Growth of Magnesium Carbonate Single Crystal in Supercritical Carbon Dioxide-Molten Sodium System
2004 VOL. 4, NO. 3 415-417
Zhengsong Lou, Qianwang Chen,* Yun Zhu, Yufeng Zhang, and Jin Gao Structure Research Laboratory and Department of Materials Science & Engineering, University of Science & Technology of China, Hefei 230026, China Received November 19, 2003;
CRYSTAL GROWTH & DESIGN
Revised Manuscript Received March 13, 2004
ABSTRACT: A novel crystal growth method has been developed to grow single crystals of magnesium carbonate from molten sodium. Single crystals of magnesium carbonate with a size up to 1.5 mm were grown at 550 °C in a closed vessel consisting of an appropriate amount of magnesium carbonate powder, metallic sodium, and carbon tetrachloride. The X-ray rocking curve of the magnesium carbonate single crystal has a full-width at half-maximum (fwhm) of 0.043°, indicating the high quality of the crystal. The possible growth mechanism of the single crystals was discussed based on the pyrolysis and recrystallization of MgCO3 in molten sodium environment at reaction temperatures. Introduction Magnesium carbonate is an abundant mineral. It is used as a translucent filler and reinforcing agent, and facilitates color development in paints and other pigmented systems. Magnesium carbonate is also used as a paper filler, thermal insulating filler, acid acceptor, polishing agent, and as an ingredient in glass and ceramic glazes. Single crystals of magnesium carbonate could be used as optical windows due to its high level of hardness and highly acidic corrosion resistant properties. However, the growth of single crystals of carbonates through a molten salt method is rather difficult due to their high melting points and easy pyrolysis of the materials. In this paper, we report the growth and characterization of single crystals of magnesium carbonate in a supercritical carbon dioxide-molten sodium system. Experimental Procedures Crystal Growth. In this method, magnesium carbonate powder, metallic sodium, and carbon tetrachloride were used as reactant to synthesize single crystals of magnesium carbonate. The reaction was carried out in a stainless steel autoclave (10 mL) that is a 110-mm-long cylindrical tube with outer diameter of 85 mm and an inner diameter of 13 mm, respectively. A typical reaction used 4.8 g of magnesium carbonate, 2.2 g of metallic sodium, and 4 mL of carbon tetrachloride, which were placed in the cell at room temperature. The operation was carried out in a glovebox. The vessel was then immediately closed tightly and heated to 500 °C, and kept at this temperature for 20 h. The reaction took place at an autogenic pressure depending on the amount of magnesium carbonate and carbon tetrachloride added. After cooling of the sample to room temperature, the solid product was collected and treated in 6.0 mol/L HCl aqueous solutions at room temperature for 12 h. A microfiliter was used to collect the precipitate that was then washed with ethanol and dried in air, yielding about 1.62 g of products. The solid precipitate was confirmed containing single crystals of magnesium carbonate, graphite, and amorphous carbon. A total of 1.46 g of crystals was picked out from the sample. In light of the weight of the sample, at 500 °C the maximum transformation ratio for magnesium carbonate to single crystals of magnesium carbonate was, referred to as carbonate, found to be as high as 30.4% (mole ratio). Crystal Characterization. The X-ray diffraction (XRD) analysis was performed on a Rigaku (Japan) D/max-rA X-ray diffraction meter equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å). The morphology of the samples was observed on a scanning electron microscope (SEM) (Hitachi X-650). The Raman * Corresponding author. E-mail:
[email protected]; fax: +86-5513631760.
spectroscopy analysis was carried out on a LABRAM-HR Confocal Laser Micro-Raman spectrometer at room temperature.
Results and Discussion Powder X-ray diffraction was performed to determine the phase of the crystal samples. The diffraction pattern of a powder sample crushed from several crystals grown at 500 °C were shown in Figure 1. All of the reflections of XRD pattern can be readily indexed with a pure hexagonal phase of magnesium carbonate, compatible with the literature values of a ) 4.632 Å and c ) 15.01 Å (PDF card, JCPDS 83-1761). An X-ray [104] direction diffraction pattern of the single crystal is shown in Figure 2A, which was obtained by mounting the crystal basal plane in the scattering plane. At the same time, careful adjustment of the initial θ and 2θ angles was also performed according to the rocking curve of the 104 reflection shown in Figure 2B, so that [104] direction main reflections were collected. All the data shown in Figure 3A have been well indexed by [104] direction reflections. The rocking curve about the peak 104 yielded an fwhm of 0.043° (Figure 2B), indicating high quality of the crystal. The result also shows (104) is the cleave face of the grown crystals, which is different from (101) for natural magnesium carbonate. Micro Raman spectroscopy was also utilized to identify single crystals of magnesium carbonate (Figure 3). The Raman spectrum of the sample exhibits characteristic peaks of magnesium carbonate (1009.3, 329.6, 210.9, 739.6,
Figure 1. The X-ray diffraction pattern of a powder sample crushed from crystals grown at 500 °C.
10.1021/cg034224f CCC: $27.50 © 2004 American Chemical Society Published on Web 04/06/2004
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Figure 4. SEM images of single crystals of magnesium carbonate grown at 500 °C (A). (B) is the enlarged image of the rectangle area shown in (A).
Figure 2. The XRD pattern for single-crystal magnesium carbonate grown at 500 °C (A) and an X-ray rocking curve of (104) reflection with 2θ at 32.82° (B).
Figure 5. Photograph of single crystals of magnesium carbonate obtained at 550 °C, taken from an optical microscope under a polarized mode.
