Vibrational spectra of Cs2Cu(SO4)

Journal of Molecular Structure 1106 (2016) 440e451

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Vibrational spectra of Cs2Cu(SO4)2$6H2O and Cs2Cu(SeO4)2$nH2O (n ¼ 4, 6) with a crystal structure determination of the Tutton salt Cs2Cu(SeO4)2$6H2O M. Wildner a, D. Marinova b, D. Stoilova b, * a b

€t Wien, Geozentrum, Althanstr. 14, A-1090 Wien, Austria Institut für Mineralogie und Kristallographie, Universita Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 August 2015 Received in revised form 8 October 2015 Accepted 8 November 2015 Available online 11 November 2015

The solubility in the three-component systems Cs2SO4eCuSO4eH2O and Cs2SeO4eCuSeO4eH2O have been studied at 25 C. The experimental results show that double salts, Cs2Cu(SO4)2$6H2O and Cs2Cu(SeO4)2$4H2O, crystallize from the ternary solutions within large concentration ranges. Crystals of Cs2Cu(SeO4)2$6H2O were synthesized at somewhat lower temperatures (7e8 C). The thermal dehydration of the title compounds was studied by TG, DTA and DSC methods and the respective dehydration schemes are proposed. The calculated enthalpies of dehydration (DHdeh) have values of: 434.2 kJ mol1 (Cs2Cu(SeO4)2$6H2O), 280.9 kJ mol1 (Cs2Cu(SeO4)2$4H2O), and 420.2 kJ mol1 (the phase transition of Cs2Cu(SO4)2$6H2O into Cs2Cu(SO4)2$H2O). The crystal structure of Cs2Cu(SeO4)2,6H2O was determined from single crystal X-ray diffraction data. It belongs to the group of Tutton salts, crystallizing isotypic to the respective sulfate in a monoclinic structure which is characterized by isolated Cu(H2O)6 octahedra and SeO4 tetrahedra, interlinked by hydrogen bonds and [9]-coordinated Csþ cations. Infrared spectra of the cesium copper compounds are presented and discussed with respect to both the normal modes of the tetrahedral ions and the water molecules. The analysis of the infrared spectra of the double compounds reveals that the distortion of the selenate tetrahedra in Cs2Cu(SeO4)2$4H2O is stronger than those in Cs2Cu(SeO4)2$6H2O in agreement with the structural data. Matrix-infrared spectroscopy was applied to confirm this claim Dn3 for SO4 2 ions matrix-isolated in Cs2Cu(SeO4)2$6H2O has a value of 35 cm1 and that of the same ions included in Cs2Cu(SeO4)2$4H2O e 84 cm1. This spectroscopic finding is due to the formation of strong covalent bands CueOSO3 on one hand, and on the other to the stronger deformation of the host SeO4 2 tetrahedra in Cs2Cu(SeO4)2$4H2O as compared to those in Cs2Cu(SeO4)2$6H2O. The strength of the hydrogen bonds as deduced from the frequencies of nOD of matrix-isolated HDO molecules (spectral range of 25002200 cm1) is discussed. An interesting feature of the spectra is the appearance of two groups of infrared bands corresponding to nOD, which are separated with 71 cm1 (sulfate compound) and about 120 cm1 (selenate compounds) due to the existence of two types of water molecules those coordinated to the copper ions via shorter CueOH2 bonds and those coordinated to the copper ions via longer CueOH2 bonds. The spectroscopic experiments reveal that the equatorial water molecules in the hexahydrates exhibit a local molecular symmetry close to C2v (at least at ambient temperature), while those forming long CueOH2 bonds are strongly asymmetrically hydrogen bonded. The differences in the wavenumbers of the bands corresponding to the wagging modes have values larger than 100 cm1, thus indicating strong distortions of the coordination sphere of the copper ions due to the JahneTeller effect. © 2015 Elsevier B.V. All rights reserved.

Keywords: Cesium copper selenates and sulfate Crystal structure of Cs2Cu(SeO4)2$6H2O Solubility diagrams Thermal dehydration X-ray powder diffraction Vibrational spectroscopy

1. Introduction

* Corresponding author. Fax: 359 2 870 50 24. E-mail address: [email protected] (D. Stoilova). http://dx.doi.org/10.1016/j.molstruc.2015.11.008 0022-2860/© 2015 Elsevier B.V. All rights reserved.

Tutton salts are reported to play a significant role for the development of materials suitable for strong energy absorbed by solar collectors. Thus, the thermal behavior of Tutton salts is of

M. Wildner et al. / Journal of Molecular Structure 1106 (2016) 440e451

considerable importance for the development of appropriate latent heat-storage materials [1,2]. The phase transitions of Tutton compounds have been the subject of intense research in recent years [3e8]. Additionally, the interest of the scientists in studying the Tutton salts is determined from the possibility of these compounds to exhibit proton conductivity. The present paper continues our infrared spectroscopic investigations of Tutton compounds [9e15]. In our previous papers a focus has been put on the vibrational behavior of SO4 2 ions incorporated in the crystals of M0 2M00 (SeO4)2,6H2O (M0 ¼ K, NH4 þ , Rb; M00 ¼ Mg, Co, Ni, Cu, Zn). The influence of different crystal chemical factors and the metal ion nature on the extent of energetic distortion of matrix-isolated sulfate ions was analyzed. Additionally, a special attention has been paid on infrared spectra of NH4 þ ions included in the crystals of M0 2M00 (XO4)2,6H2O (M0 ¼ K, Cs; M00 ¼ Mg, Co, Ni, Cu, Zn; X ¼ S, Se) [9e12,15]. In this paper we summarize our experimental results concerning the preparation and characterization of cesium copper compounds, Cs2Cu(SO4)2$6H2O and Cs2Cu(SeO4)2$nH2O (n ¼ 4, 6). The crystallization processes in the ternary systems Cs2XO4eCuXO4eH2O (X ¼ S, Se) at 25 C have been studied and the crystallization fields of the equilibrium solid phases have been determined. The crystal structure of Cs2Cu(SeO4)2$6H2O as determined from single crystal X-ray diffraction data at 250 K is reported (there is no structural data for this compound in the literature). Study on the phase transition of the cesium copper compounds using differential scanning calorimetry (DSC), differential thermal and thermogravimetric analysis (DTA and TG measurements) is reported. Infrared and Raman spectra of the double compounds are presented and discussed with respect to the normal vibrations of the anion groups and of the water molecules. The strength of the hydrogen bonds is analyzed in relation to the CueOH2 interactions (synergetic effect), proton acceptor abilities of the oxygen atoms as deduced from their bond-valence sum and proton acceptor strengths of the sulfate and selenate ions. 2. Experimental Cs2SeO4, Cs2SO4, and CuSeO4∙5H2O were prepared from cesium carbonate and copper hydroxide carbonate by treatment with dilute selenic or sulfuric acid solutions at 60e70 C. Then the resulting solutions were filtered, concentrated at 50e60 C, and cooled to room temperature. The crystals obtained were filtered, washed with ethanol and dried in air. The solubility in the three-component systems Cs2SO4eCuSO4eH2O and Cs2SeO4eCuSeO4eH2O at 25 C was studied using the method of isothermal decrease of supersaturation. The saturated solutions were vigorously stirred for two days (for more experimental details see also Ref. [14]). The analysis of the liquid phases and the wet solid phases was performed, as follows: the copper ion contents were determined complexonometrically at pH 5.5e6 using xylenol orange as indicator; the total concentrations of the sulfate ions were determined gravimetrically after precipitation as BaSO4; the selenate ions were precipitated as PbSeO4 with Pb(NO3)2 solutions and the excess of Pb2þ ions was determined complexonometrically using xylenol orange as indicator; the concentrations of the cesium sulfate and selenate were calculated by difference. Crystals of Cs2Cu(SeO4)2$6H2O were prepared by crystallization from solutions containing about 64 mass% cesium selenate and 1 mass% copper selenate at 7e8 C. The IR experiments show that the crystals are not stable in air and in several hours transform into Cs2Cu(SeO4)2$4H2O. The compositions of the solid phases from the studied systems were identified by means of both the X-ray diffraction and the infrared spectroscopy methods. Isotopic technique (matrix-isolated HDO molecules) was used to analyze the hydrogen bond strength formed in the title compounds.

