CRYSTAL GROWTH & DESIGN
Growth of Na2Ta4O11 Crystals from a Na2Mo2O7 Flux Katsuya Teshima,† Daiki Tomomatsu,† Takaomi Suzuki,† Nobuo Ishizawa,‡ and Shuji Oishi*,† Department of EnVironmental Science and Technology, Faculty of Engineering, Shinshu UniVersity, 4-17-1 Wakasato, Nagano 380-8553, Japan, and Ceramics Research Laboratory, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi, Gifu 507-0071, Japan
2006 VOL. 6, NO. 1 18-19
ReceiVed June 26, 2005; ReVised Manuscript ReceiVed September 16, 2005
ABSTRACT: Na2Ta4O11 (natrotantite) crystals were successfully grown from a Na2Mo2O7 flux by a slow-cooling method for the first time. The hexagonal plate-shaped crystals, bounded by the {0001}, {011h2} and {101h4} faces, were grown with sizes of up to 3.3 × 3.3 × 0.5 mm. The obtained crystals were light yellow and transparent. The solubility of Na2Ta4O11 in Na2Mo2O7 gradually increased with a rise in temperature. A variety of metal tantalate compounds have received much attention due to their potential applications in various industrial fields. In particular, they have been studied for photocatalysts,1-3 such as decomposition of pure water and degradation of toxic substances. Recently, metal tantalate compounds, such as ATaO3, ATaO4, ATa2O6, ATa2O7, and ATa2O9, have been reported as representative photocatalysts (A ) alkali metal, alkaline-earth metal, rare earth metal, or a couple of them). These oxides are, generally, synthesized by a conventional solid-state reaction at relatively high temperatures. Na2Ta4O11 (natrotantite) was first encountered in granitic pegmatites in association with other tantalum minerals.4 The crystals of Na2Ta4O11 belong to the rhombohedral system with space group R3c.5-8 Na2Ta4O11 has been reported to have lattice parameters a ) 0.62086(3) and c ) 3.6618(2) nm and a density of 7.71 g cm-3.8 The congruent melting point of this compound is 1655 °C.9 A crystalline powder of Na2Ta4O11 was synthesized by a solid-state reaction method.6,10,11 By calcination of Na0.5H0.5TaO3‚0.7H2O prepared under hydrothermal conditions, Na2Ta4O11 was formed.12 The single crystals of Na2Ta4O11 were grown in a NaTaO3-WO3 system7 and from MoO3 flux.9 The morphology and sizes of the crystals have not been described.7,9 Also, crystals of Na2Nb4O11 similar in chemical formula to Na2Ta4O11 have been grown from Na2Mo2O7 flux.13 In this work, Na2Mo2O7 was chosen as a flux to grow crystals of Na2Ta4O11 on the basis of our experience in growing Na2Nb4O11 crystals.13 Sodium dimolybdate has a common cation (Na+) with the solute and a low melting point with sufficient solubility in water. No report on the growth of Na2Ta4O11 crystals from the Na2Mo2O7 flux has been published. The solubility of Na2Ta4O11 in Na2Mo2O7 has not yet been reported. The present paper describes the growth of Na2Ta4O11 crystals from a Na2Mo2O7 flux by a slow cooling method. The morphology was also examined. The solubility of Na2Ta4O11 in Na2Mo2O7 was determined. The solubility of Na2Ta4O11 in Na2Mo2O7 was determined by measuring the mass loss of Na2Ta4O11 crystals in Na2Mo2O7 melts at different temperatures. Mixtures of excess crystals (1-2 mm in size and about 1 g in mass) of Na2Ta4O11 and Na2Mo2O7 powder (about 5 g) were put into platinum vessels. After the mixture was heated for 3 h at a preset temperature, undissolved crystals were present upon quenching. The undissolved crystals were separated from the solidified saturated solution in warm water and reweighed. The loss in mass due to dissolution represents the solubility at that temperature. The eutectic temperature of the Na2Ta4O11-Na2Mo2O7 system was determined on the basis of differential thermal analysis (DTA) curves. Reagent-grade Na2CO3, Ta2O5, and MoO3 were used for the growth of Na2Ta4O11 crystals. A mixture of Na2CO3 + 2Ta2O5 powders was used as a solute. A mixture of Na2CO3 + * Corresponding author. E-mail:
[email protected]. † Shinshu University. ‡ Nagoya Institute of Technology.
