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Growth of sodium chlorate crystals in the presence of potassium sulphate
Journal of Crystal Growth 426 (2015) 198–201
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of sodium chlorate crystals in the presence of potassium sulphate E.L. Kim, A.A. Tsyganova, D.A. Vorontsov n, T.I. Ovsetsina, M.R. Katkova, V.A. Lykov, V.N. Portnov Lobachevsky State University of Nizhny Novgorod, Gagarin Ave., 23, Nizhny Novgorod 603950, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 12 March 2015 Received in revised form 15 June 2015 Accepted 16 June 2015 Communicated by: S.R. Qiu Available online 24 June 2015
In this work, we investigated the morphology and growth rates of NaClO3 crystals in solutions with K2SO4 additives. NaClO3 crystals were grown using the temperature gradient technique under concentration convection. We found that the crystal habitus changed from cubic to tetrahedral, and the growth of the cubic {100}, tetrahedral {111} and rhomb-dodecahedral {110} faces decelerated with an increase in the concentration of SO24 ions. The {110} face was the most and the {100} face was the least inhibited by sulphate ions. The mechanism of SO24 ions action is their adsorption on the crystal surface, which impedes attachment of the crystal's building units. We conclude that different atomic structure and charge state of various crystal faces determine their sensitivity to the action of the SO24 ions. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal morphology A1. Impurities A2. Growth from solutions A2. Single crystal growth B1. Sodium chlorate B2. Nonlinear optic materials
1. Introduction The impact of impurities, or additives, on the crystallization in aqueous solutions has been intensively studied both in theoretical [1– 5] and experimental [6–15] work, as some compounds added even in a very small amount can essentially modify growth rate, properties and quality of growing crystals. Impurity is adsorbed from the bulk solution to the surface of the crystal; therefore, it can decelerate its growth either by blocking kinks on the face or edges on the steps [16,17], or by formation of an impurity film on the face [18,19]. The influence of different additives on the growth of non-linear optical crystals is worth studying. Thus, sodium chlorate NaClO3 crystals can be used as a new non-linear optical material for the second harmonic generation [20]. These crystals belong to a cubic syngony and have a space group P213. In the work [16], the change of the habitus of sodium chlorate crystals from cubic to tetrahedral was observed in the presence of Na2SO4 additive. The modification of the crystal shape was explained by unequal retardation of the cubic {100} and tetrahedral {111} faces during adsorption of the sulphate ions SO24 in the kinks of the steps [21]. The positive {111} and negative {111} tetrahedral faces of a NaClO3 crystal symmetrically are not identical because the {111} surfaces are terminated by a layer of Na þ cations whereas the {111} faces have ClO3 anions n
Corresponding author. Tel.: þ 7 9101316701. E-mail address: [email protected] (D.A. Vorontsov).
http://dx.doi.org/10.1016/j.jcrysgro.2015.06.016 0022-0248/& 2015 Elsevier B.V. All rights reserved.
on the surface [22]. In [23] it was found that sodium dithionate Na2S2O6 caused appearance of faces of a negative tetrahedron {111} on the shape of NaClO3 crystals. The impact of other sulfates, including potassium sulphate, on the growth of NaClO3 crystals in aqueous solutions has not been mentioned in the literature to the best of our knowledge. The authors of [24] directly observed by means of the interference technique the surface of a positive and negative tetrahedron in solution. They revealed that the {111} faces developed by the spiral dislocation mechanism, whereas the {111} faces grew as an atomically rough surface. The growth rates of the {100} and {110} faces in pure solution were measured in the study [23]. Misailovic et al. [25] investigated statistical distribution of the growth rates for the growing and nongrowing {100} faces of small NaClO3 crystals in supersaturated pure solutions. Nevertheless, we have not found any data on the growth rates of the {110} faces of NaClO3 crystal in solutions with the additives of SO24 ions. The goal of this study is investigation of the influence of potassium sulphate K2SO4 on the morphology of NaClO3 crystals and growth rates of various faces, as well as clarification of the inhibition mechanism of this additive.