441

Partially deuterated analogs (about 7e10%) were obtained employing the same crystallization procedure using H2OeD2O mixtures as a solvent (15 mass% D2O and 85 mass% H2O). In order to analyze the water librations in Cs2Cu(SeO4)2,4H2O (its spectrum is more complicated in this spectral interval as compared to those of the hexahydrates) a sample was prepared by crystallization of the tetrahydrate in pure D2O (the highest content of deuterium is about 70%). The samples of the selenates containing matrix-isolated SO4 2 ions were prepared using the same crystallization procedure in the presence of Cs2SO4. The amount of the cesium sulfate was calculated so that to obtain samples containing about 7e10 mass% sulfate ions. All reagents used were of reagent grade quality (Merck). The infrared spectra of the selenates were recorded on a Bruker model IFS 25 Fourier transform interferometer (resolution < 2 cm1) at ambient temperature using KBr discs as matrices in the case of selenate compounds. Ion exchange or other reactions with KBr have not been observed (infrared spectra using Nujol mulls were also measured). The spectra of Cs2Cu(SO4)2$6H2O were measured in Nujol mulls. In some cases Lorentz band profile for multi peak data was used to determine the correct band positions (ORIGIN PRO 6.1). The Raman spectra were recorded with a Horiba Jobin-Yvon LabRAM HR800 spectrometer using 600 1/mm grating and a 633 nm HeeNe laser line for excitation. The samples were placed under the 100 achromatic objective of a Olympus BX41 microscope and measured in back scattering configuration. The laser power on the sample was kept bellow 2.93 mW so that no heating and dehydration effects on the powder sample could be observed. The thermal behavior of the cesium copper compounds were investigated by thermogravimetric, differential thermal analysis and differential scanning calorimetry. Simultaneous TG-DTA curves were obtained applying a computerized combined apparatus for thermal analysis LABSYSEvo, SETARAM Company, France, at atmospheric pressure in a flow of synthetic air (MESSER CHIMCO GAS e OOH 1056, ADR 2, 1A) in the temperature range 25e400 С. Corundum crucibles with a volume of 100 ml were used. The sample weight in all tests was 50e60 mg. The experiments were carried out in dynamic mode, with heating rates of 5 С min1 and oxidizing gas flow rates of 20 ml min1. The DSC measurements were recorded on STA PT1600 (TG-DTA/DSC Simultaneous Thermal analysis) instrument in air up to 400 C at a heating rate of 5 C min1 using standard corundum crucibles (sample mass 20 mg). The X-ray powder diffraction data were collected within the range from 5 to 50 2q with a step 0.02 2q and counting time 35 s/ step on Bruker D8 Advance diffractometer with Cu Ka radiation and LynxEye detector. The lattice parameters of the cesium copper salts were calculated using the program ITO and refined with the program LSUCR. Single crystal X-ray diffraction data of Cs2Cu(SeO4)2$6H2O were measured on a Bruker APEX-II diffractometer equipped with a CCD area detector and an Incoatec microfocus source ImS (30 W, multilayer mirror, Mo Ka). Several sets of 4- and u-scans with 2 rotation per CCD frame were measured up to 2q ¼ 80 at a crystal to detector distance of 40 mm, covering the complete Ewald sphere with high average redundancy. In order to avoid rapid deterioration of the crystal in the X-ray beam, the data collection was performed at 250 K using a Cryostream 800 LT device from OxfordCryosystems. An absorption correction by multi-frame scaling was applied. The structure refinement on F2 with scattering curves for neutral atoms was performed with SHELXL-97 [16], atom labels and equivalent site positions were selected as in Refs. [15,17]. Table 1 summarizes crystal data and details of the data collection and structure refinement of Cs2Cu(SeO4)2$6H2O, final atomic coordinates and displacement parameters are listed in Table 2.

442


copper selenate up to solutions containing 11.68 mass% cesium selenate and 15.94 mass% copper selenate. The crystals of the double salts were dried in air and analyzed. The chemical analysis shows: for Cs2Cu(SeO4)2$6H2O (obtained at 7e8 C) e 56.41 mass% cesium selenate and 28.62 mass% copper selenate (theoretical content e 56.52 mass% cesium selenate and 28.55 mass% copper selenate); for Cs2Cu(SO4)2$6H2O e 57.36 mass % cesium sulfate and 25.44 mass% copper sulfate (theoretical content e 57.48 mass% cesium sulfate and 25.35 mass% copper sulfate); for Cs2Cu(SeO4)2$4H2O e 59.62 mass% cesium selenate and 29.92 mass% copper selenate (theoretical content e 59.47 mass% cesium selenate and 30.04 mass% copper selenate). The double salts were identified using X-ray powder diffraction. The calculated lattice parameters are, as follows: Cs2Cu(SO4)2,6H2O (SG P21/a) e a ¼ 9.432 (2) Å; b ¼ 12.754 (3) Å; c ¼ 6.311 (2) Å; b ¼ 106.20 (2) ; V ¼ 729.0 (2) Å3 (a ¼ 9.4383 (6) Å; b ¼ 12.7605 (12) Å; c ¼ 6.3130 (6) Å; b ¼ 106.199 (7) ; V ¼ 730.14 Å3 according to [18]); Cs2Cu(SeO4)2,4H2O (SG P21/n) e a ¼ 7.456 (2) Å; b ¼ 7.908 (2) Å, c ¼ 11.791 (2) Å; b ¼ 106.42 (2) ; V ¼ 666.9 (2) Å3 (a ¼ 7.427 (1) Å; b ¼ 7.878 (1) Å; c ¼ 11.743 (2) Å; b ¼ 106.31 (1) ; V ¼ 659.4 (2) Å3 according to [19]). The X-ray powder diffraction patterns of the above compounds are shown in Fig. 2.

Table 1 Crystal data and details of the X-ray data collection and structure refinement for Cs2Cu(SeO4)2$6H2O. Crystal system Space group Z a (Å) b (Å) c (Å) b ( ) V (Å3) m (mm1) Dcalc (g cm3) 2qmax Exposure time (s)/frame CCD frames measured Number of reflections Intensity data for unit cell Unique hkl's Fo > 4s(Fo) Ri (%) Variables wR2 [for all F2o] (%) R1 [for Fo > 4s(Fo)] (%) R1 [for all Fo] (%) Goodness of fit Weightinga parameter a/b Extinction coefficient Drmax/min (e Å3)

Monoclinic P21/a 2 9.592 (1) 12.925 (2) 6.416 (1) 105.44 (1) 766.7 10.9 3.134 80 20 975 52,846 9869 4745 3944 5.15 113 4.54 2.11 3.07 1.05 0.011/0.57 0.0011 (1) 1.19/0.88

Weighting scheme: w ¼ 1/[s F2o)] þ 2F2c }/3. a

2

(F2o)

3.2. Crystal structures of the cesium copper salts

2

þ (aP) þ bP]; P ¼ {[max of (0 or

3. Results and discussion 3.1. Crystallization in the Cs2SO4eCuSO4eH2O and Cs2SeO4eCuSeO4eH2O systems at 25 C The solubility diagrams of the above systems are shown in Fig. 1 (the experimental results are summarized in Tables 3 and 4). Three crystallization fields are observed in each system e crystallization fields of Cs2SO4, Cs2SeO4, CuSO4$5H2O and CuSeO4$5H2O and wide crystallization fields of double salts, Cs2Cu(SO4)2$6H2O and Cs2Cu(SeO4)2$4H2O. Fig. 1 shows that small amounts of the copper salts decrease considerably the solubility of the cesium salts a fact, which evidences that strong complex formation processes occur in the ternary solutions. Cs2Cu(SO4)2$6H2O crystallizes from solutions containing 61.07 mass% cesium sulfate and 2.58 mass% copper sulfate up to solutions containing 22.74 mass% cesium sulfate and 22.66 mass% copper sulfate. Cs2Cu(SeO4)2$4H2O e from solutions containing 76.42 mass% cesium selenate and 0.51 mass%

