Features of Phase Formation in the CsOH–H2SO4–H3PO4–H2O System and the Growth of the Cs6(SO4)3(H3PO4)4 Crystals
Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences, Moscow 119333, Russia
*
Author to whom correspondence should be addressed.
Crystals 2023, 13(8), 1163; https://doi.org/10.3390/cryst13081163
Submission received: 15 June 2023
/
Revised: 21 July 2023
/
Accepted: 25 July 2023
/
Published: 26 July 2023
(This article belongs to the Special Issue Recent Developments of Crystallography in RAS (Russian Academy of Sciences))
Abstract
:In this paper, the conditions of phase formation in the system CsOH–H2SO4–H3PO4–H2O are considered for the first time. The phase formation of Cs6(SO4)3(H3PO4)4 at t = 50 °C has been studied extensively. The main concentration boundary conditions for this compound are considered for the first time. The solubility congruence of Cs6(SO4)3(H3PO4)4 is shown. Conditions and approaches for obtaining crystals by isothermal evaporation and saturated solution temperature reduction methods are considered. The results of obtaining Cs6(SO4)3(H3PO4)4 crystals with maximum dimensions of ~20 mm are presented.
1. Introduction
The consideration of the phenomenon of proton transport in the condensed state is probably impossible without considering such objects as crystals, and in particular, superprotonics. This group includes crystals of acid salts with the general formula MmHn(AO4)(m+n)/2·xH2O (M = NH4, K, Rb, Cs; A = S, Se, P, As).
Superprotonic crystals attract the attention of researchers primarily due to the presence of order/non-order phase transitions in them. Such phase transitions occur at temperatures before melting and are accompanied by cardinal restructuring with an increase in the symmetry of the crystal. As a consequence of this restructuring, conditions for a directed proton diffusion in the crystal volume arise. Thus, during the phase transition, the conductivity in superprotonic crystals abruptly increases by several orders of magnitude and reaches values comparable with the conductivity in the melt (more than 10−3 Ω−1cm−1).
Experimentally, this phenomenon was first observed in [1,2]. By analogy with superionic phase transitions, such transitions have been termed superprotonic. At the same time, for the vast majority of the crystals belonging to the mentioned group, this phase transition is accompanied by an increase in the overall symmetry of the crystal [3].
The combination of high proton conductivity in solid state with moderate temperatures of phase transitions determines the prospects for the application of superprotonic crystals as proton-exchange membrane materials for various electrochemical devices [4,5,6].
Complex acidic cesium sulfate phosphates, along with CsHSO4 and CsH2PO4, belong to the family of superprotonic crystals. The results of the first systematic searching investigations of cesium sulfate phosphates are described in a series of works [7,8,9,10]. In these works, attention was mainly placed onto the structures of compounds and their conducting properties without the proper consideration of the conditions for obtaining crystals through a reproducible process. The study of phase formation in the system CsHSO4–CsH2PO4 from aqueous solutions [11] allowed us to obtain more reliable results. In particular, it was shown that the compounds Cs4(HSO4)3(H2PO4) and Cs3(HSO4)2(H2PO4) are separate double salts [12], although are very similar in structure, but are not the extreme members of the series of solid solutions as indicated in [8]. In addition, the existence of the compound Cs5(HSO4)3(H2PO4)2 [13] was not confirmed. At the same time, the crystal Cs6H(HSO4)3(H2PO4)4 [14], formally not belonging to the CsHSO4–CsH2PO4 system, was found. Subsequently, thanks to precision structural studies, it was found that a more correct structural formula for it would be Cs6(SO4)3(H3PO4)4 [15].
One of the features of the Cs6(SO4)3(H3PO4)4 crystal is that it is the only superprotonic crystal that is formed from an aqueous solution in a non-conducting phase in cubic syngony. The Cs6(SO4)3(H3PO4)4 crystal in the low-temperature state belongs to the I3d symmetry group with parameter a = 14.5399 (1) Å, Z = 4. In this structure, a hydrogen bond network is formed between SO4 and PO4 tetrahedra, in which the protons on the O–H...O bonds are arranged in one-minimum potentials. Such hydrogen bonding systems are typical of most superprotonic crystals, but in the Cs6(SO4)3(H3PO4)4 crystal, this hydrogen bonding system is three-dimensional even in the low-temperature state.
In addition, this crystal also exhibits a very moderate superprotonic phase transition temperature [16].
Studying the crystal structure of Cs6(SO4)3(H3PO4)4 and the peculiarities of its superprotonic phase transition would be impossible without obtaining single crystals of suitable quality. The conditions for obtaining single-crystal samples of Cs6(SO4)3(H3PO4)4 have not yet been described in detail. This is to be corrected in the present work.