2. Experimental technique The solutions were prepared from analytical grade sodium chlorate NaClO3, distilled water and additions of analytical grade
E.L. Kim et al. / Journal of Crystal Growth 426 (2015) 198–201
potassium sulphate K2SO4 in the amount of 2.1–5.2 wt%. At first, small NaClO3 crystals up to 3 mm in size were obtained in a supercooled solution in a Petry dish. Then, the crystals were used as seeds in the following experiments. Large single crystals of NaClO3 were grown by means of a temperature gradient technique with feeding in a concentration convection regime [26] at a temperature of 23.5 1C and constant solution supercooling of 1.5 1C. A crystallization vessel with a volume of 100 mL was placed inside an air thermostat. A seed crystal was attached to a glass rod connected to a vessel lid. A special container with feeding crystals was located between the seed and the lid. Crystal growth was possible due to a temperature gradient between the two zones having the growing crystal and feeding crystals. When the crystal grew, concentration convection appeared in the vessel, and depleted solution near the growing crystal was replaced by the solution which had a higher density and went down from the area with feeding crystals. The solubility of NaClO3 in water was calculated using the data of [27]: CðTÞ ¼ 79:99 þ0:67 U T þ 0:0057 U T2 g=100 mL H2 O : The average growth rates of the faces were calculated on the basis of the total growth time and the sizes of the crystals obtained at specified concentrations of K2SO4. We used in calculations all the faces present on the shape of the grown crystal. The total increment of every face was found as along the normal from the face plane to the center of the seed that was clearly visible inside the grown crystal. Measurements were performed by an optical microscope with an accuracy of 0.1 mm. Trace analysis of solution samples with a volume of 4 5 mL was performed by atomic emission spectrometry with the use of a Varian Agilent 720-ES spectrometer. The calibration was done by a standard addition method. The detection limit of sulfur atoms in a sample was 40 μg/L at a 181.972 nm line [28].
3. Results 3.1. Solubility data We revealed that the solubility of NaClO3 in water decreased in the presence of K2SO4 in the solution and there appeared precipitation, which consisted exclusively of NaClO3. We measured the weight of the precipitation to determine the decrease of the solubility ΔCNaClO3 calculated as a difference of the NaClO3 concentrations before and after the addition of K2SO4 at a solution saturation temperature of 25 1C (Fig. 1). The decrease of the saturation temperature of the solution with K2SO4 of 5.2 wt% due to the decrease of the solubility of NaClO3 was negligible, less than 0.02 1C.
199
Fig. 1. The decrease of the equilibrium concentration of NaClO3 at a saturation temperature of 25 1C as a function of the amount of K2SO4 added to the solution.
3.3. Morphology and growth rates The sodium chlorate single crystals grown from the solution without K2SO4 additive had a cubic shape with weakly pronounced faces of a positive tetrahedron {111} (Fig. 2a). We observed the development of the {111} faces and appearance of the rhomb-dodecahedral {110} faces in the habitus of the crystals (Fig. 2b) during growth in the solution containing 2% of K2SO4. Part c of Fig. 2 shows that with a further increase in the concentration of K2SO4 up to 3%, the {111} and {110} faces become more pronounced. When the amount of K2SO4 in the solution reached 5.2%, NaClO3 crystals with a tetrahedral habitus were formed (Fig. 2d). All the grown crystals were transparent, without visible cracks and inclusions of solution. The presence of faces with definite indices in the crystal shape depends on the ratio of their growth rates. The slower the face grows, the larger area it occupies in the crystal growth pattern. From the geometrical considerations it follows that the crystal of cubic symmetry will have {111} and {110} faces together with {100} faces when the ratios of their velocities satisfy the inequalities R{111}/R{100} o31/2 and R{110}/R{100} o21/2, respectively. The data of Fig. 3 demonstrate that the growth rates of the {100}, {111} and {110} faces decrease with an increace in the concentration of K2SO4. R{111}/R{100} ¼1.35 in pure solution, and the cubic faces of NaClO3 crystals are the most developed (Fig. 2a). We have not observed appearance of the rhomb-dodecahedral {110} faces in pure solution at all, and therefore, we assume that their growth rate exceeds 1.51/2 R{111}, or 28 10 6 mm/s (labeled by in Fig. 3) in this case. When the concentration of K2SO4 was equal to 2.5%, the curves for the growth rates of the cubic and tetrahedral faces intersected (Fig. 3), and the habitus changed from cubic to tetrahedral (Fig. 2a–d).
