samples with the smallest particle sizes (i.e. highest scattering
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Fig. 5. The UV–vis diffuse reflectance spectra for (a) Bi5Ti3FeO15 or (b) LaBi4Ti3FeO15 synthesized at (i) 900 8C for 1 h, (ii) 1000 8C for 1 h, (iii) 1000 8C for 2 h, (iv) 1000 8C for 5 h, and (v) 1000 8C for 20 h.
coefficients) are expected to exhibit the maximum %R, whereas samples with larger particle sizes should show a smaller %R [29]. Decreasing particle sizes increases scattering and decreases light penetration, and which enhances the diffuse reflectance while decreasing the optical absorption. In the case of Bi5Ti3FeO15, the maximum %R was exhibited by samples (i) and (ii) with the short reaction time of 1 h at 900 and 1000 8C, with both samples exhibiting a similar %R. However, samples prepared at 1000 8C with increasing time (ii, 1 h; iii, 2 h; iv/v, 5 h/20 h) showed a clear relationship between decreasing %R and increasing particle size, with a minimum in the %R found after the reactions at 5 h (iv) and 20 h (v). For the LaBi4Ti3FeO15 phase, Fig. 5b, the maximum %R is shown by the sample with the smallest particle sizes prepared at 900 8C for 1 h. For increasing reaction times at 1000 8C (ii, iii, iv, and v), the reaction times of 1, 2, and 5 h had the next lowest values for %R, but not in the expected order of increasing reaction duration. This is likely owing to the slower reaction kinetics for the LaBi4Ti3FeO15 phase, and which manifests smaller changes in particle sizes over time so that the products are more difficult to distinguish based on DRS measurements. However, the minimum in %R is found for the largest sized particles that resulted from the flux preparation at 1000 8C for 20 h. Both samples show roughly the expected trend between particle sizes and %R, though over a much greater range in the case of Bi5Ti3FeO15. The deviations from the expected trend results from the fact that the variations in particle sizes are fairly small, i.e. from 1 mm to over 10 mm, and also that a distribution of smaller and larger particles exists in every sample. These results indicate the DRS measurements could be an appropriate diagnostic tool in the future to evaluate the particle growth of other Aurivillius phases. 4. Conclusions A molten-salt flux synthesis, using a Na2SO4/K2SO4 mixture, was used to prepare high purity samples of the Bi5Ti3FeO15 and LaBi4Ti3FeO15 Aurivillius phases in a relatively short period of 1 h at 800 and 900 8C. While the
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molten flux helped accelerate the reaction kinetics, the synthesis temperature and time were found to be important variables to control the product purity and particle sizes, ranging from <1 mm to >20 mm. Both layered phases crystallized in plate-like morphologies, with those for LaBi4Ti3FeO15 requiring a longer reaction time and higher temperature to grow similarly-sized particles as that of Bi5Ti3FeO15. The UV–vis DRS exhibited a nearly regular relationship between the particle sizes and percent reflectances, with some deviations among the more similar reaction conditions and particle sizes. 5. Supporting information Energy dispersive X-ray analyses spectra for the Bi5Ti3FeO15 and LaBi4Ti3FeO15 products after reaction at 1000 8C for 20 h. Acknowledgment P.M. acknowledges support of this work by the Beckman Foundation through the Beckman Young Investigator Program. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.materresbull.2006.01.020. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
S.M. Skinner, IEEE Trans. Parts, Mater. Pack. 6 (1970) 68. H. Schmid, Int. J. Magn. 4 (1973) 337. G.A. Smolenskii, I.E. Chupis, Sov. Phys. Usp. 25 (1982) 475. H. Schmid, Ferroelectrics 162 (1994) 665. N.A. Hill, A. Filippetti, J. Magn. Magn. Mater. 242-245 (2002) 976. N.A. Hill, J. Phys. Chem. B 104 (2000) 6694. V.E. Wood, A.E. Austin, in: A.J. Freeman, H. Schmid (Eds.), Magnetoelectric Interaction Phenomena in Crystals, Gordon and Breach, Newark, NJ, 1975, p. 181. R.S. Singh, T. Bhimasankaram, G.S. Kumar, S.V. Suryanarayana, Solid State Commun. 91 (1994) 567. A.R. James, G.S. Kumar, S.V. Suryanarayana, T. Bhimasankaram, Ferroelectrics 216 (1998) 11. I.G. Ismailzade, V.I. Nesterenko, F.A. Mirishli, P.G. Rustamov, Kristallograhie 12 (1967) 468. A.R. James, G.S. Kumar, M. Kumar, S.V. Suryanarayana, T. Bhimasankaram, Mod. Phys. Lett. B 11 (1997) 633. M. Kumar, A. Srinivas, G.S. Kumar, S.V. Suryanarayana, Solid State Commun. 