Due to the fact that the crystal Cs6(SO4)3(H3PO4)4 is formed in a complex system (CsOH–H2SO4–H3PO4–H2O), the consideration of the concentration conditions of this crystal’s crystallization is impossible when in isolation from the features of phase formation in this system.
Thus, the present work is devoted to: the consideration of some features of phase formation in the system CsOH–H2SO4–H3PO4–H2O, the determination of the concentration boundary conditions of Cs6(SO4)3(H3PO4)4 crystal formation, and the conditions of its reproducible obtaining.
2. Experimental Part
To obtain single crystals of Cs6(SO4)3(H3PO4)4, we used cesium hydrosulfate (CsHSO4, Rare Metals Factory LLC, Novosibirsk, Russia, 98+%) and cesium dihydrogen orthophosphate (CsH2PO4, Rare Metals Factory LLC, 99%) without additional purification, as well as 85% phosphoric acid (H3PO4, Acros Organics, 85+%, extra pure, SLR). In order to study phase formation, we used sulfuric acid (H2SO4, Chimmed Group, 98% extra pure) and cesium sulfate (Cs2SO4, Rare Metals Factory LLC, 98+%) in addition to the listed reagents.
The interaction between the components in the CsHSO4–CsH2PO4–H2O system was investigated by the isothermal method under equilibrium conditions at 25, 50, and 75 °C. The temperature was maintained constant with the aid of a thermostat; the equilibrium time was 24 h. The equilibrium time in the solution was found by comparing the results of analysis on a cesium cation for samples selected with a step of 6 h.
Solubility isotherms were plotted by the Gibbs–Rosebom method, and the solid-phase composition was determined by the Schreinemakers method. Weights of materials were kept in a thermostat in round-bottom flasks 50 mL in volume; their mixing was implemented by the thermostat mixer drive (rocking gimbal).
Sampling of the liquid phase and solid residue was performed from the flasks located in the thermostat. At 25 °C, the liquid phase was sampled by a single-channel feeder with a removable plastic tip directly from the crystallizer. At 50 and 75 °C, the liquid phase was sampled in a similar way, but the removable plastic tip was previously heated to the thermostat temperature in this case. For each weight, solid residue samples were divided into two approximately equal parts so as to provide two solid residue samples for each liquid phase sample.
When determining the equilibrium time, the solubility of the compounds was analyzed by measuring (using the weighting method) the content of cesium (in the form of tetraphenylborates) in the saturated solution [17]. The content of phosphorus ( anion) in the form of yellow phosphorus-molybdenum-vanadium heteropolyacid was determined by spectrophotometry [18].
The interaction between the components in the Cs2SO4–H2SO4–H2O system was investigated similarly to the previous system at a temperature of 50 °C. For the analytical determination of component concentration, in addition to the procedure for the weight determination of cesium content, we used the weight determination of sulfate anion content according to the method described in [19].
X-ray diffraction analysis of polycrystalline samples and single crystals was performed at ambient temperature using Rigaku MiniFlex 600 diffractometer (Japan) (Cu Kα source, continuous scanning mode—2 deg/min, in the 2θ range between 10 and 70° with an effective step size of 0.02°, no sample rotation, in the environment atmosphere).
Crystal growth was carried out by two methods: isothermal evaporation and controlled decrease in the temperature of the saturated solution.
The method of isothermal evaporation involved the preparation of a deliberately unsaturated solution, followed by a slow evaporation of the solvent and spontaneous formation of crystals. This method was used to obtain seed crystals.
The method of controlled decrease in the solution temperature aimed at obtaining a solution with a saturation temperature higher than room temperature. Then, a seed crystal was added to this solution, and the temperature of the solution was lowered in a controlled manner according to the corresponding program. The process was carried out in a sealed glass crystallizer with reverse stirring. The seed crystal was placed on a fixed platform.
3. Results and Discussion
3.1. Variety of Crystals in the CsOH–H2SO4–H3PO4–H2O System
It is rather challenging to observe phase equilibria in the CsOH–H2SO4–H3PO4–H2O four-component system, especially in the area with a high content of CsOH. Before proceeding to the description of the phase formation of the Cs6(SO4)3(H3PO4)4 crystal, it is necessary to examine the phase formation in the CsOH–H2SO4–H2O and CsOH–H3PO4–H2O systems at least schematically.