2MoO3 powders was used as the flux. Mixtures containing solutes of 1.2 and 1.3 mol % were prepared. The mass of the mixtures was 28.1 g (25.0 g as Na2Ta4O11-Na2Mo2O7). Each of the mixtures was put into a platinum crucible of 30 cm3 capacity. After the lid was closed, the crucible was placed in an electric furnace with silicon carbide heating elements. The crucible was heated to 1100 °C at a rate of 45 °C h-1, held at this temperature for 10 h, and then cooled to 450 °C at a rate of 5 °C h-1. When the cooling program was completed, the crucible was allowed to cool to room temperature. The crystalline products were then taken out by dissolving the flux in warm water. The obtained crystals were examined using an optical microscope and a scanning electron microscope (SEM). Crystal phases were identified by the powder X-ray diffraction (XRD). The sizes of obtained crystals were measured. The morphology was investigated by the use of XRD and interfacial angle data. The lattice parameters were obtained on the basis of the powder XRD. A SEM equipped with an energydispersive X-ray spectrometer (EDS) was used to study any variations in the concentration of the major constituents in the growth crystals. The temperature dependence of the solubility of Na2Ta4O11 in Na2Mo2O7 is shown in Figure 1 (solubility curve; polynomial fitting). At 650 °C, Na2Ta4O11 was dissolved in Na2Mo2O7 at a concentration of 0.36 mol % (0.97 g in 100 g Na2Mo2O7). The solubility gradually increased with temperature, with Na2Ta4O11 reaching a solubility of 1.32 mol % (3.61 g in 100 g Na2Mo2O7) at 1100 °C. The obtained solubility curve had an appreciable temperature coefficient of solubility. Therefore, Na2Ta4O11 could be crystallized by slow cooling of the solutions. Thus, Na2Mo2O7 is expected to be a suitable flux for growing Na2Ta4O11 crystals. On the basis of the DTA data, the eutectic temperature of the Na2Ta4O11-Na2Mo2O7 system was 605 ( 5 °C. Judging from the solubility curve and eutectic temperature, the eutectic composition was considered to be around 0.3 mol % Na2Ta4O11-99.7 mol % Na2Mo2O7. Figure 1 shows that the mixture containing 1.3 mol % solute is unsaturated at a soak temperature of 1100 °C. It was expected that large Na2Ta4O11 crystals could be grown from the solution on subsequent slow cooling. Large, hexagonal plate-shaped Na2Ta4O11 crystals of widths up to 3.3 mm and thicknesses of 0.5 mm were grown from the Na2Mo2O7 flux. The obtained crystals were light yellow and transparent. They were identified as Na2Ta4O11 by their powder XRD patterns, using published data.8 Typical Na2Ta4O11 crystals are shown in Figure 2. The thickness of grown crystals was dependent on the solute concentration. At 1.3 mol % Na2Ta4O11-98.7 mol % Na2Mo2O7, the maximum platelike crystal grown reached 3.3 mm in width and 0.5 mm in thickness. Its aspect ratio was calculated at approximately 0.15. In the case of 1.2 mol % Na2Ta4O11-98.8 mol % Na2Mo2O7, the aspect ratio of the maximum crystal was 0.32 (width: 2.5 mm, thickness: 0.8 mm), twice as large as that of the crystal grown at solute of 1.3 mol %. The size of the crystals grown
10.1021/cg050291t CCC: $33.50 © 2006 American Chemical Society Published on Web 10/06/2005
Communications
Crystal Growth & Design, Vol. 6, No. 1, 2006 19
Figure 3. Schematic drawing of Na2Ta4O11 crystal with {0001}, {011h2}, and {101h4} faces.
Figure 1. Solubility of Na2Ta4O11 in Na2Mo2O7 as a function of temperature.
