A CORRELATION BETWEEN STRUCTURAL AND VIBRATIONAL SPECTROSCOPIC DATA OF SOME BERYLLIUM SULFATES AND SELENATES (REVIEW)

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1 Journal of Chemical Technology and Metallurgy, 51, 1, 2016, 5-31 A CRRELATIN BETWEEN STRUCTURAL AND VIBRATINAL SPECTRSCPIC DATA F SME BERYLLIUM SULFATES AND SELENATES (REVIEW) Department of Inorganic Chemistry University of Chemical Technology and Metallurgy 8 Kl. hridski, 1756 Sofia, Bulgaria vkar@mail.bg Received 22 April 2015 Accepted 03 September 2015 ABSTRACT The present paper summarizes experimental results on the study of some beryllium compounds and is a part of a dissertation entitled Synthesis, structure and properties of some beryllium salts sulfates and selenates. The solubility diagrams of the three-component systems BeSe 4 -K 2 Se 4, Be -Rb 2, and BeSe 4 - MSe 4 (M = Co, Ni, Cu, Zn) at 25ºC are presented and the crystallization field widths of the solid phases are determined. The experimental results are discussed with respect to the complex formation processes in the ternary solutions and the solubility of the salt components in their binary solutions. New compounds, Rb 2 Be( and K 2 Be(Se 4, have been obtained as a result of co-crystallization processes in the systems pointed above. The crystal structures of BeSe 4, M 2 Be( (M = K, Rb), and K 2 Be(Se 4 are determined from single crystal X-ray diffraction data. It has been established that the beryllium selenate tetrahydrate crystallizes in the orthorhombic space group Cmca (D 18 ). The double beryllium compounds are isostructural and crystallize in 2h the monoclinic space group P2 1 /c (C 5 ). 2h Vibrational (infrared and Raman) spectra are recorded in the regions of the normal vibrations of the different motives building up the structures of the beryllium compounds studied. The vibrational spectra are discussed in the light of both the crystal structures of the beryllium salts and the chemical properties of the different entities. Special attention is paid to the influence of different crystal chemical factors on the strength of the hydrogen bonds formed in the beryllium compounds. These are the Be interactions (synergetic effect), the proton acceptor capacity of the selenate and sulfate ions, the proton acceptor capacity of the different oxygen atoms as determined according to the Brown s bond valence theory, the compositions of the beryllium tetrahedra (acidity of the water molecules), the anti-cooperative effect (proton donor and proton acceptor competitive effect), the repulsion potential of the lattice sites of water molecules location, the size of M + cations. The intramolecular H bond distances are derived from the n D vs. r D correlation curve [H.D. Lutz, C. Jung, J. Mol. Struct., 404, 1997, 63-66]. Water librations of protiated and deuterated samples are also discussed. Crystal matrix infrared spectroscopy was applied to analyze: (i) the distribution of ions included in the structure of K 2 Be(Se 4 at two crystallographically different positions; (ii) the molecular symmetry of Se 4 ions in BeSe 4 determined from the vibrational behavior of matrix-isolated guest ions; (iii) the strength of the hydrogen bonds (with the application of the isotopic dilution method and that of the matrix-isolated HD molecules). Keywords: beryllium sulfates and selenates, solubility diagrams, crystal structures, vibrational spectroscopy, hydrogen bond strength, crystal matrix infrared spectroscopy. INTRDUCTIN The interest towards beryllium compounds is determined by their promising electro-physical and optical properties. However, the chemistry of beryllium salts is not well studied due probably to the great latent toxicity which beryllium ions exhibit to biological systems. Long-time studies on some beryllium iodates are performed at the Department of General and Inorganic Chemistry of the University of Chemical Technology 5

2 Journal of Chemical Technology and Metallurgy, 51, 1, 2016 and Metallurgy (Sofia) [1-7]. ur investigations [8-16] are a continuation of these studies and concern the structure and properties of some beryllium sulfates and selenates. Several crystal hydrates of beryllium sulfate are discussed in the literature [17-19]. Be 4H 2 and Be 2H 2 have been found to crystallize in the three-component system Be studied in large temperature and concentration intervals (up to 95ºC and 80 mass % H 2 [17]. Lower crystal hydrates are obtained by heating of the tetrahydrate at different temperatures. For example, the multistage thermal dehydration of BeSe 4 as studied [19] by simultaneous thermal analysis (DTA, TG. DTG) is described by: Be 3.8H 2 95 C Be 1.8H C Be 706 C Be C Be 0.8H 2 The crystal structure of Be as determined from single crystal X-ray and neutron diffraction data is reported in ref. [20].Values of d-spacing are published in ref. [17] for Be and Be and in ref. [18] for Be H 2 (X-ray powder diffraction data). However the lattice parameters are not calculated. The vibrational spectra (infrared and Raman) of Be are commented in refs.[21-23]. The behavior of the Be 2+ ions in aqueous solutions is widely discussed in the literature [24-27, see also the references in [16]]. The crystallization processes in some three-component systems with the participation of beryllium sulfate are reported in the literature: Be -К 2 (0, 25, 60, 75 and 99.5 С (see the references in [16]); Be - (NH 4 (0 and 50 С) [28, 29]; Be -Na 2 - H 2 (0, 25, 50, 60, 75, 86 and 99.5 С) [30]. Some double compounds have been isolated from the above systems: K 2 Be( crystallizes in the whole temperature interval and (NH 4 Be( crystallizes up to 50 С (the anhydrous compound crystallizes at temperatures higher than 50 С). Sodium sulfate forms several double salts, Na 2 Be, 3Na 2 Be, Na 2 3Be. The solubility in the three-component systems of the type Be -M (M = Li, Ca, Mg, Mn, Fe, Zn) are discussed in a number of papers (the respective solubility diagrams are of simple eutonic type) [31-33], see also the references in [16]. The literature data on the beryllium selenates are scanty (see the references in [16]). For example, Selivanova et al. reported d-spacings values for BeSe 4, BeSe 4 and BeSe 4 (the diffraction peaks are not indexed) [34]. According to [34] the thermal dehydration of BeSe 4 leads to the formation of BeSe 4 and BeSe 4 at 100 and 300ºC, respectively. The thermal behavior of (NH 4 Be(Se 4 is described in [35]. As mentioned in the abstract the present paper analyzes and summarizes our experimental results [8-16] obtained in the study of some beryllium compounds - sulfates and selenates. The correlation between the crystal structures of the beryllium compounds and their vibrational spectra is focused. Special attention is paid to the strength of the hydrogen bonds formed in these compounds. The small beryllium ions are known to display strong Be H 2 interactions (synergetic effect), i.e. a strong increase of the donor strength of water molecules hydrogen bond coordinated to beryllium ions due to the large ionic potential of the Be 2+ ions. This is revealed by the strong hydrogen bonds formed in hydrated beryllium salts (short w bond distances and considerable red-shifts of the D stretches of matrix-isolated HD molecules), even if the hydrogen bond acceptor strengths of the corresponding oxygen acceptors are small [5, 36]. Crystal matrix infrared spectroscopy is applied in several cases: (i) to determine the distribution of the ions included in the structure of K 2 Be(Se 4 at two crystallographically different positions. (ii) to determine the molecular symmetry of Se 4 ions in BeSe 4 (matrix-isolated guest ions). (iii) to analyze the influence of different crystal factors on the strength of the hydrogen bonds formed in the beryllium compounds under study (with the application of the method of isotopic dilution and that of matrix-isolated HD molecules). EXPERIMENTAL The experimental conditions are briefly commented in this part of the paper (more information is provided in refs. [8-16]). BeSe 4, K 2 Se 4, Rb 2, MSe 4 6H 2 (M = Co, Ni, Zn), and CuSe 4 5H 2 were prepared by neutralization of beryllium oxide, metal carbonates and hydroxide carbonates with dilute selenic or sulfuric acid solutions at 60-70ºC. Then the solutions were filtered, concentrated at 40-50ºC, and cooled to room temperature. The crystals were filtered, washed with alcohol 6