The system CsOH–H2SO4–H2O in the Cs2SO4–H2SO4–H2O interval, studied by us in [20], contains the Cs2SO4, Cs5H3(SO4)2⸱yH2O (where y = 0–0.5) and CsHSO4 phases (listed according to the increasing proportion of H2SO4). The listed compounds are characterized by a congruent nature of solubility and fairly wide ranges of crystallization in mole percentages (Table 1, Figure 1).
The CsOH–H3PO4–H2O system is somewhat more difficult to analyze and display. In this system, we take into account the presence of the following compounds: Cs3PO4·4H2O, Cs2HPO4·H2O, CsH2PO4, and CsH5(PO4)2 (listed according to the increasing proportion of H3PO4). We managed to obtain the last two compounds from this list in the form of fairly large single crystals. These phosphates, as well as cesium sulfates, are congruently soluble and are characterized by rather wide crystallization intervals.
Immediately in the space of the CsOH–H2SO4–H3PO4–H2O four-component system, we studied the CsHSO4–CsH2PO4–H2O cross-section in great detail [11] (Figure 2, Table 2). Displaying this part of the phase diagram of the CsOH–H2SO4–H3PO4–H2O system is also complicated by the fact that, depending on the temperature, a different set of crystallization ranges for various compounds is observed in it. For instance, the Cs2(HSO4)(H2PO4) phases known from [9] and Cs3(HSO4)2(H2PO4) [11] crystallize in a limited temperature range. The Cs2(HSO4)(H2PO4) compound crystallizes from a stoichiometric solution (CsHSO4:CsH2PO4 = 1:1) at a temperature below room temperature (about 12 °C). The Cs3(HSO4)2(H2PO4) compound crystallizes from a solution of the same composition in the temperature range of 15–40 °C, showing an incongruent nature of solubility. Finally, the Cs6(SO4)3(H3PO4)4 compound crystallizes from the solution of the same composition at a temperature above 45 °C.
The above analysis indicates the wide variety of known compounds formed in the CsOH–H2SO4–H3PO4–H2O system. Therefore, the correct display of all possible elements of the system (primarily the points of nonvariant equilibria, co-crystallization lines, solubility values) is quite complicated. Thus, in order to observe the crystallization conditions for a particular Cs6(SO4)3(H3PO4)4 compound, some simplifications must be made.
The first simplification of the display of crystallization fields in the system was the choice of a temperature of 50 °C, as this results in containing a smaller number of crystallization fields. The second simplification, which was made within the framework of this observation, was the choice of system components to display phase equilibria in the form of a tetrahedral diagram. For a simplified display of the crystallization phase field of the Cs6(SO4)3(H3PO4)4 compound, we used the following initial components: CsHSO4–CsH2PO4–H3PO4–H2O (Figure 3, Table 3). It is worth mentioning that the chosen initial components are a simplification and not an independent subsystem within a more complete system. This is due to the fact that one of the generatrices of the ternary systems, CsHSO4–CsH2PO4–H2O, contains a phase crystallization branch that does not lie in the concentration plane of this system.
There are five crystallization fields for the compounds in the chosen coordinate system of the phase diagram components: CsHSO4, Cs4(HSO4)3(H2PO4), Cs6(SO4)3(H3PO4)4, CsH2PO4, and CsH5(PO4)2. These regions of crystallization are delimited by a set of lines of monovariant equilibria (co-crystallizations). The main elements of this diagram are described in Table 2.
Thus, in the chosen concentration coordinates, during isothermal evaporation, the Cs6(SO4)3(H3PO4)4 compound is able to co-crystallize with the Cs4(HSO4)3(H2PO4) compound, provided that the solution is lacking in CsH2PO4 and H3PO4 (line E2-3). If CsH2PO4 is in excess, the Cs6(SO4)3(H3PO4)4 compound is expected to co-crystallize with CsH2PO4 (line E3-E5) until the composition of the solution reaches the E5 point of nonvariant equilibrium. At this point, the co-crystallization of three compounds, Cs6(SO4)3(H3PO4)4, CsH2PO4, and CsH5(PO4)2, will occur at once until water evaporates completely. If an excess of CsH5(PO4)2 is created in the solution composition (which is equivalent to an excess of H3PO4), the E5 point can be bypassed, and then co-crystallization will occur between the Cs6(SO4)3(H3PO4)4 and CsH5(PO4)2 compounds (line E5-4). All excesses in this case refer to the stoichiometric ratio of components relative to the Cs6(SO4)3(H3PO4)4 compound.