104 (1997) 741. T. Ko, G. Bang, J. Shin, Korean J. Ceram. 4 (1998) 83. J.-A. Deverin, Ferroelectrics 19 (1978) 9. A. Srinivas, S.V. Suryanarayana, G.S. Kumar, M.M. Kumar, J. Phys. Condens. Matt. 11 (1999) 3335. N.V. Prasad, G.S. Kumar, J. Magn. Magn. Mater. 213 (2000) 349. A. Srinivas, D.-W. Kim, K.S. Hong, S.V. Suryanarayana, Mater. Res. Bull. 39 (2004) 55. C.H. Hervoches, A. Snedden, R. Riggs, S.H. Kilcoyne, P. Manuel, P. Lightfoot, J. Solid State Chem. 164 (2002) 280. Y. Kan, X. Jin, P. Wang, Y. Li, Y. Cheng, D. Yan, Mater. Res. Bull. 38 (2003) 567. Y. Hayashi, T. Kimura, T. Yamaguchi, J. Mater. Sci. 21 (1986) 2876. R.H. Arendt, J.H. Rosolowski, J.W. Szmaszek, Mater. Res. Bull. 14 (1979) 703. T. Kimura, T. Yamaguchi, J. Mater. Sci. 17 (1982) 1863. M. Granahan, M. Holmes, W.A. Schulze, R.E. Newnham, J. Am. Ceram. Soc. 64 (1982) C68. T. Kimura, T. Yamaguchi, in: G. Messing, K.S. Mazdiyashi, J.W. McCauley, R.A. Haber (Eds.), Advances in Ceramics, Vol. 21, Ceramic Powder Science, American Ceramic Society, Westerville, OH, 1987, p. 169. T. Takenaka, K. Sakata, Jpn. J. Appl. Phys. 19 (1980) 31. N.M. Hagh, K. Nonaka, M. Allahverdi, A. Safari, J. Am. Ceram. Soc. 88 (2005) 3043. F. Kubel, H. Schmid, Ferroelectrics 129 (1992) 101. G.A. Geguzina, A.T. Shuvayev, V.G. Vlasenko, E.T. Shuvayeva, L.A. Shilkina, Cryst. Rep. 48 (2003) 406. M.G. Lagorio, J. Chem. Educ. 81 (2004) 1607.
Synthesis of textured Bi5Ti3FeO15 and LaBi4Ti3FeO15 ferroelectric layered Aurivillius phases by molten-salt flux methods Digamber G. Porob, Paul A. Maggard * Department of Chemistry, North Carolina State University, Raleigh, NC 27695, USA Received 23 September 2005; received in revised form 13 January 2006; accepted 18 January 2006 Available online 13 February 2006
Abstract The ferroelectric layered Bi5Ti3FeO15 and LaBi4Ti3FeO15 Aurivillius phases were synthesized in high purity and textured microstructures in a molten Na2SO4/K2SO4 (1:1 molar ratio) flux in much shortened reaction times, 1 h minimum compared to conventional techniques. The particle growth and microstructure of both phases were investigated as a function of temperature and reaction duration, and yielded plate-like particles that could be synthesized in sizes from <1 mm to >20 mm. The product crystallinity, purity and microstructures were characterized via powder X-ray diffraction and scanning electron microscopy. The UV–vis diffuse reflectance of the products were measured and analyzed with respect to the resultant particle sizes. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; A. Oxides; C. Electron microscopy; C. X-ray diffraction; D. Microstructure
1. Introduction Solid state materials in which long range magnetic (ferro/antiferromagnetic) and electric (ferroelectric) ordering coexists [1–4], termed ‘magnetoelectrics’, have recently become the focus of much research. This class of materials exhibits a spontaneous magnetization/polarization that can be switched by an applied magnetic/electric field, respectively, and often coupling between the two [5,6]. These solids hold promise as a new generation of electronic devices, owing to the possibility of applications such as combined information storage/logic operations, multiple-state memory devices, electric-field-controlled ferromagnetic resonance devices, and transducers with magnetically modulated piezoelectricity [7]. Among magnetoelectric solids, the Aurivillius series of compounds, e.g. Bi4Ti3O12nBiMO3 (M = Fe, Mn; n = 1, 2) or Bi4Ti3O12nLaFeO3, have been among some of the most heavily investigated for their magnetic and electronic properties [8–18]. In the former, the structure is comprised of n perovskite layers of magnetic BiMO3 (M = Fe, Mn) that have been inserted into the three-layer [Bi2Ti3O10]2 perovskite slab of the parent Bi4Ti3O12 ferroelectric [11]. The resultant perovskite sheets are 3 + n layers in thickness, with disordered Ti4+/Fe3+ (75%:25% ratio for n = 1) on the M sites, and these are in turn separated by fluorite-like [Bi2O2]2+ layers [18]. The related Bi4Ti3O12LaFeO3 phase is similarly constructed, but contains mixed Bi3+/La3+ cations in the perovskite sheet [11].