3 and dried in air. The solubility in the three-component systems K 2 Se 4 -BeSe 4, Rb 2 -Be, and BeSe 4 -MSe 4 6H 2 (M = Co, Ni, Cu, Zn) was studied at 25ºC by the method of isothermal decrease of supersaturation. Solutions containing different amount of the salt compounds corresponding to each point of the solubility diagrams were heated at about 50-60ºC and cooled to room temperature. Then the saturated solutions were vigorously stirred [37]. Equilibrium between the liquid and solid phases was reached in about 2-3 days. The content of the salt compounds in the liquid and solid phases was determined using different analytical methods previously described [10, 13, 15, 16]. The compositions of the solid phases were identified by X-ray diffraction and the infrared spectroscopy. Deuterated beryllium salts were prepared by crystallization from solutions containing different ratios of H 2 /D 2. Lower crystal hydrates and anhydrous compounds were prepared by heating of the crystal hydrates in the temperature intervals of ºC [2, 14]. Be was obtained by crystallization according to the solubility in the system Be. All reagents used were of reagent grade quality (Merck, Fluka, Aldrich). The infrared spectra were recorded on Bruker model IFS 25 and IFS 113 Fourier transform interferometers (resolution < cm -1 ) at ambient and liquid nitrogen temperature using KBr discs as matrices. Ion exchange or other reactions with KBr were not observed. Raman spectra were recorded using Bruker RFS-100\S FT interferometer (Ge-diode detector)(resolution < cm -1 ). ANd/YAG laser at 1064 nm was applied. The X-ray powder diffraction patterns were obtained using a DRN-3 diffractometer (Cu Kα radiation at a scanning speed of 1º min -1 ) and a Bruker D8 Advance diffractometer with Cu Kα radiation and SolX detector from 5 to 60 2θ with a step θ and counting time 35 s/step. The lattice parameters of some lower crystal hydrates of the beryllium sulfates and selenates as well as of the anhydrous compounds were calculated and refined using the programs IT and LSUCR, respectively. Single crystal X-ray diffraction data were measured at room temperature on a Nonius Kappa-CCD diffractometer equipped with an X-ray capillary optics collimator, using graphite mono chromatized Mo Kα radiation. The integration and correction of the intensity data, a pseudo-absorption correction by frame scaling, and the refinement of lattice parameters were done with the program DENZ-SMN [38]. The structure was solved by direct methods using the program SHELXS-97 [39] and refined on F2 with SHELXL-97 [40]. Hydrogen atoms were located by difference Fourier syntheses and refined with isotropic displacementparameters. Scattering curves for neutral atoms were used (for more experimental details for the structural data see Refs. [9-11, 13]). RESULTS AND DISCUSSIN 1. Crystallization processes in three-component salt-water systems The solubility diagrams of the three-component systems BeSe 4 -K 2 Se 4 and Be -Rb 2 are presented in Fig. 1 (the respective experimental solubility data are summarized in [10, 13]). Three Fig. 1. Solubility diagrams of the Be -Rb 2 and BeSe 4 -K 2 Se 4 systems at 25ºC. 7

4 Journal of Chemical Technology and Metallurgy, 51, 1, 2016 crystallization fields are observed in the diagram of BeSe 4 -K 2 Se 4 - two comparatively narrow crystallization fields of the simple salts, K 2 Se 4 and BeSe 4, and a wide crystallization field of the double salt K 2 Be(Se 4. It crystallizes from solutions containing mass % of beryllium selenate and 8.50 mass % of potassium selenate up to solutions containing 7.02 mass % of beryllium selenate and mass % of potassium selenate. Fig. 1 shows three comparatively wide crystallization fields in the solubility diagram of Be -Rb 2 - two crystallization fields of the simple salts, Rb 2 and Be, and a crystallization field of the double salt Rb 2 Be(. It crystallizes from solutions containing mass % of beryllium sulfate and mass % of rubidium sulfate up to solutions containing mass % of beryllium sulfate and of mass % rubidium sulfate. The existence of K 2 Be(Se 4 and Rb 2 Be( is reported for the first time in our previous papers [10, 13]. The crystallization of the double salts in comparatively large concentration ranges indicates that strong complex formation processes occur in the ternary solutions. An attempt is made to synthesize double salts between beryllium selenate and selenates of cobalt, nickel, copper and zinc. The solubility diagrams of the threecomponent systems BeSe 4 -MSe 4 (M = Co, Ni, Cu, Zn) are shown in Fig. 2 (solubility data are given [15]). The solubility curves consist of two branches corresponding to the crystallization fields of simple salts (systems of a simple eutonic type). According to the experimental results the three-component systems are divided into two groups: (i) systems in which the crystallization field of BeSe 4 is very narrow (BeSe 4 -Ni(Cu)Se 4 ) and (ii) systems in which the crystallization fields of both simple salts are of close widths (BeSe 4 -Co(Zn)Se 4 ). The widths of the Fig. 2. Solubility diagrams of the BeSe 4 -MSe 4 (M = Cu, Ni, Co, Zn) systems at 25ºC. 8