4. The mechanism of the influence of K2SO4 3.2. Atomic emission spectrometry The solutions prepared from the crystals grown in pure and 2% K2SO4 doped solution of sodium chlorate were taken for analysis to test the presence of sulphate ions by atomic emission spectrometry. The crystals were dissolved in deionized water with a resistance of 18.2 MOhm. The presence of sulphate ions in the solutions was detected by analyzing the 180.669, 181.972 and 182.562 nm sulfur lines. A possible content of the sulfur atoms in all samples and, consequently, in the structure of the NaClO3 crystal was smaller than 40 mg/L detection limit.
A possible decrease of supercooling owing to the decrease of NaClO3 solubility, the basic compound, in the presence of K2SO4 is negligible. Hence, we consider that the influence of this solubility effect on the growth rate of NaClO3 crystal may be neglected. The morphology of our NaClO3 crystals grown from solutions with K2SO4 additives is very similar to the growth patterns observed by the authors of the work [16] during crystallization of NaClO3 in the presence of Na2SO4. This confirms the fact that the SO24 ions affect the habitus of the crystal. According to the data of atomic emission spectrometry, a NaClO3 crystal has only a trace amount of
200
E.L. Kim et al. / Journal of Crystal Growth 426 (2015) 198–201
Fig. 2. The shape of sodium chlorate crystal grown from (a) pure solution (dimensions of crystal are 15 15 11 mm3) and solution with potassium sulphate added in amounts: (b) 2.0% (12 12 9 mm3), (c) 3% (9 9 6 mm3), (d) 5.2% (13 14 13 mm3).
Fig. 3. Average growth rate of {100}, {111} and {110} faces of NaClO3 crystal as a function of the concentration of K2SO4 in solution. denotes the lower estimated limit of the R110 in pure solution.
sulphate ions in the structure. Thus, we assume that the sulphate ions influence the growth kinetics due to their adsorption on the faces. The decrease of the growth rates with an increase in the
concentration of K2SO4 is evidence of the retardation effect of SO24 ions, and, furthermore, the degree of this effect is different for the faces with different indices. Ristic et al. [23] exploring the growth of NaClO3 in solutions with sodium dithionate Na2S2O6 observed unequal action of this impurity on the {111} and f111g faces. The strong decelerating effect of sodium dithionate on the growth of the f111g was explained by the structural similarity of the ClO3 ion and the 2 two SO3 tetrahedra of the S2 O6 ion. One SO3 tetrahedron of S2 O26 substituted the ClO3 ion, and the other SO3 part protruded from the surface impeding the formation of the next growth layer. The difference in the inhibition action of SO24 ions on different faces of NaClO3 crystal may be understood by considering the atomic structure of their surfaces and the geometry of sulphate ion. It follows from the structure of NaClO3 and K2SO4 crystals that the ClO3 and SO24 ions have a dimensional conformity. The ClO3 ion in the crystal structure is a trigonal pyramid that has a Cl5 þ ion at the vertex and three oxygen atoms at the base. The base of this pyramid is a regular triangle with a side of 2.38 Å. The distance between the chlorine and oxygen atoms are equal to 2.48 Å. The SO24 anion is a tetrahedron with a S6 þ ion in the center and oxygen atoms at the corners. The S6 þ ion is 1.47 Å equidistant from the faces of the tetrahedron. Every oxygen triangle of the tetrahedron has sides equal to 2.38, 2.41 and 2.43 Å. These values
E.L. Kim et al. / Journal of Crystal Growth 426 (2015) 198–201
201
Fig. 4. Atomic structure of the surface of the (a) cubic {100}, (b) tetrahedral {111} and (c) rhomb-dodecahedral {110} faces of sodium chlorate crystal.
