 Journal of Crystallization Process and Technology, 2013, 3, 136-147 http://dx.doi.org/10.4236/jcpt.2013.34022 Published Online October 2013 (http://www.scirp.org/journal/jcpt) Copyright © 2013 SciRes. JCPT Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Veronika Karadjova1, Donka Stoilova2* 1Department of Inorganic Chemistry, University of Chemical Technology and Metallurgy, Sofia, Bulgaria; 2Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria. Email: *stoilova@svr.igic.bas.bg Received June 28th, 2013; revised July 28th, 2013; accepted August 4th, 2013 Copyright © 2013 Veronika Karadjova, Donka Stoilova. This is an open access article distributed under the Creative Commons At- tribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is prop- erly cited. ABSTRACT The crystallization in the three-component systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) is studied by the method of isothermal decrease of supersaturation. It has been established that isostructural double compounds, Rb2M(SO4)2·6H2O (M = Mg, Co, Ni, Cu, Zn), 5 12 SG 2h PcC , crystallize from the ternary solutions within wide concentration ranges. The infrared spectra are discussed with respect to the normal vibrations of the sulfate ions and water molecules. The unit-cell group theoretical treatment of the double salts is presented. The extent of energetic dis- tortions of 2 4 SO guest ions (about 2 mol%) matrix-isolated in the respective selenates, 242 2 MM SeO6HO (M' = K, Rb, 4 NH; M" = Mg, Co, Ni, Cu, Zn), is commented. Keywords: Rb2Me(SO4)2·6H2O (Me = Mg, Co, Ni, Cu, Zn); Solubility Diagrams; X-Ray Powder Diffraction; Infrared Spectra; Matrix Infrared Spectroscopy 1. Introduction The rubidium double sulfates belong to a large number of isomorphous compounds with a general formula 242 2 MM XO6HO (M' = K, Rb, 4 NH, Cs; M'' = Mg, Mn, Co, Ni, Cu, Zn; X = S, Se) known as Tutton salts. They crystallize in the monoclinic space group 5 12 2h PcC with two formula units in the unit-cell. The crystal structures are built up from isolated octahedra, [M''(H2O)6], (three crystallographically different water molecules are coordinated to the M'' ions) and tetrahedra XO4. The polyhedra are linked by hydrogen bonds. All atoms, except the divalent metal ions, which lie at centre of inversion Ci, are located at general positions C1. Re- cently, the crystal structures of some rubidium Tutton sulfates have been reported in Refs. [1,2]. As an example the crystal structures of Rb2Co(SO4)2·6H2O is presented in Figure 1. In this paper we present the results on the study of the solubility in the three-component systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K. The double Tutton compounds Rb2M(SO4)2·6H2O are characterized by means of both the infrared spectroscopy and the X-ray powder diffraction methods. The vibra- tional behavior of 2 4 SO guest ions matrix-isolated in Tutton selenates, 242 2 MM SeO6HO (M' = K, Rb, 4 NH ; M'' = Mg, Co, Ni, Cu, Zn), is analyzed. A practi- cal point of studying is that the Tutton compounds could be considered as proton conductors due to the existence of comparatively strong hydrogen bonds determined by the strong proton acceptor capabilities of the sulfate ions. 2. Experimental Rb2SO4 was prepared by neutralization of Rb2CO3 with dilute sulfuric acid solutions at 333 - 343 K. Then the solutions were filtered, concentrated at 323 - 333 K, and cooled to room temperature. The crystals were filtered, washed with alcohol and dried in air. The