5 crystallization fields could be explained if the solubility of the salts in their binary solutions, their water activities, respectively, is taken into consideration. As a rule the less soluble salts have larger values of water activity and as a consequence they exhibit larger crystallization fields when compared to those having smaller values of water activity [41]. Both the strong Be H 2 interactions (due to the covalent character of the respective Be bonds) and the strong intermolecular interactions HH Se 3 (due to both the strong proton acceptor capacity of the selenate ions and the strong acidity of water molecules coordinated to the beryllium ions) result in small values of water activity, thus leading to narrower crystallization fields of the beryllium selenate tetrahydrate as compared to those of the other metal selenate crystal hydrates. This finding is much more pronounced in the case of the co-crystallization of the beryllium selenate with the selenates of nickel and copper (the latter are considerably less soluble in water than beryllium selenate tetrahydrate) and hence the selenates of nickel and copper crystallize within larger concentration ranges than the selenates of cobalt and zinc. 2. Structural data ofberyllium compounds The crystal structure of BeSe 4 was determined from single crystal X-ray diffraction data at ambient and liquid nitrogen temperatures [9]. BeSe 4 crystallizes in the orthorhombic space group Cmca (D 18 ) (at ambient temperature: a = (1), b = 2h (1), c = (1), V = Å 3, Z = 8). Interatomic bond lengths and angles are summarized in ref. [9]. BeSe 4 crystallizes with a pseudo-tetragonal orthorhombic cell and is closely related to the structure of acentric tetragonal Be (SG I-4c2 [20]). The tetragonal setting of the orthorhombic cell corresponds to a = b = Å, c = Å, a, b = 90 º, g = º, V = Å 3, and Z = 4. While the mutual position of the Be and X atoms (X = S, Se) is equivalent in both compounds (i.e. distorted CsCl type) the replacement of the sulfate by the larger selenate ions leads to pronounced polyhedral rotations and to a partial rearrangement of the hydrogen bonding scheme in BeSe 4.The crystal structure of BeSe 4 is composed of isolated Be(H 2 ) 4 and Se 4 tetrahedra which are interconnected by strong hydrogen bonds ( W distances vary from Å to Å). Contrary to the expectation, the unit cell volume of the tetrahydrate and the mean Be and Se bond lengths increase with temperature decrease. The mean Be distance of Å in beryllium selenate equals the respective value in the sulfate [9] within the limits of error (ambient temperature). The mean Se distance of Å complies well with the crystal chemical experience. The selenate ions occupy C 2 (x) positions, while the beryllium tetrahedral C s (yz) one. The three crystallographically different water molecules (H 2 (1) and H 2 (2) in C s (yz) site symmetry, H 2 (3) in C 1 site symmetry) donate very strong hydrogen bonds. Fig. 3 shows the polyhedral arrangement and the hydrogen bonding scheme in BeSe 4 and Be. Fig. 3. Crystal structures of (a) BeSe 4 and (b) Be in projections along the c-axes. 9

6 Journal of Chemical Technology and Metallurgy, 51, 1, 2016 The single crystal X-ray diffraction measurements show that K 2 Be(, K 2 Be(Se 4, and Rb 2 Be(, are isostructural [10, 11, 13, 16]. They crystallize in the monoclinic space group P2 1 /c (C 5 2h ) (Z = 4) with close lattice parameters: K 2 Be( : a = (2), b = (2), c = 7.314(1) Å, b = 95.09(1)º, V = Å 3 ; K 2 Be(Se 4 : a = (2), b = (1), c = 7.491(1) Å, b = 95.77(1)º, V = Å 3 ; Rb 2 Be( : a = (2), b = (2), c = 7.431(1) Å, b = 96.33(1)º, V = Å 3 (for more experimental details see refs. [10, 11, 13, 16]). The crystal structures are characterized by three-membered chain fragments, composed of a central Be 2 (H 2 polyhedron sharing corners with two S(Se) 4 tetrahedra. These bent [Be(X 4 (H 2 ] units are arranged to form double layers parallel to (100). Linkage within these double layers as well as between adjacent double layers is accomplished by two types of potassium and rubidium ions, respectively, as well as hydrogen bonds, resulting in three-dimensional framework structures (see Fig. 4). Selected interatomic bond lengths and angles are listed in refs.[10, 11, 13]. Two types of S(Se) 4 tetrahedra and a single type of Be 2 (H 2 tetrahedra exist in the structures (S(Se) and Be 2+ ions are located at C 1 site positions). The two crystallographically different water molecules are asymmetrically hydrogen bonded (each water molecule in C 1 site symmetry) and show W hydrogen bond distances in intervals of Å (the hydrogen bond strengths as determined from the infrared spectroscopic data will be discussed below in the text). The unit cell parameters of several beryllium sulfates and selenates have been calculated using X-ray powder diffraction data: Be and BeSe 4 are isomorphous and crystallize in the orthorhombic crystal system (Be : a = 5.752(1) Å, b = 9.605(2) Å, c = 4.520(1) Å, V = 249.7(6) Å 3 ; BeSe 4 : a = 5.843(2) Å, b = 9.790(3) Å, c = 4.692(1) Å, V = 268.4(6) Å 3 ). BeSe 4 is isostructural with the respective anhydrous beryllium sulfate [8, 14, 16] and crystallizes in the tetragonal space group I-4 with lattice parameters: a = 4.648(1) Å, c = 7.084(3) Å, V = 153.1(1) Å 3. K 2 Be(Se 4 forms monoclinic crystals with lattice parameters: a =9.217(3) Å, b = (3) Å, c = 8.989(2) Å, β = (4)º, V = Å 3, while K 2 Be( forms tetragonal crystals with lattice parameters: a =b = 7.232(2) Å, c = (2) Å, V = 741 Å 3. The respective values of d-spacings and hkl indices are listed in refs. [12, 16]. 3. Vibrational spectra (infrared and Raman) of some beryllium compounds. Correlation between structural data and vibrational spectra Fig. 4. Crystal structures of (a) K 2 Be(Se 4 in a projection along the a-axis and (b) Rb 2 Be( in a projection along the b-axis Normal vibrations of tetrahedral ions ( and Se 4 ) The free tetrahedral ions and Se 4 under perfect T d symmetry exhibit four internal vibrations: n 1 (A 1 ), symmetric X stretching modes, n 2 (E), symmetric X 4 bending modes, n 3 (F 2 ) and n 4 (F 2 ), asymmetric stretching and bending modes, respectively. Fig. 5 shows the scheme of the tetrahedral ions normal vibrations. The normal vibrations of the free tetrahedral ions in aqueous solutions are reported to appear, as follows: for the selenate ions n 1 = 833 cm -1, n 2 = 335 cm -1, n 3 = 875 cm -1, n 4 = 432 cm -1 ; for the sulfate ions n 1 = 983 cm -1, n 2 = 450 cm -1, n 3 = 1105 cm -1, n 4 = 611 cm -1, while for the ammonium ions n 1 = 3040 cm -1, n 2 = 1680 cm -1, n 3 = 3145 cm -1, n 4 = 1400 cm -1 [42]. In the course of solidification the normal modes are expected to shift to higher or lower 10