It should be mentioned that line 1-2-3-4-5 shown in Figure 3 and Table 2 is relative and was not determined experimentally. This is due to the complexity of conducting such an experiment (it is required to dissolve weighed portions of substances in a 100% H3PO4 melt in a dried atmosphere). At the same time, the results of such an experiment will not fundamentally affect the growth conditions of the Cs6(SO4)3(H3PO4)4 crystal.
A more complete description of the crystallization surface of the Cs6(SO4)3(H3PO4)4 compound (for instance, in the Cs5H3(SO4)4–CsH2PO4–H3PO4–H2O or Cs2SO4–CsH2PO4–H3PO4–H2O coordinates) will require the determination of the concentration coordinates of a far higher number of nonvariant points. For example, near the line of Cs5H3(SO4)4–CsH2PO4 compositions, there may be points of co-crystallization of the Cs5H3(SO4)4, Cs4(HSO4)3(H2PO4) and Cs2SO4 phases, and only at 50 °C. Such a study is certainly beyond the scope of this work.
3.2. Growing a Cs6(SO4)3(H3PO4)4 Crystal from Water Solutions
The Cs6(SO4)3(H3PO4)4 compound crystallizes in a fairly wide phase area in the CsOH–H2SO4–H3PO4–H2O system. In addition, this compound has a congruent solubility pattern. These properties make the Cs6(SO4)3(H3PO4)4 crystal suitable for obtaining by lowering the temperature of a saturated solution. The disadvantage of the method of temperature lowering is, albeit positive, a rather weak temperature dependence of solubility. In particular, a decrease in the temperature of the solution in the range of 75–25 °C leads to a decrease in solubility by only ~4.5% mass. This solubility difference is equivalent to ~12 g of crystal in a 100 mL volume of solution. At the same time, at least 200 g of initial reagents will be required to prepare such a solution. That is, the efficiency of crystallization in relation to the used salt mass is no more than 10–15%.
It is also noteworthy that the resulting solutions have a very high density, so high that it is necessary to use a metal washer sealed in plexiglass as a platform for placing the seed crystal, because the fluoroplastic platform floats in this solution. Nevertheless, the Cs6(SO4)3(H3PO4)4 crystal itself does not float in this solution. Given that the density of the fluoroplastic is ~2.2 g/cm3, and the X-ray density of the crystal according to [15] is 3.191 g/cm3, the density of the solution lies in the range of values 2.2–3.191 g/cm3.
A significant difficulty may be the need for a strict observance of the stoichiometric ratio of components in a rather complex solution. A saturated solution must contain a strict ratio of cesium cations to sulfate and phosphate anions, and all this at a fairly high level of acidity. If the stoichiometry is not observed, the composition of the solution during crystal growth (especially by the method of isothermal evaporation) will shift toward the co-crystallization lines mentioned in the previous section.
This difficulty can be overcome by using reliable weight forms of the initial reagents when preparing a saturated solution. These requirements are met by two formal synthesis reactions:
3CsHSO4 + 2CsH2PO4 + CsH5(PO4)2 → Cs6(SO4)3(H3PO4)4
Cs2SO4 + 2CsHSO4 + 2CsH5(PO4)2 → Cs6(SO4)3(H3PO4)4
The result of growth from the solution prepared in this way is in the form of crystals, an example of which is shown in Figure 4.
The sample in Figure 4a in particular was obtained as a result of growth on a stationary platform with reverse stirring, from a saturated solution with a volume of ~200 mL in the temperature range of 45–40 °C. As a seed crystal, we used a small crystal obtained by spontaneous crystallization upon the evaporation of a solution at room temperature (an example is shown in Figure 4b). The resulting sample was characterized by a maximum size of ~20 mm and a weight of 3 g.
The single crystal obtained in this way is characterized by the X-ray diffractogram shown in Figure 5.
The Cs6(SO4)3(H3PO4)4 crystal belongs to the cubic system, with the space group I3d, which determines the equilibrium faceting of the crystal in the form of a trigontritetrahedron formed by the {hhl} family of faces. Noteworthy is the dependence of the ratio of the face areas within this simple form on the rate of obtaining a crystal. When obtaining a sample by methods of a temperature decrease in a saturated solution, i.e., in relatively short periods of time, the edges of the tetrahedron are clearly formed. In the case of crystal growth at a constant temperature, by a slow evaporation of the solvent over several months, the shape of the crystal approaches a spheroid (Figure 4b).
4. Conclusions
By summarizing the above results, we can conclude the following.
It is rather challenging to observe the overall concept of phase formation in the CsOH–H2SO4–H3PO4–H2O system. It is much easier to study the crystallization range of a particular compound of interest using simplifying sections. The CsOH–H2SO4–H3PO4–H2O system itself contains many different complex phosphate–sulfate, and no solid solutions were found in it.