sulfates of di- valent metals were commercial products. All reagents used were of reagent grade quality (Merck). The solubil- ity in the three-component systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K was studied using the method of isothermal decrease of supersaturation. Solu- tions containing different amount of the salt compounds corresponding to each point of the solubility diagrams  Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Copyright © 2013 SciRes. JCPT 137 Figure 1. Crystal structure of Rb2Co(SO4)2·6H2O (structural data from [1]). were heated at about 333 - 343 K and cooled to room temperature. Then the saturated solutions were vigor- ously stirred [3,4]. The equilibrium between the liquid and solid phases was reached in about 20 hours. The analysis of the liquid and the wet solid phases was per- formed as follows: the M'' ion contents were determined complexonometrically at pH 9.5 - 10 using eriochrome black as indicator (magnesium ions) and at pH 5.5 - 6 using xylenol orange as indicator (cobalt, nickel, copper and zinc ions); the sulfate ions were precipitated as Ba- SO4 with Ba(NO3)2 solutions and the concentrations of the excess of Ba2+ ions were determined complexono- metrically using eriochrome black as indicator; the con- centrations of the rubidium sulfate were calculated by difference [5]. The compositions of the solid phases were identified by means of both the X-ray diffraction and the infrared spectroscopy methods. Tutton compounds Rb2M(SeO4)2·6H2O were prepared according to the solu- bility diagrams of the three-component systems Rb2SeO4-MSeO4-H2O (M = Mg, Co, Ni, Cu, Zn) [6-8]. The samples, 2442 1.98 0.02 MM SeOSO6HO (M' = K, Rb, 4 NH; M'' = Mg, Co, Ni, Cu, Zn), were prepared by crystallization from ternary selenate solutions in the presence of 2 4 SO ions (the data for the potassium and ammonium selenates are taken from [9]). The infrared spectra were recorded on a Bruker model IFS 25 Fourier transform interferometer (resolution < 2 cm−1) at ambient temperature using KBr discs as matrices. Ion exchange or other reactions with KBr have not been observed. In some cases Lorentz band profile for multi peak data was used to determine the correct band posi- tions corresponding to asymmetric stretches of the in- cluded 2 4 SO ions (ORIGIN PRO 6.1). The X-ray po- wder diffraction spectra were collected within the range from 5˚ to 50˚ 2θ with a step 0.02˚ 2θ and counting time 35 s/step on Bruker D8 Advance diffractometer with CuKα radiation and LynxEye detector. The lattice parameters of the double salts were calculated using the program ITO and refined with the program LSUCR. 3. Results and Discussions 3.1. Solubility Diagrams of the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K The solubility diagrams of the above systems are pre- sented in Figures 2-6 (the respective experimental data  Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Copyright © 2013 SciRes. JCPT 138 Figure 2. Solubility diagram of the three-component system Rb2SO4-MgSO4-H2O at 298 K. Figure 3. Solubility diagram of the three-component system Rb2SO4-CoSO4-H2O at 298 K. Figure 4. Solubility diagram of the three-component system Rb2SO4-NiSO4-H2O at 298 K. Figure 5. Solubility diagram of the three-component system Rb2SO4-CuSO4-H2O at 298 K. Figure 6. Solubility diagram of the three-component system Rb2SO4-ZnSO4-H2O at 298 K. are summarized in Tables 1-5). It is seen that the simple salts Rb2SO4, MgSO4·7H2O, CoSO4·7H2O, NiSO4·7H2O, CuSO4·5H2O and ZnSO4·7H2O crystallize within very narrow concentration ranges, whereas the rubidium dou- ble sulfates crystallize within wide concentration ranges, thus indicating that strong complex formation processes occur in the ternary solutions. 