The grown crystals of Na2Ta4O11 were bounded by two basal and 12 sided faces. The crystal surfaces were very flat as shown in Figure 2. The XRD patterns of oriented platelike crystals showed that the diffraction intensities of the (000l) in the hexagonal setting were predominant. The indices of well-developed basal faces were {0001}. The interfacial angle between (0001) and adjacent hexagonal faces was 73 ( 1°. This value was in good agreement with the calculated interfacial angle of 73.6° between (0001) and (011h2) faces. The interfacial angle between (0001) and adjacent square faces was 59 ( 1°. This value was in good agreement with the calculated interfacial angle of 59.6° between the (0001) and (101h4) faces. In addition, the interfacial angle between the (011h 2) and (101h4) faces was 56 ( 1°. This value was in good agreement with the calculated value (56.2°). The crystals were found to be bounded by the {0001}, {011h2}, and {101h4} faces (Figure 3). The EDS data showed that the sodium and tantalum atoms were distributed homogeneously in the crystals. Molybdenum atoms from the flux were not detected in the crystals. On the basis of the powder XRD data, the lattice parameters were determined as a ) 0.6210(3) nm and c ) 3.664(3) nm in the hexagonal setting. In addition, the density was pycnometrically determined to be 7.70 ( 0.02 g cm-3 (calculated: 7.67 g cm-3). Therefore, these values of the grown crystals agree approximately with those from the literature.8 In conclusion, Na2Mo2O7 was successfully used as a new flux to grow the well-formed crystals of Na2Ta4O11. Since the grown crystals in our process are relatively large and have their characteristic morphology, they should be favorable materials for various technological applications.
References
Figure 2. Na2Ta4O11 crystals grown from Na2Mo2O7 flux at a solute concentration of (a) 1.2 mol % and (b) 1.3 mol %.
might appear to be related to the presence of undissolved solute particles. Sodium dimolybdate was found to be a suitable flux to grow these crystals. The mixture containing 1.3 mol % solute produced 0.42 g of crystals. This means that about 49 mass % of the solute (0.86 g) was retrieved as platelike crystals from the solution. The theoretically expected yield of Na2Ta4O11 crystals was calculated to be 0.66 g from the lever rule between starting and eutectic compositions. The mass of the obtained crystals was about 64% of the calculated value. In the growth run, the evaporation ratio of the Na2Mo2O7 flux was less than 1 mass %. The resulting crystals could be readily separated from the flux in warm water because Na2Mo2O7 was easily soluble.
(1) Machida, M.; Yabunaka, J.; Kijima, T. Chem. Commun. 1999, 1939. (2) Tanaka, T.; Nojima, H.; Yamamoto, T.; Takenaka, S.; Funabiki, T.; Yosihda, S. Phys. Chem. Chem. Phys. 1999, 1, 5235. (3) Kato, H.; Kudo, A. Phys. Chem. Chem. Phys. 2002, 4, 2833. (4) Voloshin, A. V.; Men’shikov, Yu. P.; Pakhomovskii, Ya. A. Zap. Vseross. Mineral. O-Va. 1981, 110, 338. (5) Chaminade, J. P.; Pouchard, M.; Hagenmuller, P. ReV. Chim. Mineral. 1972, 9, 381. (6) Ercit, T. S.; Hawthorne, F. C.; Cerny, P. Bull. Mineral. 1985, 108, 541. (7) Mattes, R.; Schaper, J. ReV. Chim. Mineral. 1986, 22, 817. (8) ICCD PDF 38-0463. (9) Roth, R. S.; Parker, H. S.; Brower, W. S.; Warning, J. L. Fast Ion Transp. Solids, Solid State Batteries DeVices, Proc. NATO AdV. Study Inst. 1973, 217. (10) Al-Shukry, R. M.; Jasim, F. Thermochim. Acta 1980, 41, 281. (11) Hedden, D. B.; Torardi, C. C.; Zegarski, W. J. Solid State Chem. 1995, 118, 419. (12) Kumada, N.; Ozawa, N.; Kinomura, N.; Muto, F. Mater. Res. Bull. 1985, 20, 583. (13) Oishi, S.; Nagai, Y.; Ishizawa, N. Nippon Kagaku Kaishi 1998, 598.
CG050291T