Cs2Cu(SO4)2,6H2O is reported to crystallize in the monoclinic space group P21/a (No 14, C2h) with two formula units in the unit cell [18,20]. It belongs to a large number of isomorphous compounds with a general formula M0 2M00 (XO4)2,6H2O (M0 ¼ K, NH4 þ , Rb, Cs; M00 ¼ Mg, Mn, Co, Ni, Cu, Zn; X ¼ S, Se) known as Tutton salts. The structure consists of isolated [Cu(H2O)6] octahedra (three crystallographically different water molecules are coordinated to the Cu2þ cations) and SO4 tetrahedra which are interconnected by six hydrogen bonds with Ow/O bond lengths between 2.693 and 2.779 Å (each water molecule in C1 site symmetry) and Csþ ions. The [Cu(H2O)6] octahedron (Cu2þ in Ci site symmetry) is strongly distorted due to the JahneTeller effect (Cu2þ in [4 þ 2]-coordination; distances vary in intervals of 1.965 and 2.311 Å). The sulfate tetrahedra are slightly distorted with respect to the SeO bond distances of 1.463 and 1.484 Å. The Csþ ions are ninefold coordinated by seven O atoms belonging to the sulfate tetrahedra and by two O atoms belonging to two water molecules (mean CseO bond length of 3.238 Å). According to the structural data Cs2Cu(SeO4)2,4H2O crystallizes in the monoclinic space group P21/n (No 14, C2h) with two formula units in the unit cell [19]. The structure is composed of

Table 2 Atomic coordinates and displacement parameters (Å2) for Cs2Cu(SeO4)2$6H2O. The ADP are defined as exp(e22ijUijhihjai aj ), the Ueq as Atom

x

Cu Cs Se O1 O2 O3 O4 Ow1 Ow2 Ow3 H11 H12 H21 H22 H31 H32

0 0.11848 0.38321 0.4032 0.5291 0.2481 0.3462 0.1421 0.1875 0.0020 0.172 0.208 0.270 0.174 0.075 0.026

y (1) (1) (1) (1) (1) (1) (1) (1) (1) (3) (3) (3) (3) (3) (3)

0 0.35167 0.14490 0.2479 0.0747 0.0765 0.1797 0.1072 0.1086 0.0609 0.094 0.125 0.097 0.168 0.058 0.125

z (1) (1) (1) (1) (1) (1) (10) (1) (1) (2) (2) (2) (2) (2) (3)

0 0.35143 0.73904 0.6014 0.7884 0.5917 0.9646 0.1560 0.0176 0.2790 0.263 0.103 0.061 0.001 0.309 0.301

Uiso/eq (2) (2) (2) (2) (2) (2) (2) (2) (2) (5) (5) (5) (5) (4) (5)

0.0152 0.0237 0.0153 0.0250 0.0293 0.0219 0.0245 0.0223 0.0261 0.0191 0.053 0.045 0.045 0.049 0.038 0.055

U11 (1) (1) (1) (2) (3) (2) (2) (2) (2) (2) (9) (8) (8) (8) (7) (9)

0.0147 0.0238 0.0133 0.0327 0.0165 0.0165 0.0302 0.0217 0.0193 0.0189

U22 (1) (1) (1) (6) (5) (4) (6) (5) (5) (4)

0.0166 0.0238 0.0167 0.0188 0.0285 0.0286 0.0274 0.0256 0.0250 0.0207

U33 (1) (1) (1) (5) (6) (5) (5) (5) (6) (5)

0.0146 0.0250 0.0158 0.0252 0.0402 0.0202 0.0181 0.0190 0.0339 0.0195

1 U a a a .a . 3ij ij i j i j

U23 (1) (1) (1) (5) (7) (5) (5) (5) (6) (4)

0.0006 0.0017 0.0000 0.0033 0.0025 0.0044 0.0039 0.0004 0.0017 0.0030

U13 (1) (1) (1) (4) (5) (4) (4) (4) (5) (4)

0.0043 0.0089 0.0037 0.0108 0.0028 0.0044 0.0103 0.0046 0.0069 0.0084

U12 (1) (1) (1) (4) (4) (3) (4) (4) (4) (4)

0.0035 0.0014 0.0020 0.0046 0.0047 0.0066 0.0061 0.0086 0.0012 0.0006

(1) (1) (1) (4) (4) (4) (4) (4) (4) (4)


443

Table 4 Solubility in the Cs2SeO4eCuSeO4eH2O system at 25 C (*, solid phase compositions determined from IR and X-ray powder diffraction data). Liquid phase, mass %

Wet solid phase, mass%

Cs2SeO4

CuSeO4

Cs2SeO4

CuSeO4

e 0.51 0.38 0.97 1.03 1.55 2.39 3.46 4.92 8.17 11.89 15.38 15.94 15.41 15.06 14.35 15.72

e 88.04

e 3.98 * 21.53 * * 22.14 21.73 * * 24.71 27.48 52.58 56.61 * 51.67 -

76.70 76.42 75.87 65.08 64.06 53.89 48.79 41.53 30.64 21.46 15.72 11.73 11.68 10.86 7.79 3.87

Fig. 1. Solubility diagrams of the Cs2SO4eCuSO4eH2O and Cs2SeO4eCuSeO4eH2O systems at 25 C.

[Cu(H2O)4(SeO4)2]2 units (Cu2þ in [4 þ 2]-coordination; CueO bond distances vary in intervals of 1.964 and 2.379 Å) which are linked via cesium ions and hydrogen bonds. The two crystallographically different water molecules are involved in four hydrogen bonds with Ow/O bond lengths between 2.660 and 2.772 Å. The selenate tetrahedra are slightly distorted (the SeeO bond lengths vary between 1.632 and 1.658 Å). The Csþ cations form irregular

59.47 * * 54.36 51.67 * * 46.58 49.82 18.44 3.59 * 1.73 e

Composition of the solid phases

Cs2SeO4 Cs2SeO4 + Cs2Cu(SeO4)2$4H2O Cs2Cu(SeO4)2$4H2O “e” “e” “e” “e” “e” “e” “e”

CuSeO4$5H2O + Cs2Cu(SeO4)2$4H2O CuSeO4$5H2O “e” “e” “e”

CsO11 polyhedra (CseO bond distances vary from 3.166 to 3.581 Å). All atoms and polyatomic units, except the Cu2þ ions, which lies at the center of inversion Ci, are located at general positions C1. The present crystal structure investigation of Cs2Cu(SeO4)2,6H2O establishes its membership to the group of Tutton salts. Selected interatomic distances and angles are summarized in Table 5, a projection of the crystal structure along the c-axis is shown in Fig. 3. As to be expected, a very close relationship exists with the isotypic sulfate [18] described above, but at a 5.0% larger cell volume in the selenate. The JahneTeller elongated Cu(H2O)6 polyhedra also show a [4 þ 2]-coordination and are very similar in both compounds; the most significant changes (but nevertheless weak with ~0.9 ) are found for the polyhedral angles involving the closest water molecule Ow3. Apart from the natural volume increase, also the SeO4 group closely mirrors the respective sulfate tetrahedron with a shorter SeeO2 bond and two nearly equal longest SeeO3, O4 bonds (Table 5); however, in the selenate this weak bond length distortion is even smaller (both relative and absolute) than in the sulfate. Likewise, differences in the hydrogen bonding system between the selenate (Table 5) and the sulfate are also quite marginal. Medium strong Ow/O hydrogen bonds range in the selenate between 2.684 and 2.787 Å, and respective distances differ by at most 0.023 Å from the sulfate. The most obvious (non-tetrahedral) differences

Table 3 Solubility in the Cs2SO4eCuSO4eH2O system at 25 C. Wet solid phase, mass%

Cs2SO4

CuSO4

Cs2SO4

CuSO4

64.88 61.07 60.84 55.53 48.27 42.99 29.07 23.37 21.84 22.89 22.74 21.95 16.79 7.69

e 2.58 2.32 1.32 1.48 3.23 9.69 17.81 19.31 22.04 22.66 23.18 20.27 19.72 18.30

e 70.11 58.25 56.43 54.51 54.96 49.18 47.36 49.47 50.17 39.41 3.69 7.25 2.37

e 11.36 17.09 16.19 14.24 20.78 19.74 22.64 24.83 25.44 33.57 58.74 44.81 53.52

Composition of the solid phases

Cs2Cu(SO4)2.6H2O Cs2SO4 Cs2SO4 þ Cs2Cu(SO4)2$6H2O Cs2Cu(SO4)2$6H2O “” “” “” “” “” “” “” CuSO4$5H2O þ Cs2Cu(SO4)2$6H2O CuSO4$5H2O “” “” “”

Intensity

Liquid phase, mass%

10

Cs2Cu(SeO4)2.4H2O

20

30

40

50

60

2Θ (degree) Fig. 2. X-ray powder Cs2Cu(SeO4)2$4H2O.