3.2. X-Ray Powder Diffraction Data of Rubidium Tutton Compounds The X-ray powder diffraction patterns of the rubidium Tutton compounds are shown in Figure 7. The double salts form monoclinic crystals 5 12 SG 2h PcC . The calculated unit-cell parameters are presented in Table 6. Our results coincide well with those determined from single crystal X-ray diffraction data [1,2].  Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Copyright © 2013 SciRes. JCPT 139 Table 1. Solubility in the Rb2SO4-MgSO4-H2O system. Liquid phase mass% Wet solid phase mass% Rb2SO4 MgSO4 Rb2SO4 MgSO4 Composition of the solid phases 35.10 Rb2SO4 34.58 1.12 74.41 0.79 - 34.42 2.08 59.17 16.58 Rb2SO4 + Rb2Mg(SO4)2·6H2O 34.07 2.03 53.26 21.09 Rb2Mg(SO4)2·6H2O 29.39 2.48 50.02 19.96 - 25.85 4.13 48.62 20.14 - 21.19 6.11 49.57 21.75 - 17.17 8.66 46.68 21.24 - 13.37 12.96 47.87 23.36 - 9.45 19.25 46.71 23.96 - 6.97 24.01 48.37 24.83 - 7.38 27.54 49.58 25.68 - 7.51 28.51 20.54 37.42 MgSO4·7H2O + Rb2Mg(SO4)2·6H2O 6.87 28.73 3.19 42.21 MgSO4·7H2O 3.49 27.61 0.64 45.27 - 27.50 - Table 2. Solubility in the Rb2SO4-CoSO4-H2O system. Liquid phase mass% Wet solid phase mass% Rb2SO4 CoSO4 Rb2SO4 CoSO4 Composition of the solid phases 35.10 Rb2SO4 33.54 1.02 71.49 0.76 - 33.28 1.09 57.36 21.37 Rb2SO4 + Rb2Co(SO4)2·6H2O 33.21 1.04 47.24 21.69 Rb2Co(SO4)2·6H2O 27.43 0.98 47.21 23.56 - 20.68 0.78 45.47 23.38 - 13.32 4.93 42.78 22.84 - 8.48 8.59 43.17 25.31 - 5.44 12.97 44.25 26.83 - 3.79 17.07 47.36 29.14 - 2.83 24.21 45.28 30.07 - 2.40 26.68 14.55 42.73 CoSO4·7H2O Rb2Co(SO4)2·6H2O 2.19 26.49 0.84 44.93 CoSO4·7H2O 26.14 - Table 3. Solubility in the Rb2SO4-NiSO4-H2O system. Liquid phase mass%Wet solid phase mass% Rb2SO4NiSO4 Rb2SO4 NiSO4 Composition of the solid phases 35.10 Rb2SO4 34.72 1.53 67.42 1.17 - 34.68 1.57 63.65 18.67 Rb2SO4 + Rb2Ni(SO4)2·6H2O 31.92 0.50 49.96 25.73 Rb2Ni(SO4)2·6H2O 26.44 0.68 48.18 24.58 - 19.85 0.81 47.26 26.22 - 13.00 2.40 43.72 23.19 - 10.52 3.45 44.36 24.65 - 7.59 9.04 45.61 26.29 - 5.08 13.37 46.44 28.07 - 3.97 19.82 47.56 29.45 - 4.02 23.37 43.74 29.67 - 4.06 24.08 15.72 45.67 NiSO4·7H2O + Rb2Ni(SO4)2·6H2O 3.85 24.57 1.43 38.79 NiSO4·7H2O 26.47 - Table 4. Solubility in the Rb2SO4-CuSO4-H2O system. Liquid phase mass%Wet solid phase mass% Rb2SO4CuSO4 Rb2SO4 CuSO4 Composition of the solid phases 35.10 Rb2SO4 34.81 0.98 69.11 1.02 - 33.72 1.04 55.74 20.14 Rb2SO4 + Rb2Cu(SO4)2·6H2O 33.64 1.12 50.47 26.73 Rb2Cu(SO4)2·6H2O 28.68 1.06 48.26 26.44 - 24.59 0.96 45.49 24.36 - 19.53 0.35 43.72 24.21 - 14.46 2.48 42.64 23.55 - 10.72 4.92 44.26 25.87 - 8.75 9.28 45.82 28.33 - 4.17 16.16 47.48 29.94 - 4.43 16.57 10.45 42.48 CuSO4·5H2O + Rb2Cu(SO4)2·6H2O 3.97 17.01 1.74 38.76 CuSO4·5H2O 18.30 -  Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Copyright © 2013 SciRes. JCPT 140 Table 5. Solubility in the Rb2SO4-ZnSO4-H2O system. Liquid phase mass% Wet solid phase mass% Rb2SO4 ZnSO4 Rb2SO4 ZnSO4 Composition of the solid phases 35.10 Rb2SO4 34.61 1.98 61.48 1.28 - 34.15 1.72 56.57 20.46Rb2SO4 + Rb2Zn(SO4)2·6H2O 33.85 1.34 50.26 27.19Rb2Zn(SO4)2·6H2O 25.02 0.50 48.43 27.54- 17.72 2.63 43.67 25.71- 10.44 6.36 45.72 28.44- 4.87 15.06 43.85 29.16- 3.77 25.45 45.05 30.11- 4.01 30.86 42.67 30.85- 3.96 33.75 13.74 