It has been shown that Cs6(SO4)3(H3PO4)4 compound can be obtained in single-crystal form from water solutions. The methods of isothermal evaporation and lowering the temperature of the saturated solution have their advantages and disadvantages.
In the case of the isothermal evaporation method, the advantage is the ease of implementation and the completeness of the use of the initial reagents. The disadvantage, however, is the high sensitivity of the composition of the solution to deviation from stoichiometry and the likelihood of co-crystallization with other compounds.
The method of lowering the temperature of the saturated solution is attractive by maintaining the equilibrium composition of the mother solution throughout the entire growth process. Moreover, this method allows controlling the growth rate of the crystal and, accordingly, its quality and cut more accurately. The disadvantage of the method is the low efficiency of the use of initial reagents. Indeed, to obtain crystals weighing several tens (or hundreds) of grams, it is necessary to use hundreds (or thousands) of grams of initial reagents.
It is worth mentioning that there is a method of aqueous crystal growth that combines some of the advantages of both methods discussed above. This is a method of isothermal evaporation with feeding mother solution. It is also possible to obtain large crystals of good quality by this method while using the initial reagents more efficiently. This method alone is much more complex in instrumentation and is still sensitive to deviations from stoichiometry.
In general, the choice of the method for growing Cs6(SO4)3(H3PO4)4 crystals primarily depends on the purpose of their use. Small crystals of high diffraction quality for X-ray and synchrotron radiation experiments can be obtained by simple evaporation. For neutron diffraction or the study of physical properties, where larger crystals are needed, single crystal samples can be grown by lowering the temperature of a saturated solution.
Author Contributions
Methodology, V.A.K.; Investigation, V.A.K., I.P.M. and I.S.T.; Writing—original draft, V.A.K.; Writing—review & editing, I.P.M., I.S.T. and V.V.G.; Visualization, V.A.K. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Acknowledgments
This work was performed within the State Assignment of FSRC “Crystallography and Photonics” RAS.
Conflicts of Interest
The authors declare no conflict of interest.
References
- Ramasastry, C.; Ramaiah, K.S. Electrical conduction in Na3H(SO4)2 and (NH4)3H(SO4)2 crystals. J. Mater. Sci. 1981, 16, 2011–2016. [Google Scholar] [CrossRef]
- Baranov, A.I.; Shuvalov, L.A.; Shchagina, N.M. Superion conductivity and phase transitions in CsHSO4 and CsHSeO4 crystals. JETP Lett. 1982, 36, 459–462. [Google Scholar]
- Baranov, A.I. Crystals with disordered hydrogen-bond networks and superprotonic conductivity. Review. Crystallogr. Rep. 2003, 48, 1012–1037. [Google Scholar] [CrossRef]
- Haile, S.M.; Boysen, D.A.; Chrisholm, C.R.I.; Merle, R.V. Solid acids as fuel cell electrolytes. Nature 2001, 410, 910–913. [Google Scholar] [CrossRef] [PubMed] [Green Version]
- Norby, T. The promise of protonics. Nature 2001, 410, 877–878. [Google Scholar] [CrossRef] [PubMed]
- Fitzergerald, R. Solid acids show potential for fuel cell electrolytes. Phys. Today 2001, 54, 22–24. [Google Scholar] [CrossRef]
- Haile, S. Superprotonic conductivity in Cs3(HSO4)2(H2PO4). Solid State Ion. 1995, 77, 128–134. [Google Scholar] [CrossRef]
- Haile, S.M.; Calkins, P.M.; Boysen, D. Superprotonic conductivity in β-Cs3(HSO4)2(Hx(P,S)O4). Solid State Ion. 1997, 97, 145–157. [Google Scholar] [CrossRef]
- Chisholm, C.R.I.; Haile, S.M. Superprotonic behavior of Cs2(HSO4)(H2PO4)—A new solid acid in the CsHSO4-CsH2PO4 system. Solid State Ion. 2000, 136, 229–241. [Google Scholar] [CrossRef]