diffraction

patterns

of

Cs2Cu(SO4)2$6H2O

and

444


3.3. Dehydration of the cesium copper salts

Table 5 Selected interatomic distances (Å) and angles ( ) for Cs2Cu(SeO4)2$6H2O. SeeO2 SeeO1 SeeO4 SeeO3 CueOw3 CueOw1 CueOw2 CseO1 CseO4 CseO3 CseO1 CseO2 CseOw2 CseO2 CseOw1 CseO2 Ow1eH11 Ow1eH12 H11eOw1eH12 H11/O3 H12/O4 Ow1/O3 Ow1/O4 Ow1eH11/O3 Ow1eH12/O4

(12) (11) (12) (11)

1.9600 2.0131 2.3071 2.0934 3.081 3.113 3.157 3.203 3.231 3.231 3.283 3.429 3.611 3.260 0.69 0.82 109 2.05 1.92 2.737 2.739 171 173

(11) (12) (13) (1) (1) (1) (1) (1) (1) (2) (1) (2) (3) (3) (3) (3) (3) (2) (2) (4) (3)

O2eSeeO1 O2eSeeO4 O2eSeeO3 O1eSeeO4 O1eSeeO3 O4eSeeO3 Ow3eCueOw1 Ow3eCueOw2 Ow1eCueOw2

Ow2eH21 Ow2eH22 H21eOw2eH22 H21/O2 H22/O4 Ow2/O2 Ow2/O4 Ow2eH21/O2 Ow2eH22/O4 Ow3eH31 Ow3eH32 H31eOw3eH32 H31/O3 H32/O1 Ow3/O3 Ow3/O1 Ow3eH31/O3 Ow3eH32/O1

2a 2a 2a

109.93 110.98 108.51 109.63 108.35 109.38 89.57 90.14 89.45

(7) (7) (7) (6) (6) (6) (5) (5) (5)

0.83 0.79 104 1.93 2.00 2.760 2.787 173 175 0.78 0.87 106 1.95 1.83 2.710 2.684 169 170

(3) (3) (3) (3) (3) (2) (2) (3) (3) (3) (3) (3) (3) (3) (2) (2) (3) (3)

Plus corresponding supplementary angles.

between sulfate and selenate occur in the coordination of the Csþ cations with an average Cs [9]eO distance of 3.260 compared to 3.238 Å [18] in the sulfate. A closer inspection shows that this increase can be nearly exclusively attributed to changes of the three involved CseO2 bonds which significantly weaken in the selenate, especially the longest one, increasing by 0.084e3.611 Å. In the two corresponding rubidium-copper Tutton salts (selenate [21], sulfate [22]) this latter O2 oxygen atom drops out of the Mþ coordination, giving a clear-cut eightfold environment for the Rb cation with average Rb [8]eO distances of around 3.05 Å. Other differences with the Cs compounds are again rather moderate, apart from a remarkable tendency towards a [2 þ 2þ2]-coordination of copper just in the Rb-selenate representative (with 1.95, 2.06, and 2.26 Å [21]).

TG, DTA curves of the cesium copper salts are presented in Fig. 4 (left side TG measurements; right side e DTA measurements). The respective DSC curves are presented in Fig. 5. According to the TG and DTA curves the dehydration process of Cs2Cu(SeO4)2,4H2O occurs in one step in the temperature interval of 90e170 C, thus forming an anhydrous salt (see Fig. 4). The experimental mass loss (Dmexp ¼ 10.3 mass%) is in good agreement with the theoretical one (Dmcal ¼ 10.5 mass%). The dehydration process is registered on the DTA curve with a strong endothermic effect at 134 C. The small exothermic peak on the DTA curve at about 245 C is related probably to the recrystallization of the end anhydrous product (no mass loss is observed on the TG curve). The DSC curve (Fig. 5) displays one complex endothermic effect corresponding to the elimination of four water molecules e one strong peak with maximum at 129 C and two shoulders in the vicinity of 95 and 115 C (DHdeh has a value of 280.9 kJ mol1). Cs2Cu(SeO4)2$6H2O is reported to separate the water molecules in one step, thus forming an anhydrous salt [23]. However, no experimental evidences are presented and no comments with respect to the stability of this compound in air are made in this paper. Our experiments show that the dehydration of Cs2Cu(SeO4)2,6H2O starts at about 36 C and completes at about 170 C as seen from Fig. 4. As was mentioned above in the text the hexahydrate is unstable in air and this fact explains the unusual low start temperature of dehydration. The shapes of the TG and DTA curves indicate that the separation of the water molecules occur stepwise. The mass loss calculation from the TG curve shows that two water molecules are separated in the temperature interval of 36e75 C, thus producing Cs2Cu(SeO4)2,4H2O, which is stable in the temperature interval of 75e100 C (Dmexp ¼ 4.4 mass%; Dmcal ¼ 4.97 mass%). The formation of Cs2Cu(SeO4)2,4H2O is registered on the DTA curve with two endothermic peaks with maxima at 48 and 73 C, which could not be separated. The second stage of the dehydration registered on the TG curve between 100 and 166 C corresponds to the loss of the remaining four water molecules (Dmexp ¼ 14.8 mass%; Dmcal ¼ 14.93 mass%). However, a change in the slope of the TG curve is observed in the vicinity of 155 C (see

100

Cs Cu(SeO ) .4H O

96

90 245

92 88 Cs Cu(SeO )

84

Mass loss (%)

100 96

134

Cs Cu(SeO ) .6H O Cs Cu(SeO ) .4H O

36

92

48 73

Cs Cu(SeO ) .H O

161

Cs Cu(SeO )

88

145

84

100 96

Cs Cu(SO ) .6H O

75

Cs Cu(SO ) .H O

220

92 Cs Cu(SO ) .0.5H O

88

Cs Cu(SO )

84

0

100

200

300 0

Temperature ( C) Fig. 3. Crystal structure of Cs2Cu(SeO4)2$6H2O in a projection along the c-axis.

Endo

a

2 2 2

1.6270 1.6370 1.6417 1.6439 1.6374

400 0

110

100

200

300 0

400

Temperature ( C)

Fig. 4. TG and DTA curves of: Cs2Cu(SeO4)2$4H2O, Cs2Cu(SeO4)2$6H2O, and Cs2Cu(SO4)2$6H2O.


95

strong endothermic effect on the DTA curve centered at 110 C corresponds to the formation of the monohydrate, which transforms into an anhydrous compound at about 250 C (Dmexp ¼ 16.85 mass%; Dmcal ¼ 17.16 mass%). The comparatively small endothermic peak at 220 C appears as a result of the departure of the last water molecule. The DSC curve (Fig. 5) shows two well distinguished effects at 92 and 121 C, which are related to the separation of five water molecules (DHdeh has a value of 420.2 kJ mol1). Unfortunately, our experiments to determine the enthalpy of dehydration of the last water molecule failed. Close inspection of the TG curve shows that it changes its slope in the interval of 130e250 C from 130 C up to 200 C the monohydrate loses slowly a part of last water molecule, thus forming probably Cs2Cu(SO4)2$0.5H2O. At temperatures higher than 200 C the speed of the dehydration process increases rapidly and probably the correct scheme of dehydration is:

Cs2Cu(SeO4)2.4H2O

115

Endo

129

110 50

Cs2Cu(SeO4)2.6H2O

120 140

445

Cs2 CuðSO4 Þ2 $6H2 O/Cs2 CuðSO4 Þ2 $H2 O

150

/Cs2 CuðSO4 Þ2 0:5H2 O/Cs2 CuðSO4 Þ2 3.4. Infrared spectra of cesium copper compounds

121 Cs2Cu(SO4)2.6H2O

92

0

100

200

300 0

400

Temperature ( C) Fig. 5. DSC curves of Cs2Cu(SeO4)2$4H2O, Cs2Cu(SeO4)2$6H2O, and Cs2Cu(SO4)2$6H2O.

