Introduction

Several oxochloride compounds have previously been synthesized containing Se⁴⁺ that has a stereochemically active lone pair; e.g. Co₅(SeO₃)₄Cl₂ [1], Zn₂SeO₃Cl₂ [2], Cu₅(SeO₃)₂O₂Cl₂ [3], Cu₉Se₄O₁₄Cl₆ [4], Cu₃(SeO₂)₃Cl₂ [5], Mo₂Se₄O₂Cl₈ [6]. In such compounds Se⁴⁺ almost always only form bonds to oxygen due to its small radius and high Lewis acidity strength, while late transition metals bond to both oxygen and chlorine anions due to their weaker Lewis acidity strength. The stereochemically active lone-pair on Se⁴⁺ and the chloride ions both act as terminating groups and help to open up the crystal structures leading to an increased probability to form compounds that may show e.g. magnetic frustration when the compounds contain magnetic cations because of the increased probability for those ions to take low-dimensional arrangements; examples of such compounds are Cu₂Te₂O₅X₂ and FeTe₂O₅X (X = Cl, Br) [7]–[8]. Irregular coordination around p-element lone-pair cations also lead to an increased possibility to form non-centrosymmetric crystal structures that may show nonlinear optical second harmonic generation (SHG); examples of such compounds are BaNbO(IO₃)₅ and BiO(IO₃) [9]–[10].

The objective of this synthetic work was to search for new oxochloride compounds in the Cd²⁺-Se⁴⁺-O-Cl system. There are previously only two oxohalides described containing Cd²⁺ and Se⁴⁺; Cd₄(SeO₃)₂Cl₄(H₂O) [11] and Cd₁₀(SeO₃)₈Br₄•HBr•H₂O [12]. The study resulted in finding the compound Cd₄(SeO₃)₂OCl₂ that turned out to have a crystal structure very similar to the previously described compound Cd₄(SeO₃)₂Cl₄(H₂O) [11].

Experimental

The following chemicals were used as starting materials: CdCl₂ (BDH Chemicals, 99%) and SeO₂ (Alfa Aesar, 99.4%). Single crystals of Cd₄(SeO₃)₂OCl₂ were obtained from a mixture of CdCl₂:SeO₂ in the non-stoichiometric molar ratio 1∶2. The mixture was heated at 450°C for 96 h in an evacuated glass tube in a muffle furnace which was further cooled to room temperature at a rate of 10°C/h. The right phase was not formed at reaction temperatures lower than 400°C and reaction temperatures higher than 450°C resulted in lower yield. The synthesis product consists of white Cd₄(SeO₃)₂OCl₂ single crystals in a matrix of white undetermined powder that most likely was SeO₂. The new oxochloride was characterized in a scanning electron microscope (S3700N, HITACHI, Japan) equipped with an energy dispersive spectrometer (HORIBA, Japan) confirming the presence and stoichiometry of the elements Cd, Se, Cl and O. Thermogravimetric measurements were carried out in air using a simultaneous TGA/DSC apparatus SDT Q600 from TA instruments. The residue was further characterized using Fourier Transform Infra-Red (FTIR) spectrophotometer {FTIR-4100, JASCO-CE, PerkinElmer, UK}.

Single crystal X-ray diffraction experiments were carried out on an Oxford Diffraction Xcalibur3 diffractometer equipped with a graphite monochromator. The data collection was performed at 293 K using MoK_α radiation, λ = 0.71073 Å. Absorption correction and data reduction were performed with the software CrysAlis RED that also was employed for the analytical absorption correction [13]. The structure solution was carried out with SHELXS97 and the refinement with SHELXL97 [14] in the WINGX [15] environment. All atoms were refined anisotropically. Crystal data are reported in Table 1. The structural drawings are made with the program DIAMOND [16].

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Table 1. Crystal data and structure refinement for Cd₄(SeO₃)₂OCl₂.

https://doi.org/10.1371/journal.pone.0097175.t001

Result and Discussion

The new compound Cd₄(SeO₃)₂OCl₂ crystallizes in the orthorhombic system, space group Fmmm. A displacement ellipsoid diagram showing the coordination around the cations is given in Figure 1. EDS-data based on analyses of five different crystals determines the amount of the heavy atoms to be 26.1±0.9 at% Cd, 13.2±0.6at% Se and 13.0±0.5 at% Cl that is in reasonable agreement with the results from the refinement of single-crystal X-ray data; 26.6 at% Cd, 13.3 at% Se and 13.3 at% Cl.

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Figure 1. A displacement ellipsoid diagram for Cd₄(SeO₃)₂OCl₂ showing the coordination around the cations.

Atoms Cl(1) and Cl(2) are half occupied and the Cd(3)-Cl bonds are marked as dotted lines. Atomic displacement parameters are given at the 50% probability level. [Symmetry codes: (i) 0.5–x, y, 0.5–z; (ii) x, 0.5–y, 0.5–z; (iii) 0.5–x, 0.5–y, z; (iv) −0.5+x, −y, 0.5–z; (v) 0.5–x, −y, 0.5–z; (vi) x, −y, z; (vii) 1–x, y, z; (viii) −x, y, z; (ix) 1–x, y, −z;].