- Yamane, Y.; Yamada, K.; Inoue, K. Superprotonic solid solutions between CsHSO4 and CsH2PO4. Solid State Ion. 2008, 179, 483–488. [Google Scholar] [CrossRef]
- Komornikov, V.A.; Grebenev, V.V.; Makarova, I.P.; Selezneva, E.V.; Andreev, P.V. Production of Complex Rubidium and Cesium Hydrogen Sulfate-Phosphates. Crystallogr. Rep. 2016, 61, 665–671. [Google Scholar] [CrossRef]
- Makarova, I.; Grebenev, V.; Dmitricheva, E.; Vasiliev, I.; Komornikov, V.; Dolbinina, V.; Mikheykin, A. MmHn(XO4)(m+n)/2 crystals: Structure, phase transitions, hydrogen bonds, conductivity. II. Structure and properties of Cs3(HSO4)2(H2PO4) and Cs4(HSO4)3(H2PO4) single crystals. Acta Crystallogr. B 2016, 72, 133–141. [Google Scholar] [CrossRef] [PubMed]
- Haile, S.M.; Calkins, P.M. X-ray diffraction study of Cs5(HSO4)3(H2PO4)2, a new solid acid with a unique hydrogen-bond network. J. Solid State Chem. 1998, 140, 251–265. [Google Scholar] [CrossRef]
- Choudhury, R.R.; Chitra, R.; Makarova, I.P.; Selezneva, E.; Komornikov, V. Deterioration of hydrogen-bonded superprotonic conductors belonging to CsHSO4–CsH2PO4–H2O salt system: A single-crystal neutron diffraction investigation. Bull. Mater. Sci. 2021, 44, 108. [Google Scholar] [CrossRef]
- Makarova, I.; Selezneva, E.; Canadillas-Delgado, L.; Mossou, E.; Vasiliev, A.; Komornikov, V.; Devishvili, A. Crystal structure, hydrogen bonds and thermal transformations of superprotonic conductor Cs6(SO4)3(H3PO4)4. Acta Cryst. 2021, B77, 266–274. [Google Scholar] [CrossRef] [PubMed]
- Makarova, I.; Selezneva, E.; Grebenev, V.; Komornikov, V.; Vasil’ev, A. Structure and properties of new crystals in CsHSO4 – CsH2PO4 – H2O system. Ferroelectrics 2016, 500, 54–66. [Google Scholar] [CrossRef]
- Geilmann, W.; Gebauhr, W. Zur Fällung der Alkalimetalle als Tetraphenylborverbindungen. Anal. Chem. 1953, 139, 161. [Google Scholar] [CrossRef]
- Baibaeva, S.T.; Mirkind, L.A.; Krylova, L.P.; Navjagskaja, E.A.; Salova, A.S. Methods of Analysis of Varnish-and-Paint Materials; Khimiya: Moscow, Russia, 1974; p. 354. (In Russian) [Google Scholar]
- Scientific Council on Analytical Methods. Chemical Analytical Methods; Regulation No. 3-X; Sulfur: Moscow, Russia, 1965. [Google Scholar]
- Komornikov, V.A.; Timakov, I.S.; Zajnullin, O.B.; Grebenev, V.V.; Makarova, I.P.; Selezneva, E.V. The Study of Phase Equilibria in the Cs2SO4–Rb2SO4–H2SO4–H2O System. Crystallogr. Rep. 2018, 63, 1009–1014. [Google Scholar] [CrossRef]
Figure 1.
The variety of compounds in the CsOH–H2SO4–H3PO4–H2O system. The dotted line shows the CsHSO4–CsH2PO4–H2O pseudoternary cross-section and the choice of concentration coordinates for a simplified representation of the crystallization range of the Cs6(SO4)3(H3PO4)4 compound. The photos show crystals of compounds obtained directly by the authors of this work.
Figure 1.
The variety of compounds in the CsOH–H2SO4–H3PO4–H2O system. The dotted line shows the CsHSO4–CsH2PO4–H2O pseudoternary cross-section and the choice of concentration coordinates for a simplified representation of the crystallization range of the Cs6(SO4)3(H3PO4)4 compound. The photos show crystals of compounds obtained directly by the authors of this work.
Figure 2.
Phase equilibria in the CsHSO4–CsH2PO4–H2O pseudoternary cross-section at different temperatures: (a)—25 °C, (b)—50 °C, and (c)—75 °C.
Figure 2.
Phase equilibria in the CsHSO4–CsH2PO4–H2O pseudoternary cross-section at different temperatures: (a)—25 °C, (b)—50 °C, and (c)—75 °C.
Figure 3.
Phase equilibria in the CsHSO4–CsH2PO4–H3PO4–H2O system as part of the CsOH–H2SO4–H3PO4–H2O system.
Figure 3.
Phase equilibria in the CsHSO4–CsH2PO4–H3PO4–H2O system as part of the CsOH–H2SO4–H3PO4–H2O system.
Figure 4.
The grown crystals (a,b) and their corresponding faceting (c,d).
Figure 5.