5. Conclusions
Fig. 5. The layers of the tetrahedral {111} face of sodium chlorate crystal.
are very close to the lengths of the base of the ClO3 pyramid. Since the oxygen triangles of the SO24 and ClO3 ions are similar, the sulphate ion that is near the crystal in the solution can mimic the chlorate ion and may substitute it by depositing on the surface. The surface of the {100} face of a sodium chlorate crystal is formed by densely packed rows of alternate Na þ and ClO3 ions (Fig. 4a). As this face is electrically neutral, the adsorption of the sulphate ion is most probable at a kink. Obviously, the tetrahedron of this anion prevents deposition of the successive Na þ ion. The SO24 anion is not embedded into the lattice because it has the fourth oxygen atom. The SO24 anion is displaced from the kink by the crystallizing units of the crystal. Parallel to the {111} face, there exist layers consisting either entirely of Na þ , or of ClO3 ions (Fig. 5). The Na þ ions (Fig. 4b) emerge on the surface of a positive tetrahedron {111}, and the ClO3 anions form the surface of a negative tetrahedron f111g. The positive charge of the {111} face enhances adsorption of SO24 ions which can be attached to any site due to the electrostatic interaction and form a complete film on the surface. On the contrary, adsorption of SO24 on the f111g face is hindered because of the electrostatic repulsion. SO24 ions will not noticeably affect the growth of the negative tetrahedron, hence, these faces are not present in the crystal habitus [16,21]. Sulphate ions retard the growth of the {111} faces stronger than of the cubic faces. As regards the structure of the {110} face, Na þ ions and ClO3 anions form its surface. But within an elementary cell, there are two sodium ions and one ClO3 chlorate ion on the surface (Fig. 4c). Thus, like the tetrahedral face, the rhomb-dodecahedral face is not electrically neutral. The excess of positive charge increases adsorption energy of SO24 ions. Moreover, the {110} face is the least densely packed in comparison with the {100} and {111} faces of NaClO3 crystal. These two features simplify attachment of SO24 ions. Thus, the impact of SO24 ions on the growth of the {110} face should be greater than that for the {111} face. It was exactly what we observed in our experiments.
The morphology of NaClO3 crystals changes from cubic to tetrahedral in solutions with K2SO4 additives. Also, the {110} faces appear in the growth pattern of crystal. The growth rates of the {100}, {111} and {110} faces decelerate with an increase in the concentration of SO24 . SO24 ions retard growth due to their adsorption on the crystal surface. The rhomb-dodecahedral face is the most and the cubic face is the least inhibited by adsorbing sulphate ions. Different atomic structure and charge state of different crystal faces determine their sensitivity to the action of the SO24 ions.
References [1] N. Cabrera, D.A. Vermilyea, Growth and Perfection of Crystals, John Wiley & Sons, Inc., N.Y. (1958) 393–408, London: Chapman & Hall, Lim.. [2] N. Kubota, Cryst. Res. Technol. 36 (2001) 749–769. [3] S Yu. Potapenko, J. Cryst. Growth 133 (1993) 141–146. [4] H. Miura, K. Tsukamoto, Cryst. Growth Des. 13 (2013) 3588–3595. [5] G.M. Bliznakov, Kristallographiya 4 (1959) 150–156. [6] H.E. Buckley, Crystal Growth, Wiley, New York (1951) 571, London: Chapman & Hall. [7] D.A. Vorontsov, V.N. Portnov, E.V. Chuprunov, Crystallogr. Rep. 50 (Suppl. 1) (2005) S82–S87. [8] L.N. Rashkovich, N.V. Kronsky, J. Cryst. Growth 182 (1997) 434–441. [9] M.L. Weaver, S.R. Qiu, J.R. Hoyer, W.H. Casey, G.H. Nancollas, J.J. De Yoreo, J. Cryst. Growth 306 (2007) 135–145. [10] G. Fu, S.R. Qiu, C.A. Orme, D.E. Morse, J.J. De Yoreo, Adv. Mater. 17 (2005) 2678–2683. [11] J.J. De Yoreo, P.M. Dove, Science 306 (2004) 1301–1302. [12] D.A. Vorontsov, E.L. Kim, V.N. Portnov, E.V. Chuprunov, Crystallogr. Rep. 48 (2003) 335–338. [13] G. Bolanz, O. Schafer, J. Cryst. Growth 78 (1986) 545–548. [14] G. Pilkington, W.J. Dunning, J. Cryst. Growth 13–14 (1972) 454–457. [15] S. Al-Jibbouri, C. Strege, J.J. Urlich, J. Cryst. Growth 236 (2002) 400–406. [16] G. Blisnakov, E. Kirkova, Z. Physik. Chem. 206 (1957) 271–280. [17] G. Bliznakov, Fortschr. Miner. 