* Corresponding author. Tel.: +1 919 515 3616; fax: +1 919 515 5079. E-mail address: [email protected] (P.A. Maggard). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.01.020
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Despite the growing number of reports on their properties, all current syntheses of these magnetoelectric Aurivillius phases are based on conventional high-temperature ‘grind and heat’ solid-state techniques, which often leads to compositional and structural inhomogeneities owing to the high synthesis temperatures (>1000 8C), inadequate mixing, and long reaction times (>2–4 days). This is especially the case for solid-state reactions with relatively volatile bismuth oxides, and as well, for solids with different intergrowths that are sensitive to small compositional changes. As an alternate approach, molten-salt (or flux) synthetic methods have shown the ability to achieve improved homogeneity, purity and control over particle sizes, and has been found a suitable method of choice to synthesize pure homogeneous powders, or large single crystals, of Bi4Ti3O12 [19], ferrites [20], PZT [21], Bi2WO6 [22] and PbNb2O6 [23], for example. Some of the more common fluxes used in the syntheses of these oxides include mixtures of Na2SO4/ K2SO4, NaCl/KCl or Na2SO4/Li2SO4. Moreover, metal-oxide powders with anisotropic particle morphologies [24], i.e. textured microstructures, can be prepared by this technique and used as starting materials for grain-oriented ceramics. Compared to ceramics with a randomly oriented grain structure, those with textured microstructures and an appropriate alignment of the polar axes down the poling direction can exhibit maximal polarization responses [25,26], such as for materials with non-cubic symmetries and plate-like morphologies. It is further desirable to obtain these ceramics with good compositional and structural homogeneity for physical property reliability, and which might impose serious limitations on their study and application. In this paper, we report one of the first investigations into the molten-salt syntheses of the magnetoelectric Bi5Ti3FeO15 and LaBi4Ti3FeO15 layered Aurivillius solids, and the effects of synthesis temperatures and reaction times on the particle microstructures and sizes. Platelet-like morphologies with controlled particle sizes could be obtained for both solids that are homogeneous and in high purity. Further, the first diffuse reflectance measurements for these solids are reported herein and are analyzed in relation to the particle sizes. 2. Experimental Molten-salt syntheses of Bi5Ti3FeO15 and LaBi4Ti3FeO15 were performed by combining stoichiometric mixtures of AR grade Bi2O3, TiO2, Fe2O3 and La2O3 (preheated to 850 8C) to a eutectic mixture of Na2SO4/K2SO4 salts (1:1 molar ratio). The molar ratio of Bi5Ti3FeO15 or LaBi4Ti3FeO15 to the flux mixture was 1:7, which was excess in flux. The reactant mixtures were placed inside an alumina crucible and heated at temperatures ranging from 800 to 1000 8C for 1–20 h. Finally, the resulting powder was washed several times with hot deionized water to remove the alkali metal salts. Synthetic attempts at using a NaCl/KCl molten flux, in place of the equivalent sulfates, resulted in a detectable amount of corrosion to the alumina crucible. High-resolution powder X-ray diffraction (PXRD) spectra of the products ˚ ) radiation from a sealed tube X-ray generator were collected on an INEL diffractometer using Cu Kal (l = 1.54056 A
Fig. 1. The PXRD patterns of Bi5Ti3FeO15 synthesized at different reaction temperatures and times: (a) 800 8C for 1 h, (b) 900 8C for 1 h, (c) 1000 8C for 1 h, (d) 1000 8C for 2 h, (e) 1000 8C for 5 h, (f) 1000 8C for 20 h, and (g) the theoretical pattern of Bi5Ti3FeO15.