Fig 4), which is interpreted as a formation of a monohydrate, Cs2Cu(SeO4)2$H2O (Dmexp ¼ 12.5 mass%; Dmcal ¼ 12.44 mass%). The two strong endothermic effects on the DTA curve at higher temperatures are related to the transformation of the tetrahydrate into an anhydrous salt (maxima at 145 and 161 C, respectively). The DSC curve (Fig. 5) consists of two endothermic effects: the first one is centered at 50 C and corresponds to the separation of two water molecules (DHdeh ¼ 77.9 kJ/mol); the second one is complex and exhibits maxima at 110, 120, 140 and 150 C. The latter peaks are related to the separation of the remaining four water molecules (DHdeh ¼ 356.3 kJ/mol). Thus, the total enthalpy of dehydration of Cs2Cu(SeO4)2$6H2O to Cs2Cu(SeO4)2 has a value of 434.2 kJ mol1. Taking into account the mass loss calculation from the TG curve and the corresponding endothermic peaks registered on the DTA curve the following scheme of dehydration of Cs2Cu(SeO4)2,6H2O could be proposed:

Cs2 CuðSeO4 Þ2 $6H2 O/Cs2 CuðSeO4 Þ2 $4H2 O /Cs2 CuðSeO4 Þ2 $H2 O/Cs2 CuðSeO4 Þ2 The analysis of the TG and DTA curves of Cs2Cu(SO4)2$6H2O (see Fig. 4) shows that the dehydration process occurs according to the scheme:

Cs2 CuðSO4 Þ2 $6H2 O/Cs2 CuðSO4 Þ2 $H2 O/Cs2 CuðSO4 Þ2 Two endothermic peaks are observed on the DTA curve at 110 and 220 C, respectively, as shown in Fig. 4. According to the TG curve the separation of water molecules starts at about 75 C and completes at about 130 C, thus resulting in the formation of the monohydrate (Dmexp ¼ 14.20 mass%; Dmcal ¼ 14.30 mass%). The

3.4.1. Normal vibrations of the tetrahedral SO4 2 and SeO4 2 ions The free tetrahedral ions (XOn 4 ) under perfect Td symmetry are characterized with four internal vibrations: n1 (A1), the symmetric XeO stretching modes, n2(E), the symmetric XO4 bending modes, n3(F2) and n4(F2), the asymmetric stretching and bending modes, respectively. The normal vibrations of the free tetrahedral ions in aqueous solutions are reported to appear, as follows: for the selenate ions e n1 ¼ 833 cm1, n2 ¼ 335 cm1, n3 ¼ 875 cm1, n4 ¼ 432 cm1; for the sulfate ions e n1 ¼ 983 cm1, n2 ¼ 450 cm1, n3 ¼ 1105 cm1, n4 ¼ 611 cm1 [24]. On going into solid state, the normal modes of SO4 2 and SeO4 2 ions are expected to shift to lower or higher frequencies. The unit cell theoretical treatment for the monoclinic Tutton compounds (Z ¼ 2; factor group symmetry C2h) is presented in our previous paper [14]. The monoclinic unit cells of these compounds contains 62 atoms with 186 zone-center degrees of freedom. The 186 vibrational modes of the unit cell decompose according to the following representation: G ¼ 45Ag þ 45Bg þ 48Au þ 48Bu ; where 1Au þ 2Bu are translations (acoustic modes). The monoclinic unit cell of Cs2Cu(SeO4)2$4H2O (space group P21/n, Z ¼ 2; factor group symmetry C2h) contains 50 atoms with 150 zone-center degrees of freedom. The SeO4 2 ions (four SeO4 2 ions in the unit cell located on C1 sites) and the water molecules (eight molecules in the unit cell located on C1 sites; two crystallographically different types) contribute 60 internal modes to the 147 optical zone-center modes (each tetrahedral ion is characterized with nine normal vibrations and each water molecule with three normal vibrations, i.e. 36 internal modes for the tetrahedral ions and 24 internal modes for the water molecules). Due to the low site symmetry C1 of the SeO4 2 ions the degeneracy of both the doubly degenerate n2 modes and the triply degenerate n3 and n4 modes is removed (the non-degenerate n1 mode is activated). The nine internal modes of the tetrahedral ions are of A symmetry: one mode for the symmetric stretching vibrations (n1), two modes for the symmetric bending vibrations (n2), and three modes for both asymmetric stretching and bending vibrations (n3 and n4). The 36 optical modes for the SeO4 2 ions are subdivided into 9Ag þ 9Bg þ 9Au þ 9Bu modes due to the factor group symmetry C2h (related to interactions of identical oscillators, correlation field effect, see Fig. 6). The two different types of water molecules contribute 24 modes, as follows e 6Ag þ 6Bg þ 6Au þ 6Bu. The

446


1077 1063

1138

624

607

770 *

1105

a

550

981

451 437

653

870 881

893

b 839

877

895

d 1223

remaining 87 optical modes (external modes) are distributed between the translational and librational lattice modes. Thus, the unit cell theoretical treatment for the translational lattice modes (Csþ, SeO4 2 , Ow1 and Ow2) e all in C1 site symmetry, Cu2þ e in Ci site symmetry) and librational lattice modes (SeO4 2 , Ow1 and Ow2) yields: 51 translations (12Ag þ 12Bg þ 14Au þ 13Bu) and 36 librations (9Ag þ 9Bg þ 9Au þ 9Bu). Then the 150 vibrational modes of the unit cell decompose according to the following representation: G ¼ 36Ag þ 36Bg þ 39Au þ 39Bu ; where 1Au þ 2Bu are translations (acoustic modes). Since the crystal structures of the compounds under study are centrosymmetric, the Raman modes display g-symmetry, and their IR counterparts display u-symmetry (mutual exclusion principle). Infrared spectroscopic data for the cesium copper salts are scanty. Brown and Ross reported the vibrational frequencies of the sulfate and selenate ions in a series of Tutton compounds M0 2M00 (XO4)2,6H2O (M' ¼ Na, K, NH4, Rb, Cs; M00 ¼ Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd; X ¼ S, Se) [25]. The water librations (wagging modes) in Cs2Cu(SeO4)2,6H2O are briefly commented in Ref. [23]. Infrared spectra of the cesium copper salts in the region of the normal modes of the tetrahedral ions and water librations are presented in Fig. 7 (1400e400 cm1). Raman spectra in the interval of 1200e200 cm1 are shown in Fig. 8. According to the factor group analysis eight infrared bands and eight Raman bands corresponding to the stretching modes of the sulfate and selenate ions (six bands for the asymmetric vibrations and two bands for the symmetric ones) are expected to appear in the vibrational spectra of all compounds under study. The infrared spectrum of Cs2Cu(SO4)2,6H2O shows five infrared bands in the spectral range of 1138980 cm1, which arise from the stretching motions of the sulfate ions e the bands at 1138, 1105, 1077 and 1063 cm1 are attributed to n3 and that at 981 cm1 to n1

838 680

Fig. 6. Correlation diagrams between: (a) Td point symmetry, C1 site symmetry and C2h factor group symmetry (SO4 2 and SeO4 2 ions); (b) C2v point symmetry, C1 site symmetry and C2h factor group symmetry (two nonequivalent types of water molecules).

843

526

662

1150 738

480

554 604 535 480

878

413

636

413

504

443

c

636

843 834 420

895

738 688

Absorbance

789 713

429 409

1200 800 400 -1 Wavenumbers (cm ) Fig. 7. Infrared spectra of the cesium copper salts in the region of the internal vibrations of the tetrahedral ions and water librations: (a) Cs2Cu(SO4)2$6H2O; (b) Cs2Cu(SeO4)2$6H2O; (c) Cs2Cu(SeO4)2$4H2O; (d) Cs2Cu(SeO4)2$4H2O (about 70% D2O) (*, Nujol bands).

