The Characterization of a Newly Layered Bimetallic Hydrogen Selenite Copper-Selenium: Synthesis and Structure The Characterization of a Newly Layered Bimetallic Hydrogen Selenite Copper-Selenium: Synthesis and Structure

Single-crystal X-ray diffraction data were used to solve the structure of a newly layered copper-selenium hydrogen selenite and further refined to a final reliability factor, R 1 = 0.038. This structure was found to have an orthorhombic space group PBn2 1 , with a = 7.1753(4) Å, b = 9.0743(4) Å, c = 17.725(9) Å, V = 1154.06(10) Å 3 , and Z = 4. Although this structure may be described to exhibit a bidimensional structure, it is actually three- dimensional in shape. The bidimensional structure is made up of layers, parallel to the (010) plane, which contain copper atoms and (HSeO 3 ) (cid:1) anions with sheets interconnected by [CuCl 3 (H 2 O) 3 ] groups. Bond valence sum calculations were used to evaluate the Se and Cu oxidation states. Both the infrared (IR) and Raman spectra were obtained and employed to confirm the presence of hydrogen selenites (Se d O d H). Also, the dielectric constant at different frequencies and temperatures revealed a phase transition at 383 K.


Introduction
During the past years, lot of interest has been shown in hydrogen selenite chemistry motivating research focused on expanding the knowledge of the structural and bonding principles of this ligand. There is an important number of divalent metal hydrogen selenite crystal structures reported in the literature.  4 ] with M = Cu, Co, Mn, Ni, and Zn [8]. This family has a three-dimensional structure, but it may be considered as being derived from the [Cu(HSeO 3 ) 2 ]type structure.
Compounds exhibiting mixed valences (like Se 4+ and Cu 2+ ) are at the center of many studies owing to their potential applications in relation to the electronic exchange. For almost all of these compounds, except [Cu(HSeO 3 ) 2 ], magnetic measurements have revealed the occurrence of weak ferromagnetism at low temperature (T~10-20 K) for which a tentative explanation is offered for this peculiar property in agreement with other authors [8,9]. In this study, the crystal structure of the compound [Cu 0.335 Se 0.582 (HSeO 3 ) 2 CuCl 3 (H 2 O) 3 ], herein presented, was obtained using an X-ray single structure and various spectroscopic (IR and Raman) characterization, as well as dielectric measurements.