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(35 kV, 30 mA) in transmission mode and using a curved position sensitive detector (CPS120). Scanning electron microscopy (SEM) on a JEOL JEM 6300 was performed to examine the particle microstructures and sizes of the reaction products and energy dispersive X-ray (EDX) analyses were used to verify the elemental compositions. The UV–vis diffuse reflectance spectra were taken on a CARY 3E spectrophotometer equipped with an integrating sphere. 3. Results and discussion The powder X-ray diffraction (PXRD) patterns of the Bi5Ti3FeO15-loaded flux products, Fig. 1, shows that the targeted solid was prepared in high purity and good crystallinity in as little as 1 h at 800–1000 8C. This compares with the conventional solid-state reaction that requires 48 h at 1000 8C, or other similar multistep grind and reheat
Fig. 2. SEM micrographs of the Bi5Ti3FeO15 products synthesized at (a) 900 8C for 1h, (b) 1000 8C for 1 h, (c) 1000 8C for 2 h, (d) 1000 8C for 5 h, and (e) 1000 8C for 20 h.
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procedures [18,27,28]. Significant peak broadening is observed at 800 8C in the PXRD pattern, in particular for the [0 0 l] reflections, indicating relatively thin (<500 nm) Bi5Ti3FeO15 particle sizes. As the synthesis temperature is increased from 800 to 900 8C the peak widths become narrower, and further increases in the reaction temperature to 1000 8C, and longer reaction times, gave no further narrowing of the peak widths. The narrow peak widths at higher temperatures indicate good crystallinity even after only a 1 h reaction period. The evolution of microstructure and particle sizes in the Bi5Ti3FeO15 products was investigated using SEM, shown in Fig. 2. All micrographs revealed a distinct plate-like morphology of Bi5Ti3FeO15, typical of layered compounds belonging to the Aurivillius phases. The samples synthesized at 900 8C show the formation of aggregates of small plate-like particles, along with a few discrete platelets, and a particle size that varied approximately between 1 and 5 mm. The individual platelets were an order of magnitude thinner, corresponding to the c-axis direction of the structure that is perpendicular to the stacked perovskite layers. Samples synthesized at 1000 8C for 1 h exhibited a further increase in particle sizes with a large number of distinct platelets that are clearly visible in a size range of 5– 10 mm, as well as a few randomly distributed platelets as large as 10–15 mm. Next, the effect of the reaction duration on the particle morphologies and sizes was studied by holding the reaction at 1000 8C for 2, 5 and 20 h, shown in Fig. 2(c)–(e). At reaction times greater than 2 h, the particles transformed to discrete platelets and increasingly grew in size at longer reaction times. The particle dimensions reached much larger than 10–20 mm after a reaction time of 20 h. An energy-dispersive X-ray (EDX) analysis was performed (Section 5) and did not reveal any detectable amounts of alkali metals from the flux in the products. These observations confirm that phase purity and homogeneity of Bi5Ti3FeO15 particles can be achieved rapidly in the alkali metal sulfate flux, and that the particle growth can be controlled through an appropriate selection of the reaction temperature and reaction time. The molten-salt synthesis of the related LaBi4Ti3FeO15 Aurivillius phase was optimal under slightly different reaction conditions as compared to Bi5Ti3FeO15. The PXRD, shown in Fig. 3, revealed that the products obtained at 800 8C for a reaction time of 1 h contained a minor amount of LaFeO3 impurity along with the major phase of LaBi4Ti3FeO15. The diffraction peaks showed significant broadening, and indicated a small crystallite size within the product, similar to that found for Bi5Ti3FeO15. Raising the temperature to 900 8C at the same reaction time of 1 h gave a phase pure product with no detectable LaFeO3. Sharper diffraction peaks are observed at 900 8C and higher reaction temperatures, compared to that found at 800 8C, and demonstrates that good crystallinity and purity is obtainable in as short a reaction time as 1 h. The SEM micrographs of the LaBi4Ti3FeO15 products, shown in Fig. 4, reveal a plate-like morphology similar to that for Bi5Ti3FeO15. The products obtained at 900 8C show the formation of aggregates of small particles that are individually sized at 1–3 mm. At a higher reaction temperature of 1000 8C, the particle sizes increase and a small number of larger discrete platelets, 5–10 mm, are visible. As before, the synthesis temperature was held constant at 1000 8C, and the
Fig. 3. The PXRD patterns of LaBi4Ti3FeO15 synthesized at different reaction temperatures and times: (a) 800 8C for 1 h, (b) 900 8C for 1 h, (c) 1000 8C for 1 h, (d) 1000 8C for 2 h, (e) 1000 8C for 5 h, (f) 1000 8C for 20 h, and (g) the theoretical pattern of LaBi4Ti3FeO15.