(Fig. 7a). The Raman bands of low intensity at 1160, 1135, 1114, 1086 and 1063 cm1 correspond to n3 and that of the high intensity at 984 cm1 to n1 (Fig. 8a). The larger number of the bands corresponding to the asymmetric stretching vibrations as expected according to the site symmetry analysis is due to the crystal field effects. The bending modes of the sulfate ions lead to the appearance of bands at 624, 607, 451 and 437 cm1 in the infrared spectrum and at 621, 608, 457 and 447 cm1 in the Raman spectrum, respectively. The higher frequency bands are assigned to the asymmetric bending motions and those at lower wavenumbers to the symmetric ones. The expected six infrared bands according to the factor group analysis for the asymmetric stretching modes n3 of the selenate tetrahedra in Cs2Cu(SeO4)2$6H2O coalesce into two bands at 893 and 881 cm1. Due to the low site symmetry C1 of the selenate tetrahedra the symmetric stretching mode n1 is activated (the band of small intensity at 839 cm1 corresponds to n1) (Fig. 7b). Thus, the appearance of two infrared bands only for n3 and one band for n1 indicates that the molecular symmetry of the SeO4 2 tetrahedra is close to C2v, i.e. higher than the crystallographic one (effective spectroscopic symmetry). However, the Raman spectrum exhibits a large number of bands corresponding to n3 (905, 885, 876, 862 cm1), i.e. more than expected according to the site symmetry


984

a

1160 1114 1086

608

447

621

1135 1063

832

b

862

Intensity

457

350

885

413 428

905 876

402

c

835

827

857

374

864

419 411

877

1000

332

464

885

1200

363

800

600

400

-1

200

Wavenumbers (cm ) Fig. 8. Raman spectra of the cesium copper salts in the region of the internal vibrations of the tetrahedral ions; (a) Cs2Cu(SO4)2$6H2O; (b) Cs2Cu(SeO4)2$6H2O; (c) Cs2Cu(SeO4)2$4H2O.

rules due to the crystal field effect. The Raman band of the highest intensity at 832 cm1 is attributed to the symmetric stretching motion (Fig. 8b). The two infrared bands of middle intensity at 429 and 409 cm1 correspond to two site symmetry components of n4 of the SeO4 2 ions. The respective Raman bands appear, as follows: 428, 413 and 402 cm1 (asymmetric bending modes n4) and at 350 cm1 (symmetric bending mode n2). The analysis of the infrared and Raman spectra of the isostructural sulfate and selenate hexahydrates needs some additional comments. It is reported in the literature that the intensity of the bands corresponding to n1 of the sulfate ions in a series of salts reflects their degree of distortion [26]. The authors established that the higher the intensity of these bands is the stronger the distortion of the polyatomic ions is. Thus, having in mind this statement we claim that the tetrahedra in Cs2Cu(SeO4)2$6H2O are less distorted as compared to those in Cs2Cu(SO4)2$6H2O. This claim is confirmed when the values of the splitting of n3 are taken into consideration 97 and 43 cm1 for the sulfate and selenate compounds, respectively (calculations are made from the Raman bands). (Dr for the sulfate ions has values of 0.021 Å [18] and that for the selenate ones 0.017 Å (this paper); Dr is the difference between the longest and the shortest S(Se)eO bond lengths in the tetrahedra). The stretching vibrations of the selenate groups in Cs2Cu(SeO4)2$4H2O result in the appearance of four intensive infrared bands at 895, 877, 843 and 834 cm1 forming doublets separated from one other with 34 cm1 (the number of the bands are

447

considerably less than that predicted from the factor group analysis; eight infrared bands expected) (see Fig. 7c). The bands on the right side doublet are of same intensity and they differ in frequencies with 9 cm1 only. These bands could be attributed either to two components of n3 as a result of crystal field splitting effects or to n3 and n1. The comparison of the infrared and Raman spectra shows that the Raman band at 835 cm1 is of the highest intensity and consequently it must correspond to n1 (Fig. 8c). In our opinion the infrared band at 834 cm1 arises from the symmetric stretching modes of the selenate tetrahedra. The unusual high intensity of this band could be explained with its position e it appears very close to that corresponding to n3 (infrared band at 843 cm1). The Raman bands at 885, 877, 864, 867 and 827 cm1 (bands of smaller intensity) are assigned to the asymmetric stretching modes n3 (i.e. one of the n3 components appears at lower frequencies than n1). The same phenomenon was observed in the Raman spectra of €hnkite-type compounds Na2Me(SeO4)2$2H2O (Me ¼ Mn, Zn, Cd) kro [27]. The large splitting of n3 (Dn3 ¼ 58 cm1, Raman bands at 885 and 827 cm1) indicates that the distortion of the selenate tetrahedra is larger than that predicted from the structural data (Dr ¼ 0.026 Å; [18]). The infrared spectrum confirms this assumption the bands at 843 cm1 is separated from those at 895 cm1 and 877 cm1 with 52 and 34 cm1, respectively. For comparison this difference is 12 cm1 for Cs2Cu(SeO4)2$6H2O (infrared bands at 893 and 881 cm1). The bands in the infrared spectrum Cs2Cu(SeO4)2$4H2O at 420 and 413 cm1 are assigned to n4 of the selenate groups. As far as the infrared band at 480 cm1 is concerned it remains its position and does not decrease in intensity upon deuteration and consequently it is attributed to n4 of the selenate tetrahedra (compare Fig. 7c and d). The Raman bands at 464 and 419 cm1 and that at 332 cm1 originate from the asymmetric and symmetric bending modes of the selenate ions, respectively. The bands at 374 and 363 cm1 are due probably to lattice vibrations. Additionally, we apply matrix infrared spectroscopy to prove our assumption that the selenate tetrahedron in Cs2Cu(SeO4)2$4H2O is stronger distorted than that in Cs2Cu(SeO4)2$6H2O. The influence of different chemical and structural properties of the host compounds on the degree of energetic distortion of matrixisolated SO4 2 ions in different matrices are widely discussed in our previous papers [9e13]. Furthermore, the degree of distortion of matrix-isolated polyhedra as deduced from the spectroscopic data is reported to match roughly the distortion of the host polyhedra shown from crystal structure data [28]. Fig. 9b and d shows infrared spectra of SO4 2 ions matrixisolated (about 7e10%) in the crystals of both selenates (for comparison the spectra of SO4 2 ions included in K2Cu(SeO4)2$6H2O and (NH4)2Cu(SeO4)2$6H2O are also shown (Fig. 9a and c, respectively; the spectra are taken from Ref. [10]). It is readily seen that the sulfate guest ions are considerably stronger distorted in the case of the Cs2Cu(SeO4)2$4H2O matrix Dn3 for the sulfate ions have values of 35 and 84 cm1 in the case of the hexahydrate and the tetrahydrate matrices (bands at 1131, 1109 and 1096 cm1, compared to 1145, 1110 and 1061 cm1, respectively). The value of the ratio Dnas/nc (where nc is the centro-frequency value of the asymmetric modes) is also proposed to calculate the relative splitting of dopant ions [29]. According to this rule the values of these ratios for the sulfate ions included in the hexahydrate and tetrahydrate matrices are 3.15 and 7.60%, respectively. The sulfate ions included in the potassium and cesium hexahydrates exhibit the same values of Dn3 (Fig. 9a and c), while those included in the (NH4)2Cu(SeO4)2$4H2O matrix are stronger distorted (Dn3 and Dn3/ nc have values of 53 cm1 and 4.81%, respectively Fig. 9c). As was commented in our previous papers the formation of hydrogen bonds between the SO4 2 guest ions with both the water molecules


1098

1109

*

a

1096

1131

* *

3354

3162

1200

1079

1000

d

1110

1145

2228

3098

1061

3376

1200

-1

1000

Wavenumbers (cm )

3.4.2. Vibrations of the water molecules. Strength of the hydrogen bonds 3.4.2.1. Normal vibrations of the water molecules. The water molecules in the cesium copper compounds are characterized with three and two sets of internal vibrations (Cs2Cu(SO4)2,6H2O, Cs2Cu(SeO4)2,6H2O, and Cs2Cu(SeO4)2,4H2O, respectively) n3, n2 and n1 (water molecules in C1 site symmetry). In absence of correlation field effects the stretching modes n3 and n1 of the water molecules in the hexahydrates are expected to exhibit six infrared bands and those in the tetrahydrate four bands. On the other hand, the water molecules coordinated to the JahneTeller copper ions could also be divided into two types depending on the CueOH2 bond lengths. Thus, the water molecules bonded to Cu2þ via shorter CueOH2 bonds (equatorial water molecules) are stronger polarized due to the stronger metal-water interactions (strong synergetic effect of the copper ions) and as a result they are expected to form stronger hydrogen bonds as compared to those included in longer CueOH2 bonds. Infrared spectra of the copper compounds in the regions of the stretching and bending modes of the water molecules are shown in Fig. 10. The number of the bands in the high frequency region related to the stretching modes of the water molecules is less than expected according to the site symmetry analysis due to the interactions of the identical OH oscillators. Several well distinguished bands are observed in the infrared spectra, as follows: bands at 3354 and 3162 cm1 (Cs2Cu(SO4)2,6H2O; Fig. 10a), bands at 3376, 3098 and 3054 cm1 (Cs2Cu(SeO4)2,6H2O; Fig. 10b), and 3362, 3288 and 3050 cm1 (Cs2Cu(SeO4)2,4H2O; Fig. 10c). The inspection of the spectra of the selenates reveals that two groups of infrared bands are observed, which are separated with more than 200 cm1.