44.26ZnSO4·7H2O + Rb2Zn(SO4)2·6H2O 3.79 34.05 1.28 29.04ZnSO4·7H2O 36.43 - Table 6. Lattice parameters of the Tutton compounds. Tutton salts a (Å) b (Å) c (Å) β (˚) V (Å3) Mg 9.235(2) 12.493(2) 6.227(1) 105.95(2) 690.8(2) Co 9.198(3) 12.483(5) 6.245(1) 106.12(2) 688.9(2) Ni 9.147(4) 12.418(4) 6.223(3) 106.06(2) 679.3(3) Cu 9.269(4) 12.365(4) 6.229(2) 105.38(2) 688.4(3) Zn 9.198(2) 12.461(4) 6.248(2) 105.9(2) 688.5(1) 3.3. Infrared Spectra of Neat Rubidium Tutton Sulfates The free tetrahedral ions 4 XO n under perfect Td sym- metry exhibit four internal vibrations: 1(A1), the sym- metric X-O stretching modes, 2(E), the symmetric XO4 bending modes, 3(F2) and 4(F2), the asymmetric stret- ching and bending modes, respectively. The normal vi- brations of the free sulfate ions in aqueous solutions are reported to appear as follows: 1 = 983 cm−1, 2 = 450 cm−1, 3 = 1105 cm−1, 4 = 611 cm−1 [10]. On going into solid state, the normal modes of the 4 XO n ions are expected to shift to higher or lower frequencies. The static field (related to the symmetry of the site on which the 4 XOn ions are situated) will cause a removal of the degeneracy of both the doubly degenerate 2 mod- es and the triply degenerate 3 and 4 modes. Since the tetrahedral ions in the structures of Tutton compounds occupy site symmetry C1, two bands for 2 (2A) 10 20 30 40 50 2 (degree) Intensity Rb 2 Zn(SO 4 ) 2 ·6H 2 O Rb 2 Cu(SO 4 ) 2 ·6H 2 O Rb 2 Ni(SO 4 ) 2 ·6H 2 O Rb 2 Co(SO 4 ) 2 ·6H 2 O Rb 2 Mg(SO 4 ) 2 ·6H 2 O Figure 7. X-ray powder diffraction patterns of Rb2M(SO4)2·6H2O (M = Mg, Co, Ni, Cu, Zn). and three bands for 3 and 4 (3A), respectively, are ex- pected to appear in the vibrational spectra as predicted from the site group analysis (the nondegenerate 1 mode is activated). Additionally, the factor group analysis (C2h factor group symmetry) predicts a splitting of each spe- cies of A symmetry into Ag + Au + Bg + Bu (related to interactions of identical oscillators, correlation field ef- fects). Consequently, 18 infrared bands (9Au + 9Bu) and 18 Raman bands (9Ag + 9Bg) correspond to the normal vibrations of the tetrahedral ions. The correlation dia- gram between the Td point symmetry, C1 site symmetry of the sulfate ions and C2h factor group symmetry is pre- sented in Figure 8. Unit-cell theoretical treatment for the translational lattice modes (Rb, M'', 2 4 SO and H2O) and librational lattice modes (2 4 SO and H2O) yields: 69 modes of Ag, Au, Bg and Bu symmetry and 48 modes of Ag, Au, Bg and Bu symmetry for the translational and li- brational modes respectively [11]. Infrared spectroscopic investigations of the potassium Tutton sulfates and selenates are widely discussed in the literature [9,12-17] and those of the rubidium selenates are commented in [8,13,18]. Infrared spectra of Rb2M(SO4)2·6H2O (M = Mg, Co, Ni, Cu, Zn) in the region of 4000 - 400 cm−1 are shown in Figures 9 and 10. Some structural and spectroscopic data are summarized in Table 7 (for comparison the  Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Copyright © 2013 SciRes. JCPT 141 Figure 8. Correlation diagram between Td point symmetry, C1 site symmetry and C2h factor group symmetry (2 4 SO ions). structural and spectroscopic data for the respective po- tassium and ammonium sulfates are presented; the data are taken from [9]). It is readily seen that the shape of the spectra and the band positions are similar owing to the isostructureness of the double salts. The six infrared bands expected according to the factor group analysis for the asymmetric stretching modes of the sulfate ions coa- lesce into three bands: Rb2Mg( SO 4)2·6H2O (1141, 1111, 1097 cm−1); Rb2Co(SO4)2·6H2O (1142, 1112, 1097 cm−1); Rb2Ni(SO4)2·6H2O (1139, 1109, 1097 cm−1); Rb2Zn(SO4)2·6H2O (1139, 1111, 1099 cm−1), and into two bands for Rb2Cu(SO4)2·6H2O (1144, 1097 cm−1). The 1 modes appear at 984 cm−1. The bending modes 4 are detected in the spectral intervals of 632 - 612 cm−1. 