Materials and methods
The experiments were carried out using a single crystal of [Cu 0.335 Se 0.582 (HSeO 3 ) 2 CuCl 3 (H 2 O) 3 ] grown by slow evaporation from a mixture of hydrochloric acid containing stoichiometric CuCl 2 -SeO 2 at room temperature in the ratio 1/2. Blue thin rectangular parallelepiped crystals were grown after vaporizing in air for 15 days approximately. The determination of [Cu 0.335 Se 0.582 (HSeO 3 ) 2 CuCl 3 (H 2 O) 3 ] formula was achieved by the crystal structure refinement approach at room temperature.
For electrical impedance measurements (in the range 1-10 KHz), a Hewlett-Packard 4192 ALF automatic bridge monitored by a HP Vectra microcomputer was used. Prepared dense translucent pellets with approximately a diameter of 8 mm and a thickness of 1-1.2 mm, covered with graphite electrodes were utilized for the measurements.
The measurement of electrical impedances were equally carried out in the range, 1-10 KHz, using a Hewlett-Packard 4192 ALF automatic bridge monitored by a HP Vectra microcomputer. For these types of experiments, dense translucent pellet samples were prepared, with a diameter of 8 mm and thickness between 1 and 1.2 mm. All the pellets were then covered with graphite electrodes prior to measurements.
A modern nondispersive Fourier Transform (FT-IR) spectrometer (Perkin-Elmer 1750 spectrophotometer IR-470) was employed for the characterization of the crystalline powders after mixing with KBr, with very notable IR-active functional groups found in the samples investigated. Scans of IR spectra were recorded in in the range 400-4000 cm À1 without apodization. To record Raman spectra of the solid samples for the study of the various phases, a conventional scanning Raman instrument (Horiba Jobin Yvan HR800 microcomputer system), equipped with a Spex 1403 double monochromator (with a pair of 600 grooves/mm gratings) and a Hamamatsu 928 photomultiplier detector was used. Solid materials were sampled at different temperatures for this analysis. During the recording of prominent Raman peaks, excitation radiation from the instrument was fulfilled by a coherent radiation emitted by a He-Neon laser operating at a wavelength of 633 nm, with an output laser power of 50 mW. In order to acquire high-resolution Raman spectra, the spectral resolution of the slit width varied from 3 to 1 cm À1 .
To study the crystal structure of [Cu 0.335 Se 0.582 (HSeO 3 ) 2 CuCl 3 (H 2 O) 3 ], an APEX II diffractometer (powder XRD) fitted with graphite-crystal monochromated Mo Kα radiation (0.71073 Å) was employed. In this study, a total of 3093 reflections were collected, among which only 2803 reflections, namely those for which I > 2σ (I), were actually used in the determination and refinement of the structure. Corrections were made for Lorentz-Polarization and absorption effects. Table 1 presents the data collection procedure and structure refinement at room temperature.
A three-dimensional Patterson synthesis approach was used to determine the selenium atoms positions in the compound. On the one hand, the Fourier function allowed for the localization of the chlorine (Cl), copper (Cu), and oxygen (O) atoms. On the other hand, the hydrogen atoms were localized from a difference Fourier synthesis and introduced as fixed contributors. Conversely, all the non-hydrogen atoms were typically assigned anisotropic thermal displacements. The structure solution and refinement were carried out using SHELX programs [10,11]. The bond lengths and angles are given in Table 2 (2) is equal to 3, which would fit to 33.5% of Cu 2+ and 58.2% of Se 4+ . This outcome was confirmed by performing a calculation of bond valence sums around the centers of the cation sites. The steps and expressions used in the calculation of the bond valence are published in [12]. More specifically, the bond valence (S ij ) is expressed as S ij = exp [(R 0 À R ij )/B], where R 0 and B are the experimentally determined parameters and R ij is the bond length of the cation-anion pair [12]. The sum of the bond valence (Σ j S ij ) around an ion calculated must be equal to the formal valence (V i ) of this ion, based on the valence sum rule.
Spatially, the high anisotropic distribution of anions observed around each cation is characteristically of a strong stereochemical activity of their electron lone pair E for the Se1 and Se3 atoms. The consequence for the coordination polyhedral is the description of a distorted SeO 3 triangular pyramid, in which the SedO (7) and SedO (8) are marginally longer than the other SedO bonds ( Table 2). Thereof, the lone pair E so directed to constitute the fourth vertex of an SeO 3 E tetrahedron ( Figure 3).
The OdSedO with angles of values 98(2) and 102 (2) formed from SedO chemical bonds are situated on one side of the Se atom, whereas the other side is hitherto a "dead" zone around   the lone pair E of the Se atom. As such, the SeO 3 polyhedral has strong dipole moments, due to such a sharp asymmetry of the atomic arrangement of the first coordination spheres, which provide considerable dipole-dipole contribution to the inter-anion potentials. Taking a closer look at the structure in Figure 4, one can easily see that each Cu(1) atom is surrounded by three oxygen (O) atoms and three chlorine (Cl) atoms. This basically forms a slightly distorted octahedron with distances ranging from 1.99(7) to 2.21(3) Å for CudO and 2.30 (2) Å for CudCl.
A remarkable deviation from full occupancy was exhibited in the occupancy of the Cu(2) site during refinement. This is an indication of a substitution with Se, resulting in final occupancies constrained in sum to 1.0, and refined to 0.335(4) and 0.582(2), respectively. It should be noted that the deviation from 1.0 is due to the mixed valence between Cu and Se, for Cu(2) and Se(2), respectively. As shown in Figure 5, the Cu(2)/Se(2) atoms are surrounded by four oxygen atoms and two chlorine atoms to form an irregular octahedron.