7 Fig. 5. Scheme of normal vibrations of tetrahedral ions. frequencies due to different intra- and intermolecular interactions between different entities in the structures of solids. The unit cell theoretical treatment for the beryllium compounds under study is reported [43]. Vibrational spectra (infrared and Raman) of BeSe 4 4H 2 as well as those of the highly deuterated tetrahydrate (ca 80 % D 2 ) and of the anhydrous compound BeSe 4 are presented in Figs. 6 and 7, respectively. The pseudo-tetragonal orthorhombic unit cell of BeSe 4 (Z = 8) has an unit cell symmetry (D 2h ) and contains 144 atoms with 432 zone-centre degrees of freedom. The tetragonal setting of the orthorhombic cell corresponds to a = b = 8.264, c = Å, and a = b = 90º, g = 87.69º (Z =4). Hence, the tetragonal unit cell contains 72 atoms with 216 zone-centre degrees of freedom. The Se 4 ions (four ions in the unit cell at C 2 (x) sites; each tetrahedral ion characterized by 9 normal vibrations) contribute 36 internal modes. The water molecules (three different structural types; two types at C s (yx) sites and one at C 1 sites) contribute 48 internal modes (each water molecule with 3 normal motions). The low site symmetry C 2 (x) of Se 4 ions in the structure of BeSe 4 leads to a removal of the degeneracy of the n 2, n 3 and n 4 modes, thus resulting in two motions for n 2 of A symmetry and three motions for both n 3 and n 4 (A + 2B symmetry). The n 1 modes of the Se 4 ions are activated due to the low site symmetry of the anions. The unit cell analysis (factor group Fig. 6. Infrared spectra of BeSe 4 in the region of the normal vibrations of Se 4 ions, Be 4 skeleton vibrations and water librations: (a, b) BeSe 4 ; (c) BeSe 4 4D 2 (ca 80 % D 2 ); (d) BeSe 4 ; (a, c, d) ambient temperature; (b) liquid nitrogen temperature. Fig. 7. Raman spectrum of BeSe 4 (ambient temperature). 11

8 Journal of Chemical Technology and Metallurgy, 51, 1, 2016 symmetry D 2h ) predicts four components for the modes of A symmetry, i.e. A g + B 3g + A u + B 3u and four components for the modes of B symmetry, B 1g + B 2g + B 1u + B 2u, correspondingly (the correlation field splitting effect). Since the crystal structure is centrosymmetric, the Raman modes display g-symmetry, and their infrared counterparts display u-symmetry (mutual exclusion principle; A u is inactive in both spectra). Consequently, the 36 optical modes for Se 4 ions are subdivided into 5A g + 4B 1g + 4B 2g + 5B 3g + 5A u + 4B 1u + 4B 2u + 5B 3u. According to the unit cell analysis for the water molecules in C s symmetry the 24 motions are subdivided into: 6A g + 6B 3g + 6B 1u + 6B 2u (two different types). The water molecules in C 1 site symmetry exhibit 24 motions: 3A g + 3B 1g + 3B 2g + 3B 3g + 3A u + 3B 1u + 3B 2u + 3B 3u. The correlation diagram between T d molecular point group (of C 2 (x) site symmetry) and that of C 2v point group (of C s and C 1 site symmetry, i.e. D 2h factor group symmetry) is shown in Fig. 8. The unit cell theoretical treatment for the translational lattice modes (Be 2+ in C s (yz) symmetry, H 2 (1) and H 2 (2) in C s (yz) symmetry, H 2 (3) in C 1 symmetry, Se 4 ions in C 2 (x) symmetry) and rotational (librations) lattice modes (H 2 (1). H 2 (2), H 2 (3), and Se 4 yields: 72 translational lattice modes of - 10Ag + 8B 1g + 8B 2g + 10B 3g + 7A u + 11B 1u + 11B 2u + 7B 3u and 60 rotational lattice modes of - 6Ag + 9B 1g + 9B 2g + 6B 3g + 8A u + 7B 1u + 7B 2u + 8B 3u ). Thus, the 216 vibrational modes of the unit cell decompose in accordance with: Г = 30A g + 24B 1g + 24B 2g + 30B 2g + 23A u + 31B 1u + 31B 2u + 23B 3u where1b 1u + 1B 2u + 1B 3u are translations (acoustic modes). As mentioned above these assumptions are made taking into account the tetragonal unit cell (Z = 4). The unit cell volume for the tetragonal setting has a value of Å 3, i.e. twice less than that of the orthorhombic unit cell ( Å 3 [9]). Then all vibrations have to be multiplied by 2, which leads to 432 vibrational modes for the orthorhombic unit cell (Z = 8). They are described by: Г = 60A g +48B 1g +48B 2g +60B 2g +46A u +62B 1u +62B 2u + 46B 3u where 1B 1u + 1B 2u + 1B 3u are translations (acoustic modes). The n 3 and n 4 components of the triplets expected for the selenate ions (A + 2B symmetry) according to the site Fig. 8. Correlation diagrams between: T d molecular point group, C 2 (x) site symmetry and D 2h unit cell symmetry (Se 4 ions in BeSe 4 ); C 2v point group, C s (yz) and C 1 site symmetry and D 2h unit cell symmetry (water molecules in BeSe 4 ). symmetry analysis coalesce into a single infrared band at 877 cm -1 (the respective Raman band is observed at 878 cm -1 ; see Figs. 6b and 7) and a doublet at 442 cm -1 and 408 cm -1, respectively (the spectrum recorded at liquid nitrogen temperature). No band corresponding to n 1 (species of B 3u symmetry) is observed in the infrared spectrum of BeSe 4. This reflects the regularity of the selenite tetrahedra (four Se- bond lengths of Å [9]). The Raman spectrum shows a strong band 12

9 centered at 854 cm -1 (symmetric stretching modes n 1 ), a triplet at 460 cm -1, 443 cm -1 and 433 cm -1 (asymmetric bending mode n 4 ), and a doublet at 366 cm -1 and 343 cm -1 (symmetric bending mode n 2 ) (Fig. 7). If we assigned the three components of n 4 as n 4a, n 4b and n 4c (n 4a appears at the highest frequency, while n 4c - at the lowest one), the differences Dn ab, Dn bc and Dn ac have values of 17 cm -1, 10 cm -1, and 27 cm -1, respectively. These differences are too large to be accepted as a result of the factor group splitting and consequently the three Raman bands at 460 cm -1, 443 cm -1 and 433 cm -1 are assigned to the three site group components of n 4 (A + 2B symmetry). The same assumption is valid for the bending motions n 2 (Dn 2 has value of 17 cm -1 ; two site group components of A symmetry). The appearance of a single infrared band for the asymmetric stretches n 3 instead of the three bands expected according to the site symmetry analysis is an indication that the selenate tetrahedra exhibit a local molecular symmetry close to T d, if the Se stretching vibrations are considered (effective spectroscopic symmetry). However, the number of the Raman bands for n 4 and n 2 of the selenate ions corresponds to that predicted from the site symmetry analysis, thus showing a slight distortion of the Se 4 tetrahedra with respect to the bond angles in agreement with the structural data (small differences in the Se bond angles [9]). The infrared spectrum of the anhydrous beryllium selenate exhibits a strong narrow band at 913 cm -1 (slightly asymmetric) originated from n 3 and a band at 440 cm -1 originated from n 4 of the selenate ions, respectively (Fig. 6d). Thus, the vibrational behavior of the selenate ions in the region of the asymmetric stretching modes evidences for the selenate tetrahedra regularity in view of the Se bond lengths (the molecular symmetry of the Se 4 ions is close to T d at ambient temperature). According to the structural data the ions in Be are located in S 4 site symmetry within the unit cell of D 2d symmetry [20]. The correlation diagram between T d molecular point symmetry, S 4 site symmetry Fig. 9. Correlation diagram between T d molecular point symmetry, D 2 site symmetry and D 2d unit cell symmetry ( ions in Be ). Fig. 10. Infrared spectra of beryllium sulfates in the region of the normal vibrations of ions, Be 4 skeleton vibrations and water librations: (a) Be ; (b) Be 4D 2 (ca 80 % D 2 ); (c) Be ; (d) Be (ca 50 % D 2 ); (e) Be (spectra at ambient temperature). 13