Figure 3. Raman spectrum of a magnesium carbonate single crystal, using an excitation wavelength of 514.3 nm. The measurements were performed on a LABRAM-HR Confocal Laser MicroRaman spectrometer.
1445, and 1761.6 cm-1)1,2 and an intense peak at 1009.3 cm-1, indicating the formation of high quality single-crystal grains of magnesium carbonate. The peaks at 1009.3 and 739.6 cm-1 correspond to the symmetric stretching vibration and in-plane bending vibration of the carbon group, respectively. The two lower frequency bands at 329.6 and 210.9 cm-1 are associated with a librational motion and a translational motion of the carbonate groups relative to the divalent cation, respectively.3-5 Figure 4A shows SEM image of single-crystal grains of magnesium carbonate. The particles in the sample appear to be of distinctive shape with the sizes range from 180 to 750 µm. Figure 4B was the enlargement of the boxed area in Figure 4A. The crystal with rectangle shapes and a smooth, flat surface can be clearly seen, and the size of the crystal is about 450 µm. The crystal particles can be clearly seen by the naked eye, and they appear transparent and bright. Figure 5 shows the optical microscope (with polarized model) of single-crystal grains of magnesium carbonate grown at 550 °C. The crystal particles with an average diameter of 1.3
mm and a few larger ones with sizes around 1.5 mm are found present in the product. To examine the effects of the reaction temperature and reactant on the growth of single crystals of magnesium carbonate, a series of relevant experiments were carried out by altering the experimental parameters of the processes. The process carried out at the temperatures lower than 350 °C could not initiate the reaction; as the reaction was conducted at 450 °C, the products were mainly graphite and amorphous carbon. Single crystals of magnesium carbonate became the major products at 500 °C, as shown in Figure 3; as the temperature was increased to 550 °C, few and large crystal particles were observed by the naked eye. No crystals of magnesium carbonate formed when the reactions were conducted at 600 °C. When the temperature is maintained at 500 °C, as the amount of magnesium carbonate powder decreased from 0.0571 to 0.0298 mol, the reaction only yielded graphite and amorphous carbon, which reveals that the growth of magnesium carbonate single crystals is sensitive to the temperature and pressure of the system. It was also found that no MgCO3 crystals were formed without carbon tetrachloride or metallic sodium. The metallic sodium melts at 97.8 °C, and its boiling temperature is 892 °C. At reaction temperatures, MgCO3 pyrolyzed (350 °C, 1atm) to give off carbon dioxide and MgO. In our reaction system, the CO2 is in a supercritical state (31 °C, 73 atm). The carbon tetrachloride can be reduced by metallic sodium to produce graphite and sodium chloride. Because of the effects of sodium chloride auxiliary flux, MgCO3 can form supersaturation solution in molten sodium, from which crystals of magnesium carbonate slowly grew. At 550 °C, the solubility of magnesium carbonate increases in the molten sodium and the number of crystal nuclear decreases due to dissolving of the crystals; as a result the sizes of the crystals increase
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and the number of particles decreases. At 600 °C, MgCO3 crystals were not found in the product; the reason might be due to decomposing of magnesium carbonate to give off CO2 at the given conditions, and CO2 was reduced by metallic sodium leading to disappearing of sodium and CO2, both of which are essential for the growth of magnesium carbonate crystals. In our reaction system, the CO2 is in a supercritical state, and the pressure is about 880 atm at 500 °C. The chemical reactions can be represented as follows:
MgCO3(s) f MgO(s) + CO2(g)
(1)
CCl4(g) + 4 Na(l) f C(graphite) + 4NaCl(s)
(2)
CO2(g) + 4 Na(l) f C(graphite) + 2Na2O(s)
(3)
CO2(g) + Na2O(s) f Na2CO3(s)
(4)
CO2(g) + MgO(s) f MgCO3(single crystals)
(5)
However, the growth process of magnesium carbonate single crystals is not fully understood yet. Recently, we have synthesized diamond reaching up to 250 µm by heating CO2 to 440 °C in a sealed container with metallic sodium.6 Moreover, high-purity cubic diamond particles as large as 510 µm were synthesized by the reduction of magnesium carbonate with metallic sodium at 500 °C.7 It has been reported that diamond powder can be synthesized through a metallic reduction-pyrolysiscatalysis route with the reaction of carbon tetrachloride and sodium at 700 °C.8 It is expected that the yield of diamond in the above system can be increased by adding a small amount of carbon tetrachloride. However, no diamond but a small amount of graphite and MgCO3 single crystal were detected by XRD analysis; therefore, much work is required to exactly understand the growth mechanism of diamond. Single crystals of magnesium carbonate are insoluble in 6.0 mol/L HCl aqueous solution at room temperature, and crystals are hard, transparent, and colorless, which make
them possibly useful as the window material of instruments if the size could be further increased. In addition, single crystals of magnesium carbonate might have the same potential applications such as man-made gems. Conclusions The above studies illustrate single crystals of magnesium carbonate can be grown in a molten sodium system using magnesium carbonate powder as the starting material. Magnesium carbonate single crystals with a size up to 1.5 mm were grown at 550 °C with the addition of carbon tetrachloride. It is suggested that carbon tetrachloride was reduced by metallic sodium to produce graphite and sodium chloride. Because of the effects of auxiliary flux of sodium chloride, MgCO3 can form supersaturating solution in molten sodium, in which crystals of magnesium carbonate can slowly grow. This method may provide a means to grow single crystals of other easy pyrolysis carbonates, such as CaCO3, SrCO3, and BaCO3. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20125103 and 90206034).
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