36 (1958) 149. [18] A.S. Michaels, W. Frederick Jr., Tausch, J. Phys. Chem. 65 (1961) 1730–1737. [19] V.N. Portnov, Kristallographiya 11 (1966) 916–920. [20] M. Ajitha Sweetly, S. Ramalingam, T. Chithambarathanu, P. Selvarajan, Int. J. Rec. Sci. Res. 3 (12) (2012) 1042–1045. [21] G. Bliznakov, E. Kirkova, Krist. Tech. 4 (1969) 331–336. [22] R.W.G. Wyckoff, The Structure of Crystals, Chemical Catalog Company: John Wiley & Sons, New York (1960) 276. [23] R. Ristic, J.N. Sherwood, K. Wojciechowski, J. Phys. Chem. 97 (1993) 10774–10782. [24] R. Ristic, B. Yu. Shekunov, J.N. Sherwood, J. Cryst. Growth 139 (1994) 336–343. [25] B.M. Misailovic, D.A. Malivuk, A.A. Zekic, M.M. Mitrovic, Cryst. Growth Des. 14 (2014) 972–978. [26] A.V. Belustin, N.S. Stepanova, Kristallographiya 10 (1965) 743–745. [27] A.N. Kirgintsev, L.N. Trushnikova, V.G. Lavrentieva, Solubility of Inorganic Substances in Water, Handbook of Chemistry, 1972. [28] P.W.J.M. Boumans, J.J.A.M. Vrakking, Spectrochim. Acta Part B 42 (1987) 819–840.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of sodium chlorate crystals in the presence of potassium sulphate E.L. Kim, A.A. Tsyganova, D.A. Vorontsov n, T.I. Ovsetsina, M.R. Katkova, V.A. Lykov, V.N. Portnov Lobachevsky State University of Nizhny Novgorod, Gagarin Ave., 23, Nizhny Novgorod 603950, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 12 March 2015 Received in revised form 15 June 2015 Accepted 16 June 2015 Communicated by: S.R. Qiu Available online 24 June 2015
In this work, we investigated the morphology and growth rates of NaClO3 crystals in solutions with K2SO4 additives. NaClO3 crystals were grown using the temperature gradient technique under concentration convection. We found that the crystal habitus changed from cubic to tetrahedral, and the growth of the cubic {100}, tetrahedral {111} and rhomb-dodecahedral {110} faces decelerated with an increase in the concentration of SO24 ions. The {110} face was the most and the {100} face was the least inhibited by sulphate ions. The mechanism of SO24 ions action is their adsorption on the crystal surface, which impedes attachment of the crystal's building units. We conclude that different atomic structure and charge state of various crystal faces determine their sensitivity to the action of the SO24 ions. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal morphology A1. Impurities A2. Growth from solutions A2. Single crystal growth B1. Sodium chlorate B2. Nonlinear optic materials
1. Introduction The impact of impurities, or additives, on the crystallization in aqueous solutions has been intensively studied both in theoretical [1– 5] and experimental [6–15] work, as some compounds added even in a very small amount can essentially modify growth rate, properties and quality of growing crystals. Impurity is adsorbed from the bulk solution to the surface of the crystal; therefore, it can decelerate its growth either by blocking kinks on the face or edges on the steps [16,17], or by formation of an impurity film on the face [18,19]. The influence of different additives on the growth of non-linear optical crystals is worth studying. Thus, sodium chlorate NaClO3 crystals can be used as a new non-linear optical material for the second harmonic generation [20]. These crystals belong to a cubic syngony and have a space group P213. In the work [16], the change of the habitus of sodium chlorate crystals from cubic to tetrahedral was observed in the presence of Na2SO4 additive. The modification of the crystal shape was explained by unequal retardation of the cubic {100} and tetrahedral {111} faces during adsorption of the sulphate ions SO24 in the kinks of the steps [21]. The positive {111} and negative {111} tetrahedral faces of a NaClO3 crystal symmetrically are not identical because the {111} surfaces are terminated by a layer of Na þ cations whereas the {111} faces have ClO3 anions n
Corresponding author. Tel.: þ 7 9101316701. E-mail address: [email protected] (D.A. Vorontsov).