10 Journal of Chemical Technology and Metallurgy, 51, 1, 2016 and D 2d unit cell symmetry is shown in Fig. 9. The site symmetry analysis predicts two motions for n 2 (A + B symmetry) and two motions for both n 3 and n 4 (B + E symmetry). The unit cell analysis for Be predicts two motions for n 1 (A 1 + A 2 ), four motions for n 2 (A 1 + A 2 + B 1 + B 2 ), and three motions for both n 3 and n 4 (B 1 + B 2 + E). Thus, the normal vibrations for the sulfate ions are as follows: 2A 1 + 2A 2 + 3B 1 + 3B 2 + 2E (for the activity of the different species see Fig. 9). The unit cell theoretical treatment for the translational lattice modes (Be 2+ in S 4 symmetry, H 2 in C 1 symmetry, ions in S 4 symmetry) and rotational (librations) lattice modes (H 2 and ) yields 26 translational lattice modes (- 3A 1 + 3A 2 + 5B 1 + 5B E, where 1B 2 + 2E are acoustic modes) and 22 rotational lattice modes ( - 4A 1 + 4A 2 + 3B 1 + 3B 2 + 8E). Infrared spectra of Be, Be, and Be are presented in Fig. 10. It is seen that the infrared spectrum of Be shows two intensive infrared bands at 1125 cm -1 and 1087 cm -1, which are attributed to the site symmetry components of n 3 (B + E symmetry) of the sulfate ions. The lower frequency band shifts from 1087 cm -1 to 1094 cm -1 upon deuteration probably due to interactions with water molecules (Fig. 10b). The comparison of the spectrum of Be and that of the highly deuterated sample (ca 80 % D 2 ) provides to assign the bands at 660 cm -1 and 617 cm -1 to the site symmetry components of n 4 (B + E symmetry) (the bands do no change their positions upon deuteration - compare Fig. 10a and Fig. 10b). The motions n 1 and n 2 are active in the Raman spectrum. The differences in the frequencies of the two bands for both n 3 and n 4 (Dn 3 and Dn 4 ) have values of 31 cm -1 and 46 cm -1 (deuterated sample), respectively, thus indicating a comparatively strong energetic distortion of the sulfate tetrahedra, which could not be predicted from the structural data (according to [20] the four S- bond lengths have values of Å). The infrared spectrum of Be (the crystal structure is not known) exhibits three bands in the stretching mode region (Fig. 10c). The intensive bands at 1148 cm -1 and 1096 cm -1 are attributed to n 3 and that at lower frequencies (1054 cm -1 ) to n 1 of the sulfate ions. The slightly asymmetric band at 615 cm -1 arises from the asymmetric bending modes n 4 and that at 421 cm -1 from the symmetric bending modes n 2. Thus, the spectroscopic findings lead to the assumption that the tetrahedra in the beryllium sulfate dihydrate are comparatively regular with respect to the S bond angles and distorted with respect to the S bond lengths (Dn 3 = 52 cm -1 ). The comparatively broad asymmetric band at 1144 cm -1 and the shoulder at 1130 cm -1 in the spectrum of the anhydrous beryllium sulfate are assigned to n 3 (n 1 could not be recognized). The high intensity band at 574 cm -1 probably contributes n 4 motions of the sulfate ions and n 3 of beryllium tetrahedra (this band is too intensive if only the n 4 motions are considered (see Fig. 10e). The comparison of the spectra of Be, Be and Be reveals that the mean values of the asymmetric stretching modes n 3 of the sulfate ions are shifted to higher frequencies on going from the tetrahydrate to the anhydrous compound (1106 cm -1, 1122 cm -1 and 1137 cm -1, respectively) due probably to the increasing repulsion potential of the lattices, i.e. to the decreasing of the unit cell volumes of the respective compounds. Infrared and Raman spectra of K 2 Be(, Rb 2 Be( and K 2 Be(Se 4 are presented in Figs (infrared) and Figs (Raman). The monoclinic unit cell of the beryllium double compounds (Z = 4; unit cell symmetry C 2h ) contains 76 atoms with 228 zone-centre degrees of freedom. The 228 vibrational modes of the unit cell decompose in accordance with: Г = 57A g + 57B g + 57A u + 57B u where 1A u + 2B u are translations (acoustic modes). The X 4 ions (eight X 4 ions in the unit cell at C 1 sites; each tetrahedral ion is characterized by 9 normal vibrations) contribute 72 internal modes, while the water molecules (twelve molecules in the unit cell at C 1 sites; each water molecule with 3 normal vibrations) contribute 24 internal modes to the 225 optical zone-centre modes. The static field (related to the low symmetry C 1 of the sites at which the X 4 ions are situated) will cause a removal of the degeneracy of both the doubly degenerate n 2 modes and the triply degenerate n 3 and n 4 modes (the non-degenerate n 1 mode is activated). Thus, the 9 internal modes of tetrahedral ions are of A symmetry as predicted from the site group analysis: one mode for the symmetric stretching vibrations (n 1 ), two modes for the symmetric bending vibrations (n 2 ), and three modes for both asymmetric stretching and bending vibrations (n 3 and n 4 ). Additionally, under the factor group symmetry C 2h each species of A symmetry will split into four modes: A g + B g +A u + B u (related to the interactions of the identical oscillators, the correlation 14

