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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.53 n.2 Concepción jun. 2008

http://dx.doi.org/10.4067/S0717-97072008000200012 

 

J. Chil. Chem. Soc, 53, N° 2 (2008), pages: 1503-1506

 

SYNTHESIS AND CHARACTERIZATION OF THE LAMELLAR OXYCHLORIDE C02CU4TE4O11CL4

 

D. RUIZ - LEÓN* AND. E. SPODINE

Universidad de Chile, Facultad de Ciencias Químicas y Farmacéuticas, Departamento de Química Inorgánica y Analítica, Casilla 233, Santiago, Chile druizleo@icaro.dic.uchile.cl
Centro para la Investigación Interdisciplinaria Avanzada en Ciencias de los Materiales (CIMAT) Universidad de Chile.


ABSTRACT

In this work we describe the synthesis and characterization of the Co2Cu4Te4O11Cl4 phase prepared by the hydrothermal technique. From the polycrystalline X-ray diffraction pattern the obtained new phase was indexed in the P-l space group with cell parameters a= 9.274(7), b= 12.132(2), c= 9.252(2) Å, α = 98.18(14), ß = 108.22(13) and γ = 111.07(8) °. The nominal composition and morphology were obtained using SEM-EDX measurements. The optical band gap of the material corresponds to an insulating system with 2.7 eV, and the thermogravimetric data shows that the phase is stable up to 700 °C.


 

INTRODUCTION

From the synthetic point of view, the hydrothermal synthesis is referred to the preparation of substances in sealed containers, heated up at above room temperature and autogenerated pressures. An interesting fact of this synthesis is that it enables working at non thermodynamic conditions permitting the crystallization of compounds with different structures due to the variation of the physicochemical properties of the solutions. Basically, the mechanism of the hydrothermal reactions obeys a model of liquid nucleation1 which differs from the synthesis in solid state where the mechanism of reaction consists basically in the diffusion of atoms and ions in the interphase between the reactants. This method has been used in the synthesis of interesting types of solid phases like microporous crystals, super-ionic conductors, and chemical sensors, electronic and magnetic solids2-6.

Between current techniques used for the production of nanostructured materials, the hydrothermal synthesis presents many benefits, high degree of chemical homogeneity at molecular scale, the use of low temperatures as compared with solid state syntheses, formation of nanocrystalline materials in a one-step process. Nowadays, the hydrothermal technique represents the most promising route for the cost-reducing production of advanced ceramic materials like polycrystalline oxides, electroceramics, hybrid organic- inorganic materials and molecular systems with transition metals with characteristics adapted to technological applications.

An interesting family of compounds that presents an increasing development during the last years corresponds to the oxychlorides of transition metals, which are phases of great interest due to their intercalation properties, superconducting properties at high temperature, electric, optical and magnetic properties7-13. For example, a wide range of amines have been intercalated in FeOCl, TiOCl, VOC1 and LnOCl ( Ln = Ho, Er, Tm and Yb )7-9. On the other hand, phases like ( Ca, Na )2Cu02Cl2 and (Ca, Na )2CaCu204Cl2 have proven to be materials with a high Curie temperature14-15. The phase Ba5Co5C103 shows both antiferromagnetic and ferromagnetic phenomena16, while Sr2Mn03Cl and Sr4Mn3Cl are compounds with strong antiferromagnetic interactions17. On the other hand Bi3Pb2Nb2OnCl presents ferroelectricity18 and the phase BiSe03O8 is active in the generation of a second harmonic12, which is fundamental in nonlinear optics.

In addition, recently Okamoto's group19 has started to work on a new oxychloride family of rare earths doped with calcium, which are promising solid state conductive electrolytes. This effect is due to the migration of chloride ions across the inorganic framework. Due to the interesting and peculiar characteristics to being insoluble in water and very stable at high temperatures, they are potential candidates to be HC1 or Cl2 sensors. This type of phases have been synthesized both by ceramic, hydrothermal methods and vapor transporting reactions20-22.

In this context, telurites are particularly interesting not only from a structural point of view, since they can adopt a varied number of coordination environments, but also due to their the magnetic behavior. Despite the fact that these structures present the usual Te03-2 pyramidal fragment, which is quite common in sulfites and selenites23, telurites24 can also show a tetracoordinated fragment Te04-4 with a C2v symmetry, and can adopt a unique geometry of the type Te03+1-4, where one of the axial positions of the pseudotrigonal bipyramid is longer than the other ones25. This unusual and rich chemistry, has given a renewal interest to the production of new telurites phases like the spiroffite structure M2Te3Os (M= Co, Cu, Zn )26.

New works on oxychlorides of tellurium and copper with other transition metals do not exist in literature. Due to this fact, the aim of this work was to prepare and characterize new phases derived from oxychlorides.