http://dx.doi.org/10.1016/j.jcrysgro.2015.06.016 0022-0248/& 2015 Elsevier B.V. All rights reserved.
on the surface [22]. In [23] it was found that sodium dithionate Na2S2O6 caused appearance of faces of a negative tetrahedron {111} on the shape of NaClO3 crystals. The impact of other sulfates, including potassium sulphate, on the growth of NaClO3 crystals in aqueous solutions has not been mentioned in the literature to the best of our knowledge. The authors of [24] directly observed by means of the interference technique the surface of a positive and negative tetrahedron in solution. They revealed that the {111} faces developed by the spiral dislocation mechanism, whereas the {111} faces grew as an atomically rough surface. The growth rates of the {100} and {110} faces in pure solution were measured in the study [23]. Misailovic et al. [25] investigated statistical distribution of the growth rates for the growing and nongrowing {100} faces of small NaClO3 crystals in supersaturated pure solutions. Nevertheless, we have not found any data on the growth rates of the {110} faces of NaClO3 crystal in solutions with the additives of SO24 ions. The goal of this study is investigation of the influence of potassium sulphate K2SO4 on the morphology of NaClO3 crystals and growth rates of various faces, as well as clarification of the inhibition mechanism of this additive.
2. Experimental technique The solutions were prepared from analytical grade sodium chlorate NaClO3, distilled water and additions of analytical grade
E.L. Kim et al. / Journal of Crystal Growth 426 (2015) 198–201
potassium sulphate K2SO4 in the amount of 2.1–5.2 wt%. At first, small NaClO3 crystals up to 3 mm in size were obtained in a supercooled solution in a Petry dish. Then, the crystals were used as seeds in the following experiments. Large single crystals of NaClO3 were grown by means of a temperature gradient technique with feeding in a concentration convection regime [26] at a temperature of 23.5 1C and constant solution supercooling of 1.5 1C. A crystallization vessel with a volume of 100 mL was placed inside an air thermostat. A seed crystal was attached to a glass rod connected to a vessel lid. A special container with feeding crystals was located between the seed and the lid. Crystal growth was possible due to a temperature gradient between the two zones having the growing crystal and feeding crystals. When the crystal grew, concentration convection appeared in the vessel, and depleted solution near the growing crystal was replaced by the solution which had a higher density and went down from the area with feeding crystals. The solubility of NaClO3 in water was calculated using the data of [27]: CðTÞ ¼ 79:99 þ0:67 U T þ 0:0057 U T2 g=100 mL H2 O : The average growth rates of the faces were calculated on the basis of the total growth time and the sizes of the crystals obtained at specified concentrations of K2SO4. We used in calculations all the faces present on the shape of the grown crystal. The total increment of every face was found as along the normal from the face plane to the center of the seed that was clearly visible inside the grown crystal. Measurements were performed by an optical microscope with an accuracy of 0.1 mm. Trace analysis of solution samples with a volume of 4 5 mL was performed by atomic emission spectrometry with the use of a Varian Agilent 720-ES spectrometer. The calibration was done by a standard addition method. The detection limit of sulfur atoms in a sample was 40 μg/L at a 181.972 nm line [28].
3. Results 3.1. Solubility data We revealed that the solubility of NaClO3 in water decreased in the presence of K2SO4 in the solution and there appeared precipitation, which consisted exclusively of NaClO3. We measured the weight of the precipitation to determine the decrease of the solubility ΔCNaClO3 calculated as a difference of the NaClO3 concentrations before and after the addition of K2SO4 at a solution saturation temperature of 25 1C (Fig. 1). The decrease of the saturation temperature of the solution with K2SO4 of 5.2 wt% due to the decrease of the solubility of NaClO3 was negligible, less than 0.02 1C.