11 Fig. 11. Infrared spectra of K 2 Be( in the region of the normal vibrations of ions, Be 4 skeleton vibrations and water librations: (a, b) K 2 Be( ; (c) K 2 Be( 2D 2 (ca 80 % D 2 ); (d) K 2 Be( ; (a, c, d) ambient temperature; (b) liquid nitrogen temperature. Fig. 12. Infrared spectra of Rb 2 Be( in the region of the normal vibrations of ions, Be 4 skeleton vibrations and water librations: (a, b) Rb 2 Be( ; (c) Rb 2 Be( (ca 30 % D 2 ); (d) Rb 2 Be( (ca 50 % D 2 ); (e) Rb 2 Be( ; (a, c, d, e) ambient temperature; (b) liquid nitrogen temperature. field effect, see Fig. 17). Consequently, the 72 optical modes for the X 4 ions are subdivided into 18A g + 18B g +18A u + 18B u modes (two crystallographically different tetrahedral ions). As mentioned above the water molecules contribute 24 optical modes: 6A g + 6B g +6A u + 6B u (two crystallographically different water molecules). The rest 129 optical modes (external modes) are distributed between the translational and librational lattice modes. Thus, the unit cell theoretical treatment of the translational lattice modes (K + (1), K + (2), X 4 (1), X 4 (2), H 2 (1), and H 2 (2) all in C 1 site symmetry) and librational lattice modes (two types X 4 and two types H 2 ) yields: 81 translations (21A g + 20B g + 21A u + 19B u ) and 48 librations (12A g +12 B g + 12A u + 12B u ). The bands in the infrared regions of 1200 cm cm -1 and 1000 cm cm -1 corresponding to the n 3 and n 1 modes of the ions, respectively, appear as doublets, thus reflecting the existence of two crystallographically different tetrahedra in the structures of the potassium and rubidium compounds: in case of 15

12 Journal of Chemical Technology and Metallurgy, 51, 1, 2016 Fig. 14. Raman spectra of (a) K 2 Be( and (b) K 2 Be( (ambient temperature). Fig. 13. Infrared spectra of K 2 Be(Se 4 in the region of the normal vibrations of Se 4 ions, Be 4 skeleton vibrations and water librations: (a, b) K 2 Be(Se 4 ; (c) K 2 Be(Se 4 2D 2 (ca 80 % D 2 ); (d) K 2 Be(Se 4 ; (a, c, d) ambient temperature; (b) liquid nitrogen temperature. Fig. 15. Raman spectrum of Rb 2 Be( (ambient temperature). K 2 Be( at 1201 cm -1, 1183 cm -1, 1143 cm -1, 1128 cm -1, 1081 cm -1, 1063 cm -1 (n 3 ) and 999 cm -1 and 951 cm -1 (n 1 ), while in case of Rb 2 Be( at 1204 cm -1, 1190 cm -1, 1176 cm -1, 1149 cm -1, 1125 cm -1, 1083 cm -1 and 1060 cm -1 for n 3, and at 999 cm -1 and 992 cm -1 for n 1 (Figs. 11b and 12b). It should be added that the spectra used are recorded at liquid nitrogen temperature. The appearance of more bands for n 3 in the spectrum of the rubidium compound than expected as a result from the site group analysis is due to the correlation field effect. It is worth mentioning that the infrared bands corresponding to the symmetric stretching vibrations n 1 are of high intensity (close to that of the bands corresponding to the asymmetric ones). It is reported [44] that the intensity of the bands for n 1 is a criterion for the distortion of the tetrahedral atom groups (a higher intensity corresponds to a higher distortion). This effect could not be predicted by the structural data. The six bands in the infrared region of 630 cm cm -1 (630 cm -1, 602 cm -1, 594 cm -1, 566 cm -1 ) and the three bands 16

13 Fig. 16. Raman spectra of (a) K 2 Be(Se 4 and (b) K 2 Be(Se 4 (ambient temperature). at 465 cm -1, 452 cm -1 and 439 cm -1 arise from n 4 and n 2 motions of the sulfate ions in the double potassium compound, respectively (bands at LNT). The bands at 626 cm -1, 622 cm -1, 617 cm -1, 612 cm -1, 595 cm -1 and 565 cm -1 and those at 463 cm -1, 452 cm -1 and 443 cm -1 (bands at LNT) in the spectrum of the double rubidium sulfate originate from n 4 and n 2 motions of the sulfate ions, respectively. The doublets around 1000 cm -1 in the Raman spectra are of higher intensity than the bands at larger wavenumbers and consequently they are attributed to n 1 of the sulfate ions (1011 cm -1 and 993 cm -1 for the potassium compound, and 1007 cm -1 and 993 cm -1 for the rubidium one). The bands centered at 1214 cm -1, 1186 cm -1, 1162 cm -1, 1122 cm -1, 1084 cm -1 and 1060 cm -1 and those at 1212 cm -1, 1185 cm -1, 1156 cm -1, 1120 cm -1, 1078 cm -1 and 1060 cm -1 are assigned to n 3 of the sulfate ions in the potassium and rubidium compounds, respectively. The Raman bands in the spectral regions of 640 cm cm -1 and of 470 cm cm -1 (potassium compound), and those in the regions of 630 cm cm -1 and of 500 cm cm -1 (rubidium compound) are referred to n 4 and n 2 motions of the sulfate ions, respectively (Figs. 14a and 15). The infrared spectrum of K 2 Be( is characterized by three intensive bands in the region of the stretching modes of the sulfate ions (1230 cm cm -1 ; Fig. 11d). The comparison of the infrared and Raman spectra provide the claim that the intensive Raman bands at 1048 cm -1 and 1036 cm -1 are due to n 1 and consequently the infrared band at 1042 cm -1 could with certainty be assigned to the symmetric stretches n 1 of the sulfate ions (the infrared bands at 1229 cm -1, 1191 cm -1 and 1153 cm -1 are assigned to the asymmetric stretches). Thus, the spectroscopic finding reveals that the tetrahedra are strongly distorted if the intensity of the infrared band at 1042 cm -1 as well as the value of Dn 3 (76 cm -1 ) are taken into account [44]. The n 4 motions are detected at 626 cm -1 and 617 cm -1. The Raman bands at higher frequencies (1253 cm -1, 1230 cm -1, 1192 cm -1 and 1150 cm -1 ) are attributed to n 3. The Raman bands at 662 cm -1, 632 cm -1, 619 cm -1 and 612 cm -1 and those at 491 cm -1, 464 cm -1 and 446 cm -1 originate from the asymmetric and symmetric bending modes, respectively. The infrared spectrum of K 2 Be(Se 4 exhibits four bands of close intensities in the region of the stretching modes n 3 and n 1 of the Se 4 ions (bands at 931 cm -1, 891 cm -1, 864 cm -1 and 844 cm -1 ; liquid nitrogen temperature) (Fig. 13a and b). When the infrared and Raman spectra of K 2 Be(Se 4 are compared (Figs. 13 and 16a), it is readily seen that the Raman bands at 874 cm -1 and 856 cm -1 are of the highest intensity and consequently they are attributed to n 1. Then the Raman bands at higher frequencies 932 cm -1, 924 cm -1, 910 cm -1 and 903 cm -1 are assigned to n 3. The appearance of two Raman bands for n 1 (Dn = 18 cm -1 ) reflects the existence of two crystallographically different Se 4 ions in the structure of the double selenate. The six site group components (twelve for the two different selenate ions) expected for n 4 coalesce into four infrared bands in the region of 456 cm cm -1 (Fig. 13b) and into four Raman bands (460 cm cm -1, Fig. 16a). The four Raman bands in the region of 360 cm cm -1 are assigned to n 2. The relatively large half-widths of the infrared bands corresponding to the stretching modes are due to the L/T splitting effects of these modes with high oscillator strengths [45]. Fig. 13d displays seven bands in the region of the stretching modes for the selenate ions in the anhydrous compound (bands at 947 cm -1, 937 cm -1, 910 cm -1, 904 cm -1, 892 cm -1, 845 cm -1 and 840 cm -1 ).The most intensive Raman bands at 17