EXPERIMENTAL

Polycrystalline phase Co2Cu4Te4O11Cl4, was synthesized using the hydrothermal method. Solid reagents, CoCl2, CuO, Te02, in a relation 1:2:2 were weighted in stoichiometric amounts and placed within the reactor. The autoclave bomb was loaded with 2 ml of aqueous 5 M NH4C1 solution used as mineralizing agent. The hydrothermal bomb was placed in a furnace and then heated to 375 °C for a period of 5 days. The obtained material was filtered and washed with H20, methanol and acetone.

The elementary analysis and morphology of the sample were obtained from SEM-EDX measurements.

Room temperature diffuse reflectance measurements from 800 to 200 mil were made using a Perking Elmer spectrophotometer. Barium sulfate was used as the reflectance standard. The reflectance data was converted to absorbance data using the Kubelka-Munk function.

Thermogravimetric measurements were carried out in order to get information about the thermal stability of the sample.

Powder diffraction data were obtained using a Siemens D-5000 diffractometer, equipped with monochromated Cu Kα radiation ( λ=l.54056 A ). The lattice parameters were obtained using a minimum of 29 lines from 28 = 5-60°. For the phase analysis a STOE equipment with a molybdenum source equipped with monochromated Mo Kα radiation ( λ=0.70930 Å ) and linear PSD detector in transmission scan mode was used to decrease the high fluorescence observed in the background of the measured samples.

RESULTS AND DISCUSSION

Several reactions with different stoichiometric ratios and different temperatures were carried out, in order to obtain the optimal conditions for a high purity phase. For example at 250 °C, if the ratio of CoCl2, CuO, Te02 is 1:2:2 the principal product is the Co2Te3Os (Figure 1a) (spiroffite structure), while changing the ratios to 1:2:4, the principal products are the Co2Te3Os phase and the Te02 (Figure 1b). Stoichiometric relations between the reagents CoCl2, CuO, Te02, in a ratio 1:2:2 was found to be the optimal, at the reaction temperature of 375°C The results are summarized in table 1.



Chemical composition was determined using SEM-EDX. The secondary and retro-dispersed electrons confirm the stoichiometry and the homogeneity of the sample. The nominal composition was found to be Co24Cu38Te42O105Cl43 (Table 2).The morphology of the obtained phase is shown in the micrograph given in Figure 2.



For Co2Cu4Te4O11Cl4 the lattice parameters were obtained indexing the sample from powder diffraction data in the space group P-1 with cell parameters of a= 9.274(7), b= 12.132(2), c= 9.252(2) Å, α = 98.18, ß = 108.22 and γ = 111.07°, which are related to the isoestructural compound Ba2Cu4Te4O11Cl4. The indexed data are summarized in table 3.



Recently Kolis et. al27 synthesized two new phases Ba2Cu4Te4O11Cl4 and BaCu2Te2O6Cl2, and were able to reach the interesting structural properties of the telurites with the physical properties of the oxychlorides. These phases correspond to lamellar telurites of copper and barium, with the incorporation of chloride as mineralizing agent. In this way it is possible to obtain oxychlorides of these phases, where it is possible to find unusual square planar coordinations due to the presence of tellurium( IV ) oxoanions. From the structural point of view, in the Ba2Cu4Te4O11Cl4 phase Cu (I) atoms co-exist in tetrahedral or trigonal planar configurations and the Cu (II) atoms are in the square planar configuration, separated by chloride and oxide layers respectively. Barium atoms are located in the interlamellar space (Figure 3 ).


It is interesting to remark that the ionic radii of Co(II) and Ba(II) are 0.78 and 1.35 Å respectively. However, both oxychloride phases which contain these divalent cations are isoestructural. This can be explained by the fact that these ions must have a weak influence on the interlamellar space, and consequently on the lattice parameters.

Diffuse reflectance spectroscopy was used to determine the optical band gap of the obtained compound. A plot of absorbance v/s energy for Co2Cu4Te4O11Cl4is shown in Figure 4. The band gap was obtained by determining the inflection point of the first derivative curve of reflectance v/s energy. The obtained band gap was 2.7 eV, indicative of an essentially insulating material, and can be compared with previous data reported for Ba2Cu4Te4O11Cl4(3.0 eV). Thus a less insulating behavior is observed for the studied Co (II) phase.


Thermogravimetric data were obtained between room temperature and 800 °C, showing that the phase is stable in air to 700 °C with a loose of weight of 11% associated to a decomposition process ( Figure 5 ).


CONCLUSION

After several attempts we were able to obtain the exact experimental conditions to prepare the Co2Cu4Te4O11Cl4 phase.

It is clear that the variation in the ratios of the solid reagents and the mineralizer, promotes in most cases the formation of the spiroffite structure like the most stable compound, and a detailed control of the experimental conditions is needed.

ACKNOWLEDGMENTS

The authors acknowledge the financial support of FONDECYT GRANT 3060073 and FONDAP GRANT 11980002. The authors thank Prof. A. West for the diffraction facilities at the University of Sheffield, UK and Prof. J. Llanos for diffuse reflectance measurements and thermogravimetric analyses.

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(Received: 7 December 2007 - Accepted: 9 April 2008)

 

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