https://doi.org/10.1371/journal.pone.0097175.g001

All the cadmium polyhedra are connected to each other to form a 3D network. An overview of the crystal structure of Cd₄(SeO₃)₂OCl₂ along [100] is shown in Figure 2. There are three unique Cd²⁺ ions having different coordination polyhedra; [Cd(1)O₆], [Cd(2)O₈] and [Cd(3)O₄Cl₂]. The distorted octahedron [Cd(1)O₆] has Cd – O distances in the range 2.182(2) –2.379(2) Å and the distorted cube [Cd(2)O₈] has bond distances in the range 2.379(7)–2.476(2) Å. Those two Cd-polyhedra are connected via corner- and edge sharing to form oxide layers in the crystal structure, see Figure 3. The Cd(3)-atom coordinate four oxygen and four half occupied Cl-positions, two Cl(1) and two Cl(2), that altogether form a square antiprism, see Figure 1. The half occupied Cl(1) atoms show a very short Cl(1)-Cl(1) distance that is 2.179(4) Å and also a very short Cl(2)-Cl(2) distance that is 1.496(5) Å. For the structural drawings the half occupied Cl(1) and Cl(2) atoms are treated as one fully occupied Cl-atom positioned in between Cl(1) and Cl(2) to yield an irregular [Cd(3)O₄Cl₂] polyhedron with Cd-O bond lengths in the range 2.146(2)–2.614(2) Å and Cd-Cl distances in the range 2.591(2)–2.698(2).

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Figure 2. Cd₄(SeO₃)₂OCl₂ along [100].

The Cl atoms are replaced with a dummy atom inserted halfway in between the half occupied Cl(1) and Cl(2) to replace those two atomic positions in the polyhedra.

https://doi.org/10.1371/journal.pone.0097175.g002

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Figure 3. Oxide layer made up of Cd(1)O₆ and Cd(2)O₈ polyhedra and SeO₃ groups, view along [001].

https://doi.org/10.1371/journal.pone.0097175.g003

There is only one crystallographically unique Se atom having the classic trigonal pyramidal [SeO₃] coordination. The Se-O distances are in the range 1.690(2)–1.733(3) Å. Those polyhedra are isolated and do not polymerize. This is in accordance with earlier observations that SeO₃ only seldom link to other Se-coordination polyhedra [17].

The [Cd(1)O₆] polyhedra share corners with two other polyhedra of the same kind to form chains along [100]. Those chains are further connected by edge sharing to chains of edge sharing [Cd(2)O₈] polyhedra to form oxide layers parallel to (001), see Figure 3. These layers are further linked to the [Cd(3)O₄Cl₂] polyhedra via edge sharing. The [Cd(3)O₄Cl₂] connect to each other via the Cl-atoms to build the 3D structure, see Figure 2. The [SeO₃] polyhedra are positioned so that the lone-pairs point out from the oxide layers. Each [SeO₃] group share corners with two [Cd(1)O₆] and three [Cd(3)O₄Cl₂] and via edge sharing to two [Cd(2)O₈] polyhedra.

Bond-valence sum calculations yield the following reasonable values; 2.05 for Cd(1), 1.96 for Cd(2), 1.90 for Cd(3), 4.01 for Se(1), 1.98 for O(3), 2.17 for O(4), 2.02 for O(5). Calculations are not made for the Cl-ions due to their half occupancy. The calculations are carried out according to Brown and the r₀ values used were 1.811 for Se-O and 1.904 for Cd-O and with B = 0.37 [18].

The crystal structure of Cd₄(SeO₃)₂OCl₂ is very similar to the previously described compound Cd₄(SeO₃)₂Cl₄(H₂O) [11]. The two structures are considered to be isostructural and a main difference is that the half occupied Cl-positions in Cd₄(SeO₃)₂OCl₂ are fully occupied in Cd₄(SeO₃)₂Cl₄(H₂O), it is, however, not clear from the structure description of the latter compound where the water molecules are located.

TGA analysis in air show that Cd₄(SeO₃)₂OCl₂ is stable up to 500°C. There is an initial weight loss due to evaporation of adsorbed water (≈1.5%) up to this temperature. The decomposition of the compound takes place in one main step in the temperature range 500–750°C which corresponds to release of two mole SeO₂ and one mole CdCl₂ to yield amorphous CdO as the remaining solid product. Powder X-ray diffraction show that the residuals after thermal decomposition is amorphous and IR-spectra show that there are Co-O vibrations, see Supporting Information, Figure S3. The observed weight loss ≈52.5% is close to the calculated (51.3%) for the product CdO. The decomposition curve is available in Supporting Information, Figure S1.

Conclusions

A new compound Cd₄(SeO₃)₂OCl₂ was obtained via solid state reactions. It crystallizes in the orthorhombic space group Fmmm with the unit cell a = 7.3610(3) Å, b = 15.4936(2) Å, c = 17.5603(3) Å and Z = 8. The building units in the crystal structure are [Cd(1)O₆], [Cd(2)O₈], [Cd(3)O₆Cl₂] and [SeO₃] polyhedra. The Cd-polyhedra are linked by corner and edge sharing to build the 3D framework. The crystal structure can be regarded as layered where the oxide layers are connected via Cl-bridges. Thermal gravimetric analysis show that the compound is stable up to 500°C where it starts to decompose.

Supplementary Material

Crystallographic information files has been sent to Fachinformationzentrum Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany (fax +49-7247-808-666; email: crysdata@fiz-karlsruhe.de), and can be obtained on quoting the deposit number CSD-426650. Atomic coordinates, selected atomic distances, angles, a graph showing the thermogravimetric decomposition and analysis of the decomposition product are shown in the Supporting Information; Table S1–S3, Figure S1–S3.

Supporting Information

Acknowledgments

This work has been carried out with financial support from the Higher Education Commission (HEC) in Pakistan and from the Swedish Research Council.