199
Fig. 1. The decrease of the equilibrium concentration of NaClO3 at a saturation temperature of 25 1C as a function of the amount of K2SO4 added to the solution.
3.3. Morphology and growth rates The sodium chlorate single crystals grown from the solution without K2SO4 additive had a cubic shape with weakly pronounced faces of a positive tetrahedron {111} (Fig. 2a). We observed the development of the {111} faces and appearance of the rhomb-dodecahedral {110} faces in the habitus of the crystals (Fig. 2b) during growth in the solution containing 2% of K2SO4. Part c of Fig. 2 shows that with a further increase in the concentration of K2SO4 up to 3%, the {111} and {110} faces become more pronounced. When the amount of K2SO4 in the solution reached 5.2%, NaClO3 crystals with a tetrahedral habitus were formed (Fig. 2d). All the grown crystals were transparent, without visible cracks and inclusions of solution. The presence of faces with definite indices in the crystal shape depends on the ratio of their growth rates. The slower the face grows, the larger area it occupies in the crystal growth pattern. From the geometrical considerations it follows that the crystal of cubic symmetry will have {111} and {110} faces together with {100} faces when the ratios of their velocities satisfy the inequalities R{111}/R{100} o31/2 and R{110}/R{100} o21/2, respectively. The data of Fig. 3 demonstrate that the growth rates of the {100}, {111} and {110} faces decrease with an increace in the concentration of K2SO4. R{111}/R{100} ¼1.35 in pure solution, and the cubic faces of NaClO3 crystals are the most developed (Fig. 2a). We have not observed appearance of the rhomb-dodecahedral {110} faces in pure solution at all, and therefore, we assume that their growth rate exceeds 1.51/2 R{111}, or 28 10 6 mm/s (labeled by in Fig. 3) in this case. When the concentration of K2SO4 was equal to 2.5%, the curves for the growth rates of the cubic and tetrahedral faces intersected (Fig. 3), and the habitus changed from cubic to tetrahedral (Fig. 2a–d).
4. The mechanism of the influence of K2SO4 3.2. Atomic emission spectrometry The solutions prepared from the crystals grown in pure and 2% K2SO4 doped solution of sodium chlorate were taken for analysis to test the presence of sulphate ions by atomic emission spectrometry. The crystals were dissolved in deionized water with a resistance of 18.2 MOhm. The presence of sulphate ions in the solutions was detected by analyzing the 180.669, 181.972 and 182.562 nm sulfur lines. A possible content of the sulfur atoms in all samples and, consequently, in the structure of the NaClO3 crystal was smaller than 40 mg/L detection limit.
A possible decrease of supercooling owing to the decrease of NaClO3 solubility, the basic compound, in the presence of K2SO4 is negligible. Hence, we consider that the influence of this solubility effect on the growth rate of NaClO3 crystal may be neglected. The morphology of our NaClO3 crystals grown from solutions with K2SO4 additives is very similar to the growth patterns observed by the authors of the work [16] during crystallization of NaClO3 in the presence of Na2SO4. This confirms the fact that the SO24 ions affect the habitus of the crystal. According to the data of atomic emission spectrometry, a NaClO3 crystal has only a trace amount of
200
E.L. Kim et al. / Journal of Crystal Growth 426 (2015) 198–201
Fig. 2. The shape of sodium chlorate crystal grown from (a) pure solution (dimensions of crystal are 15 15 11 mm3) and solution with potassium sulphate added in amounts: (b) 2.0% (12 12 9 mm3), (c) 3% (9 9 6 mm3), (d) 5.2% (13 14 13 mm3).
Fig. 3. Average growth rate of {100}, {111} and {110} faces of NaClO3 crystal as a function of the concentration of K2SO4 in solution. denotes the lower estimated limit of the R110 in pure solution.
