Crystal structure data and experimental conditions for the structure determination of [Cu0.335Se0.582(HSeO3)2CuCl3(H2O)3].
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, R1 = 0.038. This structure was found to have an orthorhombic space group PBn21, with a = 7.1753(4) Å, b = 9.0743(4) Å, c = 17.725(9) Å, V = 1154.06(10) Å3, 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 (HSeO3)− anions with sheets interconnected by [CuCl3(H2O)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─O─H). Also, the dielectric constant at different frequencies and temperatures revealed a phase transition at 383 K.
- structural study
- dielectric properties
- bimetallic hydrogen selenites
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. For example, M(HSeO3)2 (where M: Cu, Mg, Sr, Ba) [1, 2]; M(HSeO3)2
Compounds exhibiting mixed valences (like Se4+ and Cu2+) 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(HSeO3)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 [Cu0.335Se0.582(HSeO3)2CuCl3(H2O)3], herein presented, was obtained using an X-ray single structure and various spectroscopic (IR and Raman) characterization, as well as dielectric measurements.
2. Materials and methods
The experiments were carried out using a single crystal of [Cu0.335Se0.582(HSeO3)2CuCl3(H2O)3] grown by slow evaporation from a mixture of hydrochloric acid containing stoichiometric CuCl2-SeO2 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 [Cu0.335Se0.582(HSeO3)2CuCl3(H2O)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−1 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−1.
To study the crystal structure of [Cu0.335Se0.582(HSeO3)2CuCl3(H2O)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.
|Crystallographic data||T = 296(2) K|
|Crystal size (mm3)||0.05 × 0.04 × 0.03|
|Data collection instrument||Kappa-APEX II|
|Radiation, graphite||λMo Kα (0.71073 Å)|
|θ range for data collection (°)||6.80–30.57|
|Index ranges||−10 ≤ h ≤ 9; −12 ≤ k ≤ 12; −23 ≤ l ≤ 25|
|Reflection with (F > 4σ(F))||3093|
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.
|a: SeO3 polyhedron|
|c: CuCl3(H2O) octahedron|
3. Results and discussion
3.1. Structure description
From the charge balance in [Cu0.335Se0.582(HSeO3)2CuCl3(H2O)3], it can be suggested that the average oxidation state of Cu(2)/Se(2) is equal to 3, which would fit to 33.5% of Cu2+ and 58.2% of Se4+. 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 . More specifically, the bond valence (Sij) is expressed as Sij = exp[(R0
In this work, our calculation shows that, for the pyramidal sites (Se1 and Se3), the sum of bond valence is ~4, which is equal to selenium formal valence. Thus, the bond valence sums around the octahedral site of Cu(2) are typically consistent with the value +2.7, confirming the presence of selenium Se4+ and cuprite Cu2+ in the same site. It is also observed that the blue single crystal [Cu0.335Se0.582(HSeO3)2CuCl3(H2O)3] crystallizes in the orthorhombic system, space group Pbn21. Structurally, the crystal structure of [Cu0.332Se0.582(HSeO3)2CuCl3(H2O)3] represents a new type of structure for complexes of hydrogen selenites (Figure 1). The building blocks [Cu0.335Se0.582(HSeO3)2] and [CuCl3(H2O)3], hereunder drawn, are arranged to form layers in the structure parallel to the (001) plane between which the lone pairs E are located (Figure 2). Due to the stereochemical activity of the lone pairs E, Se has very asymmetric coordination polyhedral SeO3 pyramids.
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 SeO3 triangular pyramid, in which the Se
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 . Therein, the sequential metal atoms in the chain trend schematically following Cu(1)
3.2. Spectroscopic studies
In order to confirm the crystallographic results of the following compound: [Cu0.335Se0.582 (HSeO3)2CuCl3(H2O)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−1.
In this chapter, the band corresponding to the symmetric stretching vibrations of SeO2 groups was observed at around 825 cm−l 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−1 [13, 14, 15].
From Figure 7, a band of very weak intensity is observed at 710 cm−1, accompanied by a shoulder at 738 cm−1, which is ascribed to asymmetric stretching vibrations of SeO2 groups. In the 686–740 cm−1 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 CuO4Cl2 coordination octahedra. The axial Cu
Typically, the stretching vibrations of the HSeO3− ion (νSe
|825 vs||825 s||νsSeO2|
|425 vw||442 w||δSeO2|
|288 w||Cu─O stretching|
|225 vw||stretching modes of Cu─Cl|
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
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−l are noticeable for this mode. The bending mode of H2O that appears at around 1606 cm−1 in the IR is noteworthy. The considerable shifting of stretching and bending frequencies from those of a free water molecule (H2O)  may be an indication of the presence of strong hydrogen bonding in the new crystal. The external modes of the HSeO3 ion, lattice modes of water, and metal-oxygen stretching modes appear approximately below 200 cm−l .
3.3. Dielectric studies
Figure 8 shows an illustration of the temperature dependence of the dielectric constant (ε′) 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 [Cu0.335Se0.582(HSeO3)2CuCl3(H2O)3]. These curves (Figure 8) exhibit the following characteristics:
(1) There is one anomaly in the dielectric constant ε′ 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 HSeO3 groups, as reported in the literature [16, 17].
In conclusion, the author ascertains that a novel substituted hydrogen selenites [Cu0.335Se0.582 (HSeO3)2CuCl3(H2O)3], have been successfully prepared via slow evaporation method. The crystal structure of the novel compound is characterized by the presence of structural blocs with structures as such [Cu0.335Se0.582(HSeO3)2] and [CuCl3(H2O)3]. The principal compound is arranged to form layers in the structure parallel to the (001) plane between which the lone pairs E are located. So, the main feature of the structure of this compound is based on different coordination polyhedral, SeO3 pyramids, and [CuCl3(H2O)3] groups. The presence of hydrogen selenites (Se
The author gratefully acknowledges the support of the University of Sfax.