The obtained powder X-ray diffraction pattern of the grown crystal Cs6(SO4)3(H3PO4)4. The indexing was carried out in accordance with the data [15].
Figure 5.
The obtained powder X-ray diffraction pattern of the grown crystal Cs6(SO4)3(H3PO4)4. The indexing was carried out in accordance with the data [15].
Table 1.
Concentration coordinates of the researched points and phase equilibria in the Cs2SO4–H2SO4–H2O system.
Table 1.
Concentration coordinates of the researched points and phase equilibria in the Cs2SO4–H2SO4–H2O system.
| Crys-n № | Composition of Liquid Phase, wt.% (Excluding Water). | Phase Equilibria | |
|---|---|---|---|
| Cs2SO4 | H2SO4 | ||
| 1 | 64.59 | 4.37 | Cs2SO4 ↔ L |
| 2 | 70.74 | 8.21 | |
| 3 | 71.54 | 10.43 | Cs2SO4 + Cs5H3(SO4)4 ↔ L |
| 4 | 72.61 | 13.11 | Cs5H3(SO4)4 ↔ L |
| 5 | 69.18 | 15.33 | Cs5H3(SO4)4 + CsHSO4 ↔ L |
| 6 | 69.43 | 18.80 | CsHSO4 ↔ L |
| 7 | 59.31 | 19.63 | |
| 8 | 54.70 | 22.22 | |
| 9 | 48.68 | 24.49 | |
Table 2.
Concentration coordinates of the researched points and phase equilibria in the CsHSO4–CsH2PO4–H2O system at different temperatures.
Table 2.
Concentration coordinates of the researched points and phase equilibria in the CsHSO4–CsH2PO4–H2O system at different temperatures.
| 25 °C | 50 °C | 75 °C | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Crys-n № | Composition of the Liquid Phase, wt.%. | Phase Equilibria | Crys-n № | Composition of the Liquid Phase, wt.%. | Phase Equilibria | Crys-n № | Composition of the Liquid Phase, wt.%. | Phase Equilibria | |||
| CsHSO4 | CsH2PO4 | CsHSO4 | CsH2PO4 | CsHSO4 | CsH2PO4 | ||||||
| CsHSO4 | 79.03 | - | CsHSO4 ↔ L | CsHSO4 | 80.58 | - | CsHSO4 ↔ L | CsHSO4 | 86.93 | - | CsHSO4 ↔ L |
| 1 | 75.37 | 8.44 | 1 | 78.917 | 8.986 | 1 | 77.14 | 14.203 | |||
| E1 | 73.72 | 11.67 | CsHSO4 + Cs4(HSO4)3(H2PO4) | 2 | 77.865 | 10.311 | E1 | 75.753 | 16.53 | CsHSO4+ Cs4(HSO4)3(H2PO4) ↔L | |
| 3 | 73.02 | 13.40 | Cs4(HSO4)3(H2PO4) ↔ L | E1 | 76.586 | 12.807 | CsHSO4 + Cs4(HSO4)3(H2PO4) ↔ L | 3 | 69.747 | 21.321 | Cs4(HSO4)3(H2PO4) ↔L |
| 4 | 62.31 | 24.75 | 4 | 66.644 | 20.062 | Cs4(HSO4)3(H2PO4) ↔ L | 4 | 64.553 | 27.91 | ||
| 5 | 56.83 | 34.93 | 5 | 65.194 | 22.323 | E2 | 55.716 | 32.31 | Cs4(HSO4)3(H2PO4) + Cs6(HSO4)3(H3PO4)4 ↔ L | ||
| 6 | 54.50 | 36.95 | 6 | 62.852 | 27.152 | 6 | 37.316 | 50.13 | Cs6(HSO4)3(H3PO4)4 ↔ L | ||
| E2 | 54.19 | 36.65 | Cs4(HSO4)3(H2PO4) + Cs3(HSO4)2(H2PO4) ↔ L | E2 | 56.368 | 32.033 | Cs4(HSO4)3(H2PO4) + Cs6(HSO4)3(H3PO4)4 ↔ L | 7 | 31.811 | 58.731 | |
| 8 | 48.61 | 42.55 | Cs3(HSO4)2(H2PO4) ↔ L | 8 | 41.668 | 45.429 | Cs6(HSO4)3(H3PO4)4 ↔ L | E3 | 29.067 | 61.92 | Cs6(HSO4)3(H3PO4)4 + CsH2PO4 ↔ L |
| 9 | 42.66 | 53.09 | 9 | 28.724 | 61.703 | 9 | 22.47 | 65.375 | CsH2PO4 ↔ L | ||
| E3 | 40.11 | 54.01 | Cs3(HSO4)2(H2PO4) + Cs6(HSO4)3(H3PO4)4 ↔ L | 10 | 15.869 | 68.528 | 10 | 13.817 | 72.256 | ||
| 11 | 37.30 | 54.58 | Cs6(HSO4)3(H3PO4)4 ↔ L | E3 | 14.479 | 68.827 | Cs6(HSO4)3(H3PO4)4 + CsH2PO4 ↔ L | 11 | 9.771 | 76.561 | |
| E4 | 34.24 | 58.04 | Cs6(HSO4)3(H3PO4)4 + CsH2PO4 ↔ L | 12 | 11.358 | 70.204 | CsH2PO4 ↔ L | CsH2PO4 | - | 70.8 | |
| 13 | 24.39 | 52.69 | CsH2PO4 ↔ L | 13 | 7.18 | 69.73 | |||||
| 14 | 17.54 | 53.93 | 14 | 2.87 | 69.07 | ||||||
| 15 | 11.07 | 53.33 | CsH2PO4 | - | 64.91 | ||||||
| 16 | 5.85 | 52.84 | |||||||||
| CsH2PO4 | - | 52.39 | |||||||||
Bold denotes concentration conditions for the points of noninvariant equilibrium.