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Fig. 4. SEM micrographs of the LaBi4Ti3FeO15 products synthesized at (a) 900 8C for 1 h, (b) 1000 8C for 1 h, (c) 1000 8C for 2 h, (d) 1000 8C for 5 h, and (e) 1000 8C for 20 h.
reaction durations were increased from 2, 5 to 20 h. The micrographs show that as the reaction time increases the percentage of discrete plate-like particles increases and which paralleled an increase in their sizes. For the LaBi4Ti3FeO15 products synthesized at 1000 8C for 20 h, all particles had transformed to discrete platelets and particle sizes of approximately 10–20 mm were observed. No detectable amounts of alkali metal from the flux were found in the products by EDX analyses. A comparison of the LaBi4Ti3FeO15 and Bi5Ti3FeO15 products, Figs. 2 and 4, reveals that under identical reaction conditions the particles of the latter were 2 larger and crystallized more quickly. Molten-salt syntheses of pure LaBi4Ti3FeO15 was possible at the slightly higher temperature 900 8C, while increasing reaction times and temperatures had slightly smaller effects on the particle growth processes. The UV–vis diffuse reflectance spectra (DRS) were taken for each sample in order to probe the relationship between particle sizes and percent reflectance (%R), shown in Fig. 5(a) and (b) for Bi5Ti3FeO15 and LaBi4Ti3FeO15, respectively. For weakly absorbing solids, the samples with the smallest particle sizes (i.e. highest scattering
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Fig. 5. The UV–vis diffuse reflectance spectra for (a) Bi5Ti3FeO15 or (b) LaBi4Ti3FeO15 synthesized at (i) 900 8C for 1 h, (ii) 1000 8C for 1 h, (iii) 1000 8C for 2 h, (iv) 1000 8C for 5 h, and (v) 1000 8C for 20 h.
coefficients) are expected to exhibit the maximum %R, whereas samples with larger particle sizes should show a smaller %R [29]. Decreasing particle sizes increases scattering and decreases light penetration, and which enhances the diffuse reflectance while decreasing the optical absorption. In the case of Bi5Ti3FeO15, the maximum %R was exhibited by samples (i) and (ii) with the short reaction time of 1 h at 900 and 1000 8C, with both samples exhibiting a similar %R. However, samples prepared at 1000 8C with increasing time (ii, 1 h; iii, 2 h; iv/v, 5 h/20 h) showed a clear relationship between decreasing %R and increasing particle size, with a minimum in the %R found after the reactions at 5 h (iv) and 20 h (v). For the LaBi4Ti3FeO15 phase, Fig. 5b, the maximum %R is shown by the sample with the smallest particle sizes prepared at 900 8C for 1 h. For increasing reaction times at 1000 8C (ii, iii, iv, and v), the reaction times of 1, 2, and 5 h had the next lowest values for %R, but not in the expected order of increasing reaction duration. This is likely owing to the slower reaction kinetics for the LaBi4Ti3FeO15 phase, and which manifests smaller changes in particle sizes over time so that the products are more difficult to distinguish based on DRS measurements. However, the minimum in %R is found for the largest sized particles that resulted from the flux preparation at 1000 8C for 20 h. Both samples show roughly the expected trend between particle sizes and %R, though over a much greater range in the case of Bi5Ti3FeO15. The deviations from the expected trend results from the fact that the variations in particle sizes are fairly small, i.e. from 1 mm to over 10 mm, and also that a distribution of smaller and larger particles exists in every sample. These results indicate the DRS measurements could be an appropriate diagnostic tool in the future to evaluate the particle growth of other Aurivillius phases. 4. Conclusions A molten-salt flux synthesis, using a Na2SO4/K2SO4 mixture, was used to prepare high purity samples of the Bi5Ti3FeO15 and LaBi4Ti3FeO15 Aurivillius phases in a relatively short period of 1 h at 800 and 900 8C. While the