From the earlier arguments, the structure depicted in Figure 2 can be ascribed to being formed by Cu (1), Se(2)/Cu(2) polyhedral that structurally shares chlorine (Cl) corners in infinite chains along the direction [001]. Therein, the sequential metal atoms in the chain trend schematically following Cu(1)dSe(2)/Cu(2)dCu(1) 0 dSe(2)/Cu (2)dCu (1), whereby the prime refers to the polyhedral generated by the symmetry operation: 1 -x, 2 -y, À0.5 + z, in the [010] direction. As a result, the three-dimensional network is thus formed. It can be argued that the lattice cohesion may be strengthened by the hydrogen bonds within the layer (O(8)dH (7)dO (4)) and (O(7)dH(8)dO (3)) or outside the layer.   3 ], IR, and Raman spectroscopy were used. Figure 6 shows that the IR spectrum is restricted to the mid-infrared frequency range: 400-4000 cm À1 . In this chapter, the band corresponding to the symmetric stretching vibrations of SeO 2 groups was observed at around 825 cm Àl in the Raman spectrum (Figure 7). Similarly, a strong intense broad band is observed in the infrared (IR) spectrum for this mode. These findings are in agreement with those reported by Cody and al. and Micka et al. for vibrational analysis on a series of alkali hydrogen selenites. From the work of these authors, the symmetric stretching vibrations are around 850 cm À1 [13][14][15].
From Figure 7, a band of very weak intensity is observed at 710 cm À1 , accompanied by a shoulder at 738 cm À1 , which is ascribed to asymmetric stretching vibrations of SeO 2 groups. In the 686-740 cm À1 region, a corresponding IR spectrum with an intense (broad) frequency absorption is present. In the literature [16,17], these modes have been observed at a very much lower wave numbers than those in alkali hydrogen selenites.
Another observation is that copper (selenium) atoms are located at the center of CuO 4 Cl 2 coordination octahedra. The axial CudCl bonds are longer than the others, and they are coordinated to water molecules. It can be stated that the high spin d-configuration of Cu leads to Jahn-Teller distortions by Jahn and Teller [18] hitherto producing planar CudO bonds in the 1.91-1.98 Å range and an axial CudCl distance of 2.77-2.80 Å. It can be ascertained that the planar oxygen (O) atoms are shared by Cu and Se atoms which can lead to the observed reduction in the stretching frequencies of the SeO 2 groups.
Typically, the stretching vibrations of the HSeO 3 À ion (νSedOH), which often characteristically appear in the 600-650 cm Àl region [16,17], are also appearing at lower wave numbers (~627 cm À1 ). In the IR spectrum, this mode is observed as an intense broad band at 532 cm À1 . The OdO distance involving the SedOH system with one of the equatorial oxygen atoms of the neighboring Cu(2)/Se(2)O 4 Cl 2 group is 2.662 Å. The observed lowering of the SedO(H) vibrations from the free-state values is a confirmation of the corresponding strong hydrogen bonds determined in the X-ray diffraction data. As presented in Table 3, it is seen that symmetric deformation vibrations of the HSeO 3 À ion have given only weak bands of the Raman spectrum, while a medium intense broad band is obtained in the IR. In the corresponding asymmetric bending vibration, Raman spectrum shows medium intense bands with a weak band in the IR. A reduction in the symmetry of the HSeO 3 ion may be causing the changes in the activity of these modes. From the observed strong hydrogen bonding and the distortion of Cu(Se)O 4 Cl 2 octahedra, we can deduce that the Jahn-Teller distortion affects the HSeO 3 vibrations.
Fundamentally, the hydrogen-bonded OH groups may lead to three vibrations, namely: ν(OH) stretching, the in-plane (OH), and the out-of-plane (OH) deformation vibrations. In fact, the stretching bands of strongly H-bonded systems are intense and usually built up of a number of unresolved components owing to strong interaction between the proton vibration and the ν(O…O) vibrations [19,20]. It is also clearly elucidated from the literature [21,22] that the broad ν(OH) band in Fermi resonance with the overtones of the ν(OH) modes splits into three bands A, B, and C. The A mode is typically observed as a strong broad band at 2850 cm À1 in the IR spectrum, while the B mode is obtained as a medium intense band at 2360 cm À1 . A medium intense broad band at 1900 cm À1 and a weak one at 1730 cm À1 are generally being assigned to C bands. From a practical viewpoint, the appearance of these bands confirms the existence of strong hydrogen bonds in the crystal. The in-plane bending ν(OH) vibrations are less sensitive to the hydrogen bond strength than the ν(OH) mode [15]. A medium band observed at 1280 cm Àl in the IR spectrum is attributed to the in-plane ν(OH) bending mode and a medium intense broad band in the 870-940 cm À1 region to the ν(OH) mode.
Two prominent broad bands were observed in the stretching region of the typical water in the Raman spectra of the main (title) compound. Similarly, in the IR spectrum, a corresponding strong broad band with two distinct peaks exhibited at 3553 and 3170 cm Àl are noticeable for this mode. The bending mode of H 2 O that appears at around 1606 cm À1 in the IR is noteworthy. The considerable shifting of stretching and bending frequencies from those of a free water   Figure 8 shows an illustration of the temperature dependence of the dielectric constant (ε 0 ) in the frequency range [1][2][3][4][5][6][7][8][9][10] KHz, and in the temperature region of 300-500 K obtained for [Cu 0.335 Se 0.582 (HSeO 3 ) 2 CuCl 3 (H 2 O) 3 ]. These curves (Figure 8) exhibit the following characteristics:

Dielectric studies
(1) There is one anomaly in the dielectric constant ε 0 observed at about 383 K, (2) there is a maxima in the permittivity curves, displaced to higher temperatures with increasing frequency, and (3) apparently, this is a transition which can be attributed to the "order-disorder" phase transition, probably characterizing the motion of H + diffusion related to the motion of HSeO 3 groups, as reported in the literature [16,17].

Conclusion
In conclusion, the author ascertains that a novel substituted hydrogen selenites [ 3 ] is that it undergoes a phase transition on heating at 383 K. High temperature structure investigation of the new compound is in our future plans in order to confirm the nature of this transformation.