sulphate ions in the structure. Thus, we assume that the sulphate ions influence the growth kinetics due to their adsorption on the faces. The decrease of the growth rates with an increase in the
concentration of K2SO4 is evidence of the retardation effect of SO24 ions, and, furthermore, the degree of this effect is different for the faces with different indices. Ristic et al. [23] exploring the growth of NaClO3 in solutions with sodium dithionate Na2S2O6 observed unequal action of this impurity on the {111} and f111g faces. The strong decelerating effect of sodium dithionate on the growth of the f111g was explained by the structural similarity of the ClO3 ion and the 2 two SO3 tetrahedra of the S2 O6 ion. One SO3 tetrahedron of S2 O26 substituted the ClO3 ion, and the other SO3 part protruded from the surface impeding the formation of the next growth layer. The difference in the inhibition action of SO24 ions on different faces of NaClO3 crystal may be understood by considering the atomic structure of their surfaces and the geometry of sulphate ion. It follows from the structure of NaClO3 and K2SO4 crystals that the ClO3 and SO24 ions have a dimensional conformity. The ClO3 ion in the crystal structure is a trigonal pyramid that has a Cl5 þ ion at the vertex and three oxygen atoms at the base. The base of this pyramid is a regular triangle with a side of 2.38 Å. The distance between the chlorine and oxygen atoms are equal to 2.48 Å. The SO24 anion is a tetrahedron with a S6 þ ion in the center and oxygen atoms at the corners. The S6 þ ion is 1.47 Å equidistant from the faces of the tetrahedron. Every oxygen triangle of the tetrahedron has sides equal to 2.38, 2.41 and 2.43 Å. These values
E.L. Kim et al. / Journal of Crystal Growth 426 (2015) 198–201
201
Fig. 4. Atomic structure of the surface of the (a) cubic {100}, (b) tetrahedral {111} and (c) rhomb-dodecahedral {110} faces of sodium chlorate crystal.
5. Conclusions
Fig. 5. The layers of the tetrahedral {111} face of sodium chlorate crystal.
are very close to the lengths of the base of the ClO3 pyramid. Since the oxygen triangles of the SO24 and ClO3 ions are similar, the sulphate ion that is near the crystal in the solution can mimic the chlorate ion and may substitute it by depositing on the surface. The surface of the {100} face of a sodium chlorate crystal is formed by densely packed rows of alternate Na þ and ClO3 ions (Fig. 4a). As this face is electrically neutral, the adsorption of the sulphate ion is most probable at a kink. Obviously, the tetrahedron of this anion prevents deposition of the successive Na þ ion. The SO24 anion is not embedded into the lattice because it has the fourth oxygen atom. The SO24 anion is displaced from the kink by the crystallizing units of the crystal. Parallel to the {111} face, there exist layers consisting either entirely of Na þ , or of ClO3 ions (Fig. 5). The Na þ ions (Fig. 4b) emerge on the surface of a positive tetrahedron {111}, and the ClO3 anions form the surface of a negative tetrahedron f111g. The positive charge of the {111} face enhances adsorption of SO24 ions which can be attached to any site due to the electrostatic interaction and form a complete film on the surface. On the contrary, adsorption of SO24 on the f111g face is hindered because of the electrostatic repulsion. SO24 ions will not noticeably affect the growth of the negative tetrahedron, hence, these faces are not present in the crystal habitus [16,21]. Sulphate ions retard the growth of the {111} faces stronger than of the cubic faces. As regards the structure of the {110} face, Na þ ions and ClO3 anions form its surface. But within an elementary cell, there are two sodium ions and one ClO3 chlorate ion on the surface (Fig. 4c). Thus, like the tetrahedral face, the rhomb-dodecahedral face is not electrically neutral. The excess of positive charge increases adsorption energy of SO24 ions. Moreover, the {110} face is the least densely packed in comparison with the {100} and {111} faces of NaClO3 crystal. These two features simplify attachment of SO24 ions. Thus, the impact of SO24 ions on the growth of the {110} face should be greater than that for the {111} face. It was exactly what we observed in our experiments.
The morphology of NaClO3 crystals changes from cubic to tetrahedral in solutions with K2SO4 additives. Also, the {110} faces appear in the growth pattern of crystal. The growth rates of the {100}, {111} and {110} faces decelerate with an increase in the concentration of SO24 . SO24 ions retard growth due to their adsorption on the crystal surface. The rhomb-dodecahedral face is the most and the cubic face is the least inhibited by adsorbing sulphate ions. Different atomic structure and charge state of different crystal faces determine their sensitivity to the action of the SO24 ions.
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