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molten flux helped accelerate the reaction kinetics, the synthesis temperature and time were found to be important variables to control the product purity and particle sizes, ranging from <1 mm to >20 mm. Both layered phases crystallized in plate-like morphologies, with those for LaBi4Ti3FeO15 requiring a longer reaction time and higher temperature to grow similarly-sized particles as that of Bi5Ti3FeO15. The UV–vis DRS exhibited a nearly regular relationship between the particle sizes and percent reflectances, with some deviations among the more similar reaction conditions and particle sizes. 5. Supporting information Energy dispersive X-ray analyses spectra for the Bi5Ti3FeO15 and LaBi4Ti3FeO15 products after reaction at 1000 8C for 20 h. Acknowledgment P.M. acknowledges support of this work by the Beckman Foundation through the Beckman Young Investigator Program. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.materresbull.2006.01.020. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
S.M. Skinner, IEEE Trans. Parts, Mater. Pack. 6 (1970) 68. H. Schmid, Int. J. Magn. 4 (1973) 337. G.A. Smolenskii, I.E. Chupis, Sov. Phys. Usp. 25 (1982) 475. H. Schmid, Ferroelectrics 162 (1994) 665. N.A. Hill, A. Filippetti, J. Magn. Magn. Mater. 242-245 (2002) 976. N.A. Hill, J. Phys. Chem. B 104 (2000) 6694. V.E. Wood, A.E. Austin, in: A.J. Freeman, H. Schmid (Eds.), Magnetoelectric Interaction Phenomena in Crystals, Gordon and Breach, Newark, NJ, 1975, p. 181. R.S. Singh, T. Bhimasankaram, G.S. Kumar, S.V. Suryanarayana, Solid State Commun. 91 (1994) 567. A.R. James, G.S. Kumar, S.V. Suryanarayana, T. Bhimasankaram, Ferroelectrics 216 (1998) 11. I.G. Ismailzade, V.I. Nesterenko, F.A. Mirishli, P.G. Rustamov, Kristallograhie 12 (1967) 468. A.R. James, G.S. Kumar, M. Kumar, S.V. Suryanarayana, T. Bhimasankaram, Mod. Phys. Lett. B 11 (1997) 633. M. Kumar, A. Srinivas, G.S. Kumar, S.V. Suryanarayana, Solid State Commun. 104 (1997) 741. T. Ko, G. Bang, J. Shin, Korean J. Ceram. 4 (1998) 83. J.-A. Deverin, Ferroelectrics 19 (1978) 9. A. Srinivas, S.V. Suryanarayana, G.S. Kumar, M.M. Kumar, J. Phys. Condens. Matt. 11 (1999) 3335. N.V. Prasad, G.S. Kumar, J. Magn. Magn. Mater. 213 (2000) 349. A. Srinivas, D.-W. Kim, K.S. Hong, S.V. Suryanarayana, Mater. Res. Bull. 39 (2004) 55. C.H. Hervoches, A. Snedden, R. Riggs, S.H. Kilcoyne, P. Manuel, P. Lightfoot, J. Solid State Chem. 164 (2002) 280. Y. Kan, X. Jin, P. Wang, Y. Li, Y. Cheng, D. Yan, Mater. Res. Bull. 38 (2003) 567. Y. Hayashi, T. Kimura, T. Yamaguchi, J. Mater. Sci. 21 (1986) 2876. R.H. Arendt, J.H. Rosolowski, J.W. Szmaszek, Mater. Res. Bull. 14 (1979) 703. T. Kimura, T. Yamaguchi, J. Mater. Sci. 17 (1982) 1863. M. Granahan, M. Holmes, W.A. Schulze, R.E. Newnham, J. Am. Ceram. Soc. 64 (1982) C68. T. Kimura, T. Yamaguchi, in: G. Messing, K.S. Mazdiyashi, J.W. McCauley, R.A. Haber (Eds.), Advances in Ceramics, Vol. 21, Ceramic Powder Science, American Ceramic Society, Westerville, OH, 1987, p. 169. T. Takenaka, K. Sakata, Jpn. J. Appl. Phys. 19 (1980) 31. N.M. Hagh, K. Nonaka, M. Allahverdi, A. Safari, J. Am. Ceram. Soc. 88 (2005) 3043. F. Kubel, H. Schmid, Ferroelectrics 129 (1992) 101. G.A. Geguzina, A.T. Shuvayev, V.G. Vlasenko, E.T. Shuvayeva, L.A. Shilkina, Cryst. Rep. 48 (2003) 406. M.G. Lagorio, J. Chem. Educ. 81 (2004) 1607.