2 appear at about 450 cm−1 (see Figure 9). Our infrared spectroscopic findings differ slightly from those reported by Brown and Ross with respect to the number of the bands corresponding to 3 of the sul- fate ions. The spectra of the rubidium compounds com- mented in [13] show more bands for 3 (some of them assigned as shoulders) as compared to our results. We believe that the difference between our spectra and those discussed by Brown and Ross is due probably to the temperatures at which the spectra are recorded. It is men- tioned in [13] that some spectra are run at liquid nitrogen temperature. However, there is no indication which spec- tra are obtained at LNT. The normal vibrations of the water molecules appear in the high frequency region of 3000 - 4500 cm−1. The three crystallographically different water molecules (C1 site symmetry) in the structures of the Tutton sulfates are expected to display six infrared bands corresponding to the asymmetric and symmetric modes, 3 and 1 respec- tively. However, due to the strong interactions of identi- cal oscillators O-H the different normal modes overlap and as a result one broad band centered at about 3230 cm−1 is observed in the spectra of the double Tutton salts 1400 1200 1000800600400 459 509 579 619 631 755 851 984 1099 1111 1139 Wavenumber (cm-1) Absorbance 445 561 631 619 771 895 981 1144 1097 Zn Cu Ni Co 460 507 617 586 632 760 875 984 1097 1139 1109 450 579 616 631 870 751 984 1112 1142 1097 445 514 589 612 631 753 860 984 1111 1097 1141 Mg Figure 9. Infrared spectra of Rb2M(SO4)2·6H2O (M = Mg, Co, Ni, Cu, Zn) in the region of 1400 - 400 cm−1 (normal vibrations of the 2 4 SO ions and water librations). (with exception of the copper compound) (Figure 10). Three bands corresponding to 2 of the three crystal- lographically different water molecules are observed in the spectra of the compounds under study: 1709, 1630 and 1553 cm−1 (magnesium); 1734, 1641 and 1560 cm−1 (cobalt); 1734, 1631 and 1574 cm−1 (nickel); 1734, 1626 and 1590 cm−1 (copper), and 1734, 1621 and 1548 cm−1 (zinc). The band positions of the stretching modes indi- cate that comparatively strong hydrogen bonds are formed in the sulfates and the hydrogen bond strengths  Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Copyright © 2013 SciRes. JCPT 142 1600 1800 1560 1641 1734 3245 1553 1630 1709 3230 1574 1631 1734 3234 1590 1626 1734 3245 3100 3348 1548 1621 1734 3225 Cu 3500 3000 Absorbance Wavenumber (cm -1 ) Mg Co Ni Zn // Figure 10. Infrared spectra of Rb2M(SO4)2·6H2O (M = Mg, Co, Ni, Cu, Zn) in the region of the stretching and bending modes of the water molecules. do not depend on the M'' chemical nature. The appear- ance of a band at a lower frequency (3100 cm−1) in the spectrum of the copper compound shows that stronger hydrogen bonds are formed in this salt as compared to other rubidium compounds. This spectroscopic finding is owing to the stronger synergetic effect of the Cu2+ ions, i.e. to the strong Cu-OH2 interactions (increasing of the acidity of the