3362

2240

1674

of the host compounds and the NH4 þ host ions obviously increases the field strength at the lattice sites where the guest ions are located and as a result the extent of energetic distortion of the guest ions increases as compared to the SO4 2 guest ions included in the potassium, rubidium and cesium selenate hexahydrates [9e12,15]. The remarkably strong distortion of the sulfate ions included in Cs2Cu(SeO4)2$4H2O (even stronger than in the ammonium salt) is owing to two main reasons the formation of strong CueOSO3 covalent bonds on one hand, and on the other the strong energetic distortion of the SeO4 2 host ions as deduced from the spectroscopic experiments commented above.

1665 1560

b

Fig. 9. Infrared spectra of the stretching modes of SO4 2 matrix-isolated (about 7e10%) in: (a) K2Cu(SeO4)2$6H2O; (b) Cs2Cu(SeO4)2$6H2O; (c) (NH4)2Cu(SeO4)2$6H2O; (d) Cs2Cu(SeO4)2$4H2O.

1573

3054

3050

1132

1092

3288

c

Absorbance

Absorbance

b

1108 1130

c

1547

a

1695

448

2210

4000

3200

2400

-1

1600

Wavenumbers (cm ) Fig. 10. Infrared spectra of the cesium copper salts in the region of OH vibrations and bending modes of the water molecules: (a) Cs2Cu(SO4)2$6H2O; (b) Cs2Cu(SeO4)2$6H2O; (c) Cs2Cu(SeO4)2$4H2O (*, Nujol bands).

This finding is much more pronounced in the case of Cs2Cu(SeO4)2,6H2O and is not observed in the spectrum of Cs2Cu(SO4)2,6H2O because probably of the broad Nujol band. Thus, the higher wavenumbered bands are attributed to water molecules involving in longer CueOH2 bonds (CueOH2 bond lengths have values of 2.379 and 2.307 Å in Cs2Cu(SeO4)2,4H2O and Cs2Cu(SeO4)2,6H2O, respectively) and those observed at lower frequencies to water molecules forming shorter CueOH2 bonds (CueOH2 bond lengths have values of 1.964 Å in Cs2Cu(SeO4)2,4H2O, and 1.960 and 2.013 Å in Cs2Cu(SeO4)2,6H2O, respectively). The bending modes n2 are observed at 1695, 1665 and 1674 cm1 (see Fig. 10a, b and c). Broad bands of low intensity appear in the vicinity of the bending modes: 1573 cm1 (Cs2Cu(SO4)2,6H2O), 1560 cm1 (Cs2Cu(SeO4)2,6H2O), and 1547 cm1 (Cs2Cu(SeO4)2,4H2O). Soptrajanov and Petrusevski [30] reported that in the case of Tutton compounds the complex spectral pictures observed in the region around 1500 cm1 are due to vibrational interactions between the bending modes n2 and overtones or combinations of water librations (especially those which appear in the 900700 cm1 region, i.e. rocking modes). According to the classification of Novak [31] hydrogen bonds larger than 2.70 Å are assigned as weak (the nOH stretching modes are expected to appear at wavenumbers larger than 3200 cm1). According to Soptrajanov and Petrusevski [30] the hydrogen bonds in Tutton compounds have to be assigned as medium strong. Thus, our experiments show that the equatorial water molecules form strong hydrogen bonds and those linked to copper ions via longer bonds form hydrogen bonds of medium strength. As far as the weak broad bands of small intensity at 2228 cm1 (Cs2Cu(SO4)2,6H2O), 2240 cm1 (Cs2Cu(SeO4)2,6H2O), and


2210 cm1 (Cs2Cu(SeO4)2,4H2O) are concerned, they originate probably from second order transitions (combinations of bending modes of the water molecules and some librations of the same species). 3.4.2.2. OD vibrations of matrix-isolated HDO molecules. The assignments of the bands corresponding to the OD vibrations of matrix-isolated HDO molecules (about 7e10% D2O) are made on the basis of the experimentally determined Ow/O bond distances [[18,19], this paper]. According to the authors the structural data for the respective H/O bond distances are reasonable and consequently, these bond lengths could be taken into considerations when the hydrogen bond strengths are discussed. The Brown's bond-valence theory is applied to estimate the hydrogen bond acceptor capacity of the oxygen atoms (calculations of the bondvalence sums of the proton acceptors are made according to [32]). Thus, oxygen atoms, which exhibit smaller values of the bond-valence sum act as acceptors of two hydrogen bonds. On the other hand, oxygen atoms included in two or more hydrogen bonds are reported to act as weaker proton acceptors as compared to those included in one hydrogen bond [[33,34] and Refs. therein]. Fig. 11 shows infrared spectra of the cesium copper compounds in the region of OD vibrations. It is readily seen that the bands corresponding to nOD of the HDO molecules form well recognized doublets analogically to those observed in the high frequency region, thus illustrating the influence of the JahneTeller effect on the hydrogen bond strength. According to the structural data the water molecules in Cs2Cu(SO4)2,6H2O form six hydrogen bonds with bond lengths, as follows: Ow1 e 2.714 and 2.738 Å, Ow2 e 2.777 and 2.779 Å, and Ow3 e 2.725 and 2.693 Å [18]. The bond-valence sums of the different oxygen atoms belonging to the sulfate tetrahedra have values of: O1 e 1.787 v.u., O2 e 1.833 v.u., O3 e 1.607 v.u., and O4 e 1.613 v.u. Thus, the oxygen atoms O3 and O4 are acceptors of two hydrogen bonds and consequently, they are expected to be

2354

2391 2462

a

2482

2493

b

2464

c

2482

2344 2304

2410

Absorbance

2336 2300 2462

2211

2800

2400

2000

-1

Wavenumbers (cm ) Fig. 11. Infrared spectra of the cesium copper salts in the region of uncoupled OD stretches (about 7e10% D2O) in: (a) Cs2Cu(SO4)2$6H2O; (b) Cs2Cu(SeO4)2$6H2O; (c) Cs2Cu(SeO4)2$4H2O.

449

included in weaker hydrogen bonds. The Ow2 molecules are linked to the copper ions via longer CuOw2 bonds and as results they form the weakest hydrogen bond (weaker synergetic effect). So, the bands at the highest wavenumbers 2482 and 2462 cm1 are assigned to hydrogen bonds formed by Ow2 (Ow2eH21/O4 and Ow2eH22/O2 hydrogen bonds; 2.04 and 1.91 Å, respectively; Fig. 11a). It is important to mention that irrespective of the same lengths of the Ow2/O2 and Ow2/O4 hydrogen bonds the wavenumbers of the bands corresponding to these bonds differ with 20 cm1, thus indicating that the Ow2 molecules are asymmetrically hydrogen bonded, i.e. their local molecular symmetry deviates from C2v molecular symmetry. The reason for this spectroscopic finding is the different proton acceptor strength of the oxygen atoms (O4 is included in two hydrogen bonds). The Ow1 molecules exhibit molecular symmetry close to C2v if the bond distances Ow1H11 and Ow1H12 are taken into account (0.77 and 0.75 Å, respectively) and they are symmetrically hydrogen bonded (H11/O3 and H12/O4 have values of 1.97 and 1.99 Å). The same is valid for the Ow3 molecules Ow3H31 and Ow3H32 have values of 0.74 and 0.71 Å, and H31/O3 and H32/O1 have values of 2.02 and 1.99 Å, respectively. So, the band at 2354 cm1 is related to both the Ow3/O3 and the Ow3/O1 hydrogen bonds and that at 2391 cm1 to both the Ow1/O3 and the Ow1/O4 hydrogen bonds. The isotopic ratios nOH/nOD calculated on the basis of the higher frequency bands (3354 and 3162 cm1) vary in the interval of 1.321.36. Four bands corresponding to nOD of HDO molecules are seen in the spectrum of Cs2Cu(SeO4)2,6H2O e 2493, 2462, 2336 and 2300 cm1 (Fig. 11b). The bands on the left side of the spectrum originate from the motions of water molecules Ow2 connected to the copper ions via the longest bond lengths (CueOH2 is 2.307 Å), thus resulting in the formation of the weakest hydrogen bonds (weaker synergetic effect). The hydrogen bonds Ow2/O2 and Ow2/O4 exhibit bond distances of 2.760 and 2.787 Å, respectively. Then the band at 2493 cm1 is attributed to the Ow2/O4 hydrogen bond and that at 2462 cm1 to the Ow2/O2 hydrogen bond in agreement with the structural data (for the Ow2eH21, Ow2eH22, H21/O2 and H22/O4 bond lengths see Table 5). The difference in the wavenumbers of the bands corresponding to the hydrogen bonds formed by Ow2 has value of 31 cm1, thus indicating that the local symmetry of Ow2 deviates considerably from C2v molecular symmetry, irrespective of the close hydrogen bond lengths. The formation of hydrogen bonds of different strengths as deduced from the frequencies of the respective infrared bands is explained with the smaller proton acceptor capability of the O4 atoms (O4 act as proton acceptors of two hydrogen atoms; bond-valence sum 1.638 v.u.), as compared to that of the O2 atoms, which are involved in one hydrogen bond only (bond-valence sum 1.794 v.u.). The band at 2300 cm1 is attributed to water molecules Ow3 forming the shortest hydrogen bonds (2.710 and 2.684 Å) and that at 2336 cm1 to water molecules Ow1 forming hydrogen bonds of middle lengths (2.737 and 2.739 Å). Thus, the spectroscopic experiments show that the equatorial water molecules Ow1 and Ow3 are symmetrically hydrogen bonded (at least at ambient temperature), i.e. their local molecular symmetry is close to C2v. The isotopic ratios nOH/nOD have values in the interval of 1.331.37. When the spectra of the isostructural sulfate and selenate are compared it is readily seen that the equatorial water molecules in the selenate compounds form stronger hydrogen bonds than those formed in the sulfate due to the stronger proton acceptor strength of the selenate ions [14,33e37]. The existence of two crystallographically different water molecules in Cs2Cu(SeO4)2,4H2O, Ow1 and Ow2, (each type in C1 site symmetry) will result in the appearance of four infrared bands corresponding to four uncoupled OD stretches of matrix-isolated