14 Journal of Chemical Technology and Metallurgy, 51, 1, cm -1 and 887 cm -1 arise from the n 1 motions of the selenate ions. Thus, the comparison of the infrared and Raman spectra leads to the conclusion that some of the components of n 3 in the infrared spectrum appear at lower frequencies than the n 1 motions, i.e. n 1 >n 3. The appearance of Raman bands of smaller intensity at 860 cm -1 and 836 cm -1 confirms this assumption (they are attributed to n 3 ). The infrared bands at 449 cm -1, 432 cm -1 and 401 cm -1 are assigned to n 4 (the respective Raman bands appear at 458 cm -1, 443 cm -1 and 412 cm -1 ; the bands at 394 cm -1, 378 cm -1, 364 cm -1 and 337 cm -1 are assigned to n 2 ). The large number of the infrared bands corresponding to the stretching modes as well as that of the Raman bans corresponding to n 2 indicates that probably more than one structural type of selenate ions exists in the structure of the anhydrous potassium beryllium selenate (however, the crystal structure of this compound is not known and this assumption is open to discussion) Lattice modes of the beryllium tetrahedra The Be 4 tetrahedra are reported to exhibit lattice modes n 1, n 2, n 3 andn 4 in the range of 540 cm -1, 250 cm cm -1, 700 cm cm -1 and 350 cm -1, respectively [5, 22, 23]. The infrared bands corresponding to the lattice modes of the Be(H 2 ) 4 tetrahedra are expected to display high intensities, while the Raman bands are expected to be very weak due to the small polarization of Be 2+ ions and the water molecules [5]. As far as the n 3 and n 1 motions of the beryllium tetrahedra are concerned a strong overlapping of these modes with water librations occurs and the bands could not be assigned unambiguously. However, the bands arising from the lattice motions of the Be 4 tetrahedra could be assigned more precisely if the spectra of the beryllium hydrates are compared to those of the highly deuterated samples and the anhydrous compounds. Be 2+ ions occupy C s (yz) site symmetry in the structure of BeSe 4. Thus, the site group analysis for the lattice modes of the beryllium tetrahedra predicts: n 1 (A 1 ) A, n 2 (E) A + A, both n 3 and n 4 (F) 2A + A. The correlation diagram between T d molecular point symmetry, C s site symmetry and D 2h factor group symmetry is shown in Fig. 18. It is seen that both species of A and A symmetry are subdivided into four components as follows: A A g + B 1u + B 2u + B 3g ; A A u + B 1g + B 2g + B 3u. The bands at 760 cm -1 and 770 cm -1 (ambient and liquid nitrogen temperatures, respectively) preserve their intensities upon deuteration and consequently they originate from n 3 (compare Fig. 6a, b and c). The small shift of the band at 760 cm -1 to higher frequencies upon deuteration (770 cm -1 ) evidences for the interactions between n 3 and water librations. The appearance of one symmetric band corresponding to n 3 instead of three bands expected in accordance with the site symmetry analysis (2A + A ) is due to the close bond distances Be (effective spectroscopic symmetry close to T d ) [9]. The bands of small intensity in the region of 550 cm cm -1 arise probably from the symmetric stretches of the n 1 of the beryllium tetrahedra (Fig. 6b). The intensive infrared bands at 703 cm -1 and 666 cm -1 in the spectrum of the anhydrous compound are assigned to n 3 vibrations of the Be 4 tetrahedra (Fig. 6d). The beryllium tetrahedra in the structure of Be are located on S 4 site symmetry and consequently the correlation diagram for the [Be(H 2 ) 4 ] 2+ ions is identical with that for the ions (see Fig. 9). The bands at 776 cm -1 (Be, Fig. 10a) and 777 cm -1 (Be, Fig. 10c) do not change their positions upon deuteration and consequently they could be attributed to the n 3 motions. According to the structural data the four Be H 2 bonds have lengths of Å [20] and this fact explains the appearance of one symmetrical band only for n 3 instead of two bands expected (B + E symmetry) according to the site symmetry analysis. The symmetric n 1 mode of the Be(H 2 ) 4 tetrahedra in Be is not infrared active. The appearance of one symmetric band corresponding to n 3 of the beryllium tetrahedra in Be suggests that these tetrahedra exhibit a local molecular symmetry close to T d as far as the Be bond lengths are concerned (as mentioned before the crystal structure of the dehydrate is not known). The symmetric stretching mode n 1 is observed at 536 cm -1 (see Fig. 10c and d). The anhydrous compound shows three intensive bands corresponding to the stretches of the beryllium tetrahedra - two strong bands at 768 cm -1 and 736 cm -1 (n 3 ) and a band at 515 cm -1 (n 1 ) (see Fig. 10e). The comparison of the spectra of the double salts, K 2 Be( and Rb 2 Be( with those of the highly deuterated samples (about 80 % D 2 for the potassium salt and 50 % D 2 for the rubidium one) on one hand and with the anhydrous compounds on the other shows that the broad bands with two maxima at 770 cm -1 and 746 cm -1 (potassium salt) and 776 cm -1 and 18