Table 3.
Main elements of the CsHSO4–CsH2PO4–H3PO4–H2O diagram.
| Element | Description |
|---|---|
| ω(CsHSO4) | CsHSO4 solubility at 50 °C, 80.6% mass |
| ω(CsH2PO4) | CsH2PO4 solubility at 50 °C, 64.9% mass |
| E1 | CsHSO4—73.1, CsH2PO4—12.4, H2O—14.5% mass |
| E2 | CsHSO4—54.3, CsH2PO4—30.3, H2O—15.4% mass |
| E3 | CsHSO4—28.8, CsH2PO4—56.6, H2O—14.6% mass |
| E4 | CsH2PO4—51.2, H3PO4—29.6, H2O—19.2% mass |
| E5 | CsHSO4—22.3, CsH2PO4—45.8, H3PO4—18.2, H2O—13.7% mass. Nonvariant equilibrium Cs6(SO4)3(H3PO4)4+CsH2PO4+CsH5(PO4)2↔L |
| Line E1-2 | Equilibrium CsHSO4+Cs4(HSO4)3(H2PO4)↔L |
| Line E2-3 | Equilibrium Cs4(HSO4)3(H2PO4)+Cs6(SO4)3(H3PO4)4↔L |
| Line E3-E5 | Equilibrium Cs6(SO4)3(H3PO4)4+CsH2PO4↔L |
| Line E4-E5 | Equilibrium CsH2PO4+CsH5(PO4)2↔L |
| Line E5-4 | Equilibrium Cs6(SO4)3(H3PO4)4+CsH5(PO4)2↔L |
| Line 1-2-3-4-5 | Solid phase solubility in 100% H3PO4 |
| Surface ω(CsHSO4)-E1-2-1 | Crystallization range CsHSO4 |
| Surface E1-E2-3-2 | Crystallization range Cs4(HSO4)3(H2PO4) |
| Surface E2-E3-E5-4-3 | Crystallization range Cs6(SO4)3(H3PO4)4 |
| Surface ω(CsH2PO4)-E4-E5-E3 | Crystallization range CsH2PO4 |
| Surface E4-E5-4-5 | Crystallization range CsH5(PO4)2 |
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content. |
© 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
Share and Cite
MDPI and ACS Style
Komornikov, V.A.; Makarova, I.P.; Timakov, I.S.; Grebenev, V.V. Features of Phase Formation in the CsOH–H2SO4–H3PO4–H2O System and the Growth of the Cs6(SO4)3(H3PO4)4 Crystals. Crystals 2023, 13, 1163. https://doi.org/10.3390/cryst13081163
AMA Style
Komornikov VA, Makarova IP, Timakov IS, Grebenev VV. Features of Phase Formation in the CsOH–H2SO4–H3PO4–H2O System and the Growth of the Cs6(SO4)3(H3PO4)4 Crystals. Crystals. 2023; 13(8):1163. https://doi.org/10.3390/cryst13081163
Chicago/Turabian StyleKomornikov, V. A., I. P. Makarova, I. S. Timakov, and V. V. Grebenev. 2023. "Features of Phase Formation in the CsOH–H2SO4–H3PO4–H2O System and the Growth of the Cs6(SO4)3(H3PO4)4 Crystals" Crystals 13, no. 8: 1163. https://doi.org/10.3390/cryst13081163
Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.