water molecules) [19,20]. The formation of comparatively strong hydrogen bonds in the rubidium compounds is due to the strong proton acceptor capacity of the sulfate ions [19,20]. The water librations (rocking, twisting and wagging) appear in the region below 1000 cm−1 and a strong over- lapping with vibrations of other entities in the structure is expected. Two types of water librations for the Tutton sulfates are discussed briefly in the literature—rocking and wagging, the former observed at higher frequencies [14]. Each type is characterized with two broad bands. The water molecules bonded to the M'' ions via shorter M''-OH2 bonds display water librations at higher fre- quencies as compared to those forming longer M''-OH2 bonds (equatorial water molecules). The former M''-OH2 bonds are much more polarized due to the stronger syn- ergetic effect of the M'' ions (stronger metal-water inter- actions). The mean wavenumbers for the rocking libra- tions are reported to have values of 855 and 740 cm−1, and 770 and 680 cm−1 for the potassium and ammonium sulfates, respectively. The respective wagging modes have mean values of 570 and 441 cm−1 for the potassium compounds, and 544 and 425 cm−1 for the ammonium ones [14]. Thus, the bands in the interval of 895 - 751 cm−1 are attributed to the rocking modes of the water molecules and the bands in the region of 589 - 507 cm−1 to the wagging modes. The close wavenumbers of the water librations confirm the claim that the hydrogen bonds formed in the rubidium Tutton sulfates are of close strength (see Figure 9). 3.4. Infrared Spectra of 2 4 SO Ions Matrix Iso- lated in Tutton Selenates The method of crystal matrix-spectroscopy provides im- portant information about the local potential at the lattice sites where the guest ions are located as deduced from their extent of distortion and the chemical nature of the ligand environments in the lattice. When the polyatomic ions are doped in host lattices at low concentration (up to 2 - 7 mol%) the correlation field splitting, the dispersion of phonon curves (due to the interactions between iden- tical oscillators) and LO/TO splitting effects (due to the long-range forces of electrostatic origin) are neglected. Thus, the vibrational spectra of the guest ions are deter- mined by the site symmetry, which is assumed to be the same as that of the respective host ions (substitutional mixed crystals). The spectra of matrix-isolated polya- tomic ions are an excellent probe of the local potential at called an energetic distortion in order to distinguish it from the geometrical distortion revealed by structural data [21-23]. Both the site group splitting of the asym- metric modes ( as) and the value of max (the differ- ence between the highest and the lowest wavenumbered components of the stretching and bending modes, respec- tively) are an adequate measure for the degree of ener- getic distortion of the polyatomic ions [22,24,25]. Re- cently, the value of the ratio as/ c (where c is the cen- tro-frequency value of the asymmetric modes) has been proposed to calculate the relative splitting of the dopant ions [26]. Numerous papers in the literature are devoted to the vibrational spectroscopic studies of polyatomic ions (for example, 3 XO , 2 4 XO , 3 4 XO ) doped in various host lattices [21,23-40]. Lutz et al. performed solid solution spectroscopic experiments on monoclinic Ba(ClO3)2·H2O-type and orthorhombic Sr(ClO3)2-type  Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Copyright © 2013 SciRes. JCPT 143 Table 7. Some structural and spectroscopic characteristics for the 2 4 SO ions in the neat Tutton salts (V/n, unit-cell volumes divided by the numbers of the 2 4 SO ions; S-O, mean values of the S-O bond lengths; r(SO4), the difference between the longest and the shortest S-O bond lengths in the respective tetrahedra; max, the difference between the highest and the low- est wave numbered components of the stretches of the 2 4 SO ions; the structural data are taken from [1,2,9]). Compounds V/n (Å3) S-O (Å) r(SO4) (Å) 3 (cm−1) 3 (cm-1) 1 (cm−1) 3 (cm−1) max (cm−1) K2Mg(SO4)26H2O 164 1.474 0.065 1147, 1108, 10981115 984 49 163 Rb2Mg(SO4)26H2O 172 1.470 0.014 1141, 1111, 10971116 984 45 160 (NH4)2Mg(SO4)26H2O 174 1.473 0.021 1147, 1108, 10981118 984 49 163 K2Co(SO4)26H2O 165 1.474 0.020 1144, 1100 1122 984 44 160 Rb2Co(SO4)26H2O 172 1.472 0.015 1142, 1112, 10971117 984 45 158 (NH4)2Co(SO4)26H2O 175 1.510 0.065 1146, 1102 1124 982 44 164 K2Ni(SO4)26H2O 162 1.473 0.016 1144, 1111, 11011119 982 43 164 Rb2Ni(SO4)26H2O 169 1.470 0.012 1139, 1109, 10971115 984 42 155 (NH4)2Ni(SO4)26H2O 171 1.476 0.020 1144, 1108, 11001117 982 44 164 K2Cu(SO4)26H2O 164 1.471 0.024 1144, 1102, 10971114 984 47 160 Rb2Cu(SO4)26H2O 171 1.473 0.024 1144, 1097 1121 981 47 163 (NH4)2Cu(SO4)26H2O 173 1.473 0.022 1144, 1102, 10951114 981 49 163 K2Zn(SO4)26H2O 164 1.470 0.018 1141, 1108, 11021117 982 39 159 Rb2Zn(SO4)26H2O 169 1.472 0.013 1139, 1111, 10991116 984 40 155 (NH4)2Zn(SO4)26H2O 173 1.474 0.021 1144, 1105 1124 984 39 160 host crystals with incorporated 3 ClO, 3 BrO and 3 IO ions [21,24,25]. The energetic distortions of tetrahedral ions matrix-isolated in different alkali metal lattices are widely discussed in [26-34]. Recently, infrared spectra of isomorphously included species—4 NH ions isolated in KMPO4·6H2O (M = Mg, Ni) and 3 4 PO ions isolated in MgNH4AsO4·6H2O have been reported [37]. Infrared spectra of mixed crystals 2442 2 MM SeOSO6HO xx (M' = K, Rb, 4 NH ; M'' = Mg, Co, Ni, Cu, Zn; x is approximately 0.02) are presented in Figure 11. The infrared spectroscopic char- acteristics of the matrix-isolated 2 4 SO ions are summa- rized in Table 8 (the data for potassium and ammonium Tutton salts are taken from [9]). The matrix-isolated 2 4 SO ions exhibit three bands corresponding to 3 in agreement with the low site sym- metry C1 of the 2 4 SeO host ions. Bands of small inten- sities around 980 cm−1 appear in the spectra which are assigned to 1 of the guest sulfate ions (the spectra are recorded at higher concentrations of the samples in KBr in order to distinguish the 1 mode; the spectra are not shown; see Table 8). When the larger 2 4 SeO host ions are replaced by the smaller 2 4 SO guest ions the mean values of the asymmetric stretching modes 3 of the 2 4 SO guest ions are slightly shifted to lower frequencies as compared to those of the same ions in the neat sulfate compounds due to the smaller repulsion potential of the selenate matrices, i.e. to the larger unit-cell volumes of the respective selenates (compare Tables 7 and 8). Several factors are expected to influence on the values of 3 and 3/ c: 1) the chemical nature of the metal ions; 2) the repulsion potential of the selenate matrices, and 3) the strength of the hydrogen bonds. The spectro- scopic experiments show that the distortion of the 2 4 SO guest ions as deduced from the values of 3 and 3/ c increase on going from the potassium to the ammonium compounds (see Table 8). The formation of hydrogen bonds between the 2 4 SO guest ions and both the water molecules of the host compounds and the 4 NH host ions obviously facilitate the extent of energetic distortion of the guest ions (for example, 3 of the sulfate guest ions have values of 30 and 51 cm−1 in the nickel potas- sium and ammonium matrices, and 33 and 49 cm−1 in the zinc potassium and ammonium matrices, respectively). The larger values of 3 and 3/ c of the sulfate guest ions matrix-isolated in rubidium selenate matrices as compared to those of the 2 4 SO ions included in the potassium matrices are owing probably to the smaller  Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Copyright © 2013 SciRes. JCPT 144 c b a Mg 1100 1119 1134 Co 1097 1121 1133 Ni 1102 1113 1132 Cu 1098 1108 1130 Zn 1100 1119 1133 Absorbance 1101 1132 1089 1097 1134 1137 1117 11331095 1086 1119 1135 1079 1092 1132 1086 1120 1140 1080 // // 1131 1090 1140 1080 // // 1129 1089 1101 Wavenumber (cm -1 ) 1140 1080 1093 1103 1140 1080 1079 1092 1132 1140 1080 1090 1100 1132 Figure 11. Infrared spectra of 2 4 SO ions matrix-isolated (about 2 mol%) in 2442 1.98 0.02 MM SeOSO6HO (M' = K, Rb, + 4 NH ; M'' = Mg, Co, Ni, Cu, Zn) in the region of 3: potassium selenate matrices (row a); ammonium selenate matrices (row b); rubidium selenate matrices (row c). Table 8. Some spectroscopic characteristics of 2 4 SO guest ions matrix-isolated in selenate matrices (for the assignments see Table 1; the data for the potassium and ammonium compounds are taken from [9]). 2 4 SO guest ions (approximately 2 mol%) Host compounds 3 (cm−1) 3 (cm−1) 1 (cm−1) 3 (cm−1) max (cm−1) 3/c (%) K2Mg(SeO4)26H2O 1134, 1119, 1100 1118 980 34 154 3.04 Rb2Mg(SeO4)26H2O 1132, 1101, 1090 1108 981 42 151 3.79 (NH4)2Mg(SeO4)26H2O 1134, 1097, 1089, 1107 980 45 154 4.06 K2Co(SeO4)26H2O 1133, 1121, 1097 1117 982 36 151 3.22 Rb2Co(SeO4)26H2O 1131, 1101,1089 1107 980 42 151 3.79 (NH4)2Co(SeO4)26H2O 1133, 1117, 1095 1115 981 38 152 3.40 K2Ni(SeO4)26H2O 1132, 1113, 1102 1116 982 30 150 2.68 Rb2Ni(SeO4)26H2O 1129, 1103, 1093 1108 982 36 147 3.25 (NH4)2Ni(SeO4)26H2O 1137, 1119, 1086 1114 979 51 158 4.58 K2Cu(SeO4)26H2O 1130, 1198, 1098 1112 981 32 149 2.88 Rb2Cu(SeO4)26H2O 1135, 1100, 1084 1106 980 51 155 4.61 (NH4)2Cu(SeO4)26H2O 1132, 1092, 1079 1101 981 53 151 4.81 K2Zn(SeO4)26H2O 1133, 1119, 1100 1117 982 33 151 2.95 Rb2Zn(SeO4)26H2O 1132, 1100, 1090 1107 983 42 149 3.79 (NH4)2Zn(SeO4)26H2O 1135, 1120, 1086 1114 982 49 153 4.40  Crystallization in the Three-Component Systems Rb2SO4-MSO4-H2O (M = Mg, Co, Ni, Cu, Zn) at 298 K Copyright © 2013 SciRes. JCPT 145 local potential at the lattice sites where the sulfate ions are located (i.e. to the larger unit-cell volumes of the ru- bidium selenates) [39]. As far as the influence of the M'' ion nature on the values of 3 and 3/ c of the sulfate ions is concerned the spectroscopic experiments show that the sulfate ions are stronger distorted in the case of (NH4)2M(SeO4)1.98(SO4)0.02·6H2O (M = Ni, Cu, Zn) and Rb2Cu(SeO4)1.98(SO4)0.02·6H2O. These findings are due probably to the formation of stronger hydrogen bonds in these compounds, i.e. to the stronger interaction between the water molecules of the host compounds and the sul- fate guest tetrahedra (stronger proton donor capacity of the water molecules coordinated to the copper, zinc and nickel cations, i.e. to the stronger synergetic effect of these ions) [19,20]. 4. 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