15 Fig. 17. Correlation diagram between T d molecular point group, C 1 site symmetry and C 2h unit cell symmetry (, Se 4 and Be 4 tetrahedra in K 2 Be(, Rb 2 Be( and K 2 Be(Se 4 ). Fig. 18. Correlation diagram between T d molecular point symmetry, C s site symmetry and D 2h unit cell symmetry (Be 4 tetrahedra in BeSe 4 ). 736 cm -1 (rubidium salt) are probably determined by vibrations of beryllium tetrahedra and water librations (see Figs. 11a and 12a, respectively). The band 770 cm -1 in the spectrum of the potassium compound (ambient temperature) does not disappear in the spectrum of the highly deuterated sample and consequently this band is attributed to the lattice modes n 3 of the beryllium tetrahedral (compare Fig. 11a and c). The assignments of the bands in the region of n 3 for the rubidium compound could not be assigned correctly due to the strong overlapping of this mode with librations of heavy water (deuteration about 50 %). With a great deal of precaution the band at 748 cm -1 could be attributed to n' 3 of the beryllium tetrahedra in the rubidium compounds (deuterated sample, see Fig. 12d).The comparatively strong bands in the spectra of the anhydrous double sulfates 782 cm -1 and 744 cm -1 (potassium salt, see Fig. 11d) as well as 772 cm -1 and 743 cm -1 (rubidium salt, see Fig. 12e) are assigned to the asymmetric stretching modes of the beryllium tetrahedra. The ν 3 lattice modes of the beryllium tetrahedra in K 2 Be(Se 4 are observed at 731 cm -1 - the band preserves its position and intensity upon deuteration (Fig. 13a and c). The beryllium tetrahedra in the anhydrous K 2 Be(Se 4 exhibit a complicated infrared spectrum in the region of 750 cm cm -1 and the respective bands are assigned to ν 3 (Fig. 13d). The Raman bands at 524 cm -1 and 505 cm -1 are probably due to ν 1 in the tetrahydrate and the anhydrous salt, respectively (Fig. 16) Bending modes of water molecules and water librations The water molecules in C 2v molecular symmetry exhibit three normal vibrations - symmetric stretching modes, n 1 (A 1 ); asymmetric stretching modes (water molecules in plane yz, n 3 (B 2 ) and bending modes, n 2 (A 1 ). The water molecules in salt crystal hydrates display additionally rotational motions (librations) - wagging (B 1 symmetry), rocking (B 2 symmetry), twisting (A 2 symmetry) and translational motions. All modes are active in the Raman spectra, while the twisting modes are inactive in the infrared spectra. However, in the case of strongly distorted water molecules (C 1 site symmetry) these modes become infrared active. Fig. 19 shows a scheme of water molecules normal vibrations and librations. The water librations appear in the spectral region below 1000 cm -1 and exhibit a strong overlapping with vibrations of other entities in the structure. Consequently, due to strong interactions of the water molecules with other atoms and molecular groups the assignments of the water librations have to be made with a great deal of precaution. Single-crystal infrared and Raman measure- Fig. 19. Scheme of the normal vibrations and librations of water molecules. 19

16 Journal of Chemical Technology and Metallurgy, 51, 1, 2016 ments are needed for precise assignments of the water librations. Usually infrared spectra of crystal hydrates and those of highly deuterated samples and anhydrous compounds are compared in order to assign the bands arising from the water librations. The latter are reported to shift to higher frequencies upon cooling and to lower frequencies upon deuteration [36, 46, 47]. Each structural type water molecule will exhibit a set of rocking, twisting and wagging motions. The assignments of the bands corresponding to water librations in the beryllium compounds under study are made according to those reported by Lutz and coworkers [5]. The authors claim that in the case of strong hydrogen bonds formed in beryllium crystal hydrates and water molecules trigonal planar environments the rocking modes display considerable blue-shifts and small values of isotopic ratios upon deuteration (close to 1). For example, the rocking and the wagging modes of H 2 in Be(I 3 appear in the range of 1015 cm cm -1 and shift to 890 cm -1 for D 2 [5]. The experimental infrared spectroscopic measurements in the spectral regions of the bending modes n 2 of the water molecules and water librations are analyzed. The comparison of the spectra of BeSe 4, BeSe 4 4D 2 (ca 80 % D 2 ) and BeSe 4 allows us to claim that the bands at 994 cm -1 and 977 cm -1 (liquid nitrogen temperature) and 990 cm -1 (ambient temperature) correspond to rocking modes of the water molecules in BeSe 4 (three crystallographically different water molecules) (see Fig. 6a, c and d). Then, the new band at 855 cm -1, which appears in the spectrum of the highly deuterated sample (Fig. 6c) is assigned to rocking modes of D 2 (the isotopic ratio is1.16; ambient temperature). The band at 714 cm -1 (liquid nitrogen temperature) is attributed to twisting modes of H 2 (3) (C 1 site symmetry; twisting modes are activated). The comparatively intensive band centered at 530 cm -1 in the spectrum of the highly deuterated sample is attributed to wagging and twisting modes of D 2 (the corresponding wagging modes of H 2 occur at 670 cm -1 (isotopic ratio 1.26) (Figs. 6a and c). The water molecules exhibit two bands in the region of n 2 (1682 cm -1 and 1509 cm -1, ambient temperature). The spectrum is much more complicated at liquid nitrogen temperature. Šoptrajanov and Petruševski [48] reported that complex spectral pictures are observed in the infrared spectra of Tutton compounds, M 2 M (X 4 6H 2 (M = K, Rb; M = Mg, Fe, Co, Ni, Cu; X = S, Se) in the region of the water bending modes. According to the authors the differences in the band frequencies extend over the region of hundreds of wavenumbers and the origin of such complex spectra could not be explained with the structural differences between the crystallographically different water molecules. They suggest that the vibrational interactions between the bending modes n 2 and overtones or combinations arising from water librations are responsible for the complex spectral pictures. We assume that the same phenomenon is observed in the case of BeSe 4 (the differences in the wavenumbers is 173 cm -1 (ambient temperature). The band at 1236 cm -1 could be attributed to n 2 of D 2 and that at 1159 cm -1 probably to water librations of D 2 (see Fig. 6c). The band at 977 cm -1 in the spectrum of Be disappears upon deuteration (ca 80 % D 2 ) and is not observed in the spectrum of the anhydrous beryllium sulfate. Consequently this band could be attributed to water librations (rocking modes of H 2 ) (see Fig. 9a and b). The new band observed in the spectrum of the deuterated sample (855 cm -1 ) is assigned to rocking modes of D 2 (the isotopic ratio is 1.14). According to the structural data the water molecules in Be are strongly asymmetrically hydrogen bonded (one type in C 1 site symmetry [20]) and as a result the twisting modes are expected to appear in the infrared spectra. The bands at 734 cm -1 and 690 cm -1 are not observed in the spectra of both the deuterated sample and the anhydrous sulfate and consequently they are assigned to water librations (probably twisting and wagging). A new comparatively intensive band at 592 cm -1 (Fig. 9b) is attributed to librations of D 2 (the isotopic ratios have values of 1.24 and 1.16). Two bands at 1687 cm -1 and 1656 cm -1 are observed in the spectrum of Be in the region of n 2 of the water molecules. However, according to the factor group analysis a single band is expected for the n 2 motion (one structural type water molecule in C 1 site symmetry; n 2 of B 2 symmetry). The appearance of two bands is due probably to overtones or combinations of water librations. Indeed, the spectrum of the highly deuterated sample exhibits one band at 1242 cm -1 assigned to the bending mode n 2 of D 2 (Fig. 9b). Figs. 9c and 9d (spectra of Be ) show that the bands at 908 cm -1 and 730 cm -1 decrease in intensity upon deuteration and a new band appears at 829 cm -1 in the spectrum of the deuterated sample (ca 50 % D 2 ; 20