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HDO molecules. Indeed, Fig. 11c shows four infrared bands, as follows: 2482, 2464, 2344, and 2304 cm1. Analogically to the hexahydrates, the bands at higher frequencies arise from OD vibrations of water molecules linked to copper ions via longer CuOw2 bonds (2.379 Å) and those at lower frequencies to water molecules coordinated to copper ions via shorter CuOw1 bonds (1.964 Å). As above the bands corresponding to the OD motions are assigned taking into account both the Ow1/O and the H/O hydrogen bond lengths. The water molecules form four hydrogen bonds, as follows: Ow1/O3 (2.704 Å) and H11/O3 (1.94 Å), and Ow1/O2 (2.660 Å) and H12/O2 (1.83 Å); Ow2/O3 (2.762 Å) and H21,,,O3 (1.99 Å), and Ow2/O1 (2.772 Å) and H22/O1 (2.09 Å). The band at 2482 cm1 is attributed to the H22/O1 hydrogen bonds and that at 2464 cm1 is attributed to H22/O3 hydrogen bonds. The Ow1 molecules form stronger hydrogen bonds the band at 2304 cm1 is assigned to the H12/O2 hydrogen bonds and that at 2344 cm1 to the H11/O3 hydrogen bonds. According to the spectroscopic experiments the water molecules in Cs2Cu(SeO4)2,4H2O are distorted, especially the water molecules forming stronger hydrogen bonds (Dn has a value of 40 cm1) a finding, which could not be predicted from the structural data [19]. The values of the calculated ratios nOH/nOD vary in the interval 1.301.35. As far as the band at 2410 is concerned it reflects interactions between n2 and water librations (n2 þ LR). 3.4.2.3. Water librations. The water librations in the Tutton compounds are discussed in Refs. [23,38,39] as well as in our previous paper [9e14]. Two types of water librations are commented in the literature e rocking and wagging, the former observed at higher frequencies. Each type of water librations is characterized with two broad bands the first group of bands appear in the range of about 900700 cm1 (rocking) and 650500 cm1 (wagging) [38]. The bands belonging to each group which are observed at higher frequencies are assigned to the equatorial water molecules, i.e. those forming stronger hydrogen bonds. Infrared spectra of the cesium copper compounds in the region of water librations are shown in Fig. 7. The bands at 870 and 770 cm1 are assigned to rocking modes and those of 653 and 550 cm1 to wagging modes of the water molecules in Cs2Cu(SO4)2,6H2O, respectively (see Fig. 7a). The bands at 789 and 713 cm1, and 636 and 526 cm1 result from rocking and wagging modes of the water molecules in Cs2Cu(SeO4)2,6H2O, respectively (Fig. 7b). As far as the selenate tetrahydrate is concerned several bands are observed in its spectrum in the region below 1000 cm1 738, 688, 662, 554, and 480 cm1 (Fig. 7c). In order to assign correctly the bands corresponding to water librations the spectrum of Cs2Cu(SeO4)2,4H2O is compared with that of a highly deuterated sample (about 70% D2O; see Fig. 7d; our attempt to prepare a fully deuterated sample failed). The comparison of Fig. 7c and d shows that the bands at 662 and 554 cm1 disappear in the spectrum of the deuterated sample and consequently they are attributed to librations of H2O. The band at 738 cm1 is due to the rocking motions and those at 662 and 554 cm1 to the wagging ones of H2O. The new band at 604 cm1 is assigned to rocking vibrations of D2O (LR(H2O)/LR(D2O) has a value of 1.22). The wagging modes of D2O appear at 535 and 443 cm1 (LW(H2O)/LW(D2O)) has values of 1.24 and 1.25, respectively). The weak bands at 636 and 504 cm1 correspond to wagging modes of HDO molecules the respective ratios are 1.04 and 1.1 (LW(H2O)/LW(HDO)). However, the band at 688 cm1 (Fig. 7c) does not change its intensity and shifts slightly to 680 cm1 upon deuteration (Fig. 7d) and could not be related to water librations. In our opinion this band originates from CueO motions. The band at 480 cm1 is related to the asymmetric stretching modes of the selenate tetrahedra as was commented above in the text. The n2 mode of heavy water is recognized at

1223 cm1. It is reported in the literature that the splitting of the bands corresponding to the wagging modes is a measure for the deformation of the octahedral hydrate sphere. Indeed, the data presented in Ref. [23] show that with exception of two copper Tutton compounds the value of DLW is not more than 60 cm1 (DLW for K2Cu(SeO4)2,6H2O is 115 cm1 and for (NH4)2Cu(SeO4)2,6H2O 165 cm1, respectively). The spectroscopic experiments reported in this paper confirm this claim DLW for Cs2Cu(SO4)2,6H2O is 103 cm1, that for Cs2Cu(SeO4)2,6H2O is 110 cm1, and that for Cs2Cu(SeO4)2,4H2O 109 cm1 (Dr have values of 0.345, 0.347 and 0.415 Å, respectively; Dr is the difference between the longest and the shortest CueO bond lengths in the respective octahedra). 4. Conclusions Cs2Cu(SO4)2,6H2O and Cs2Cu(SeO4)2,4H2O crystallize from the respective ternary solutions within wide concentrations ranges. Cs2Cu(SeO4)2,6H2O belongs to the group of Tutton compounds, i.e. it crystallizes isotypic to the respective sulfate in a monoclinic structure type which is characterized by isolated Cu(H2O)6 and SeO4 tetrahedra interlinked with six hydrogen bonds and cesium cations. The analysis of the infrared and Raman spectra reveals: (i) The selenate tetrahedron in Cs2Cu(SeO4)2,4H2O is comparatively stronger distorted as compared to that in Cs2Cu(SeO4)2,6H2O as deduced from the positions of the bands correspondent to the normal motions of the respective tetrahedra. (ii) The infrared spectra of SO4 2 ions matrix-isolated in the structures of both selenates confirm this claim Dn3 for the sulfate ions have values of 35 and 84 cm1 in the case of the hexahydrate and tetrahydrate matrices. (iii) The equatorial water molecules of the JahneTeller distorted Cu2þ polyhedra are symmetrically hydrogen bonded (molecular symmetry close to C2v) at least at ambient temperature. (iv) The equatorial water molecules in the selenates form stronger hydrogen bonds as compared to that formed in the sulfate compound in agreement with the well-known rule that the selenate ions are stronger proton acceptors than the sulfate ones. (v) The large differences in the wavenumbers of the bands corresponding to the wagging modes (more than 100 cm1) reflect the strong deformation of the Cu(H2O)6 octahedra. (vi) The spectroscopic experiments of the cesium copper compounds are an excellent example for the influence of the JahneTeller